HETEROCYCLIC METHYL FREE RADICALS AS IMAGE ENHANCING AGENTS

Abstract: The invention provides novel persistent free radicals useful in Overhauser MRI being triarylmethyl radicals having as at least one aryl group a group of formula (1) or (2), wherein each X which may be the same or different is an oxygen or sulphur atom or a group CO or S(O)n (where n is 1 to 3) with the proviso that at least one group X is a sulphur atom or an S(O)n group; R<1> is a hydrogen atom or group of formula -M, -XM, -X-Ar<2> where M is a water solubilising group, and Ar<2> represents a 5-10 membered aromatic ring optionally substituted by a water solubilising group M; and each of the groups R<7>, which may be the same or different, is a hydrogen atom, or a hydrocarbon group or a water solubilising group M, or two groups R<7> together with the atom to which they are bound represent a carbonyl group or a 5 to 8 membered cycloalkylidene, mono- or di-oxacycloalkylidene, mono or di-azacycloalkylidene or mono- or di-thiacycloalkylidene group optionally wit the ring attachment carbon replaced by a silicon atom, and R<7> where it is other than hydrogen, is optionally substituted by a hydroxyl group, an optionally alkoxylated, optionally hydroxylated acyloxy or alkyl group or a water solubilising group M. The novel radicals have good stability, long relaxation times and good relaxivity.

Description: HETEROCYCLIC METHYL FREE RADICALS AS IMAGE ENHANCING AGENTS Field of the Invention The present invention relates to certain novel triaryl methyl free radicals and their use as image enhancing agents in magnetic resonance imaging ( RI) as well as to contrast media containing such radicals and to the use of such radicals and their non-radical precursors in the manufacture of MRI contrast media. Background of the Invention MRI is a diagnostic technique that has become particularly attractive to physicians as it is non- invasive and does not involve exposing the patient under study to potentially harmful radiation, such as for example the X-radiation of conventional radiography. This technique, however suffers from several serious drawbacks, including in particular the expense of manufacture and operation of the MRI apparatus, the relatively long scanning time required to produce an image of acceptable spatial resolution, and the problem of achieving contrast in the magnetic resonance (MR) images between tissue types having the same or closely similar imaging parameters, for example in order to cause a tissue abnormality to show up clearly in the images. The expense of manufacture and operation of an MRI apparatus is closely associated with the strength of the magnetic field that the primary magnet in the apparatus is required to generate in order to produce images of acceptable spatial resolution in an acceptable time. MR images are generated by manipulation of the MR signals detected from the sample, for example a human or animal body, placed in a magnetic field and exposed to pulses of radiation of a frequency (typically radiofrequency (RF) ) selected to excite MR transitions in selected non-zero spin nuclei (the "imaging nuclei", which are generally water protons in body fluids) in the sample. The amplitude of the induced MR signals is dependent upon various factors such as the strength of the magnetic field experienced by the sample, the temperature of the sample, the density of the imaging nuclei within the sample, the isotopic nature and chemical environment of the imaging nuclei and the local inhomogeneities in magnetic field experienced by the imaging nuclei. Thus many techniques have been proposed for enhancing MR image quality, for example by increasing MR signal amplitude or by increasing the difference in MR signal amplitude between different tissue types. The imaging parameters (nuclear density, T and T 2 ) for tissues of interest may be altered and many proposals have been made for doing this by the administration of MRI contrast agents into patients under study (see for example US-A-4647447 (Gries/Schering) , US-A-4925652 (Gries/Schering) and US-A-4863715 (Jacobsen/Nycomed) ) . Where such MRI contrast agents are paramagnetic they produce significant reduction in the T x of the water protons in the body zones into which they are administered or at which they congregate, and where they are ferromagnetic or superparamagnetic (for example as suggested by Jacobsen) they produce a significant reduction in the T 2 of the water protons. In either case the result is enhanced (positive or negative) contrast in the MR images of such zones. The contrast enhancement achievable by such agents in conventional MRI is relatively limited and it is generally not such as to allow a reduction in the image acquisition period or in the field strength of the primary magnet. Utilisation of the spin transition coupling phenomenon known as dynamic nuclear polarisation or as the Overhauser effect to amplify the population difference between the ground and excited spin states of the imaging nuclei by the excitation of a coupled ESR transition in a paramagnetic species present in the sample being imaged has been described in US-A-4984573 (Leunbach/Nycorned Innovation) . This new technique for generating a MR image of the sample, which is hereinafter termed Overhauser MRI (OMRI) , involves exposing the sample to a first radiation of a frequency selected to excite nuclear spin transitions in selected nuclei in the sample (radiation which is generally of radiofrequency or thereabouts and thus for convenience will be referred to hereinafter as RF radiation) and also exposing the sample to a second radiation of a frequency selected to excite electron spin transitions coupled to nuclear spin transitions for at least some of the selected nuclei (radiation which is generally of microwave frequency or thereabouts and thus for convenience is referred to hereinafter as MW or UHF radiation) , the MR images being generated from the resulting amplified MR signals (free induction decay signals) emitted by the sample. The paramagnetic substance which possesses the ESR transition which couples with the NMR transition of the image nuclei may be naturally present within the imaging sample or more usually may be administered as an OMRI contrast agent. A number of "oxygen free radicals" that is to say radicals where the unpaired electron or electrons are associated with the oxygen atom have been proposed as OMRI contrast agents including for example nitroxide stable free radicals, chloranil semiquinone radical and Fremy's salt (US-A-4984573) and deuterated stable free radicals, in particular deuterated nitroxide stable free radicals (WO-A-90/00904) . Such radicals have not however been found to be entirely satisfactory due to problems associated with stability, toxicity or poor coupling of the electron and nuclear spin transitions. In WO-A-91/12024 Nycomed Innovation AB proposed persistant carbon free radicals, i.e. radicals where the unpaired electron or electrons are primarily associated with carbon atoms, including triaryl methyl radicals, for use as OMRI contrast agents. Summary of the Invention We have now found that a particular group of novel triaryl methyl radicals has certain advantageous properties making them particularly suitable for use as OMRI contrast agents. Viewed from one aspect the present invention thus provides a radical compound of formula I .C(ArX (I) (where each group Ar 1 , which may be the same or different is an optionally substituted aromatic group, preferably an optionally substituted 5-7 membered carbocyclic or heterocyclic aromatic ring optionally carrying one or more fused carbocyclic or heterocyclic rings, preferably a benzyl ring and at least one of said Ar 1 groups is a group Ar 3 of formula (wherein each X which may be the same or different represents an oxygen or sulphur atom or a group CO or S(0) n (where n is 1 to 3) with the proviso that at least one group X is a sulphur atom or a S(0) n group; R 1 represents a hydrogen atom or group of formula -M, -XM, -X-Ar 2 , or -Ar 2 where M is a water solubilising group, and Ar 2 represents a 5-10 membered aromatic ring optionally substituted by a water solubilising group M; and each of the groups R 7 , which may be the same or different represents a hydrogen atom, or a hydrocarbon group such as an alkyl, hydroxyalkyl, alkoxyalkyl, alkoxycarbonyl, or carbamoyl group, or a water solubilising group M or two groups R 7 together with the atom to which they are bound represent a carbonyl group or a 5 to 8 membered cycloalkylidene, mono- or di-oxacycloalkylidene, mono- or di-azacycloalkylidene or mono- or di-thiacycloalkylidene group optionally with the ring attachment carbon replaced by a silicon atom (preferably however in any spiro structure the ring linking atom will be bonded to no more than three heteroatoms) and R 7 where it is other than hydrogen, is optionally substituted by a hydroxyl group, an optionally alkoxylated, optionally hydroxylated acyloxy or alkyl group or a water solubilising group M) ) or a perdeuterated analogue or salt thereof. The optionally substituted aromatic group Ar 1 may be selected from any of the aromatic groups described for the triarylmethyl radicals of WO-A-91/12024. Preferably however in compounds of Formula I where not all Ar 1 groups are groups Ar 3 , those Ar 1 groups which are other than groups Ar 3 are groups Ar 1 ' of formula (wherein X, R 1 and R 7 are as defined above excluding the proviso that at least one X is S or S(0) n ). In such preferred groups Ar 1 ', X is preferably an oxygen atom and R 7 is a hydrogen atom or an optionally hydroxylated alkyl, preferably methyl group. Preferred radical compounds of formula I are those in which each group Ar 1 is an aromatic group substituted by a solubilising group M and carrying two fused sulphur- containing heterocyclic rings each substituted by at least one, preferably two, solubilising groups M. Especially preferred radical compounds are those of formula I, wherein each of said Ar 1 groups is of formula: (wherein R 1 represents a group of formula COOR 5 or COOM 1 (wherein R 5 and M 1 are as defined above) ; each of the groups R 7 , which may be the same or different represents a hydrogen atom, or a hydrocarbon group such as an alkyl, hydroxyalkyl, alkoxyalkyl, alkoxycarbonyl, or carbamoyl group, and R 7 where it is other than hydrogen, is optionally substituted by a hydroxyl group, an optionally alkoxylated, optionally hydroxylated acyloxy or alkyl group,- and at least one group R 7 on each fused ring is a water solubilizing group, i.e other than hydrogen and unsubstituted alkyl) or a deuterated analog, precursor or salt thereof. Especially preferred radical compounds of formula I according to the invention are tris(8-carboxy-2,2, 6,6- tetrahydroxymethylbenzotl,2-d:4,5-d 1 ]bis(1,3)dithiole-4- yl)methyl and the salts (eg sodium salt) and + COO ' Na bis- (8-sodium carboxylate-2,2,6,6-tetrakis- ( 2 H 3 -methyl) - benzo[1,2-d:4,5-d 1 ] -bis(1,3)dithiole-4-yl) -mono- (8-sodium carboxylate-2,2,6,6-tetrakis- ( 2 H 3 -methyl) -benzo[1,2-d:4,5- d'] -bis(1,3)dioxole-4-yl)methyl (herein referred to as perdeuterated trityl) and bis- (8-sodium carboxylate-2,2,6,6-tetrakis- (hydroxy- 2 H 2 - methyl) -benzo[1,2-d:4,5-d 1 ] -bis(l,3)dithiole-4-yl) -mono- (8-sodium carboxylate-2,2,6,6-tetramethylbenzo[1,2-d:4,5- d'] -bis(1,3)dioxole-4-yl)methyl (herein referred to as deuterated hydroxy trityl) . Particularly preferred radical compounds of Formula I include those wherein at least one, preferably two, and more preferably all three Ar 1 groups are groups Ar 3 and any remaining Ar 1 groups are Ar 1 ' groups. Especially preferably in each group Ar 3 or Ar 1 ', the two fused rings are the same. In the radical compounds of formula I, the solubilising groups M may be any of the solubilising groups conventionally used in diagnostic and pharmaceutical products. Particularly preferred solubilizing groups M include optionally hydroxylated, optionally alkoxylated alkyl or oxo-alkyl groups and groups of formulae R 5 , COOR 5 , OCOR 5 , CHO, CN, CH 2 S(0)R 5 , CONR 5 2 , NR 5 COR 5 , NR 5 2 , S0 2 NR 5 2 , OR 5 , P0 3 2" ; SOR 5 , S0 2 R 5 , S0 3 M\ COOM 1 (where R 5 represents a hydrogen atom or an optionally hydroxylated, optionally aminated, optionally alkoxylated, optionally carboxylated alkyl, oxo-alkyl, alkenyl or alkaryl group and M 1 is one equivalent of a physiologically tolerable cation, for example an alkali or alkaline earth metal cation, an ammonium ion or an organic amine cation, for example a meglumine ion), -(0(CH 2 ) n ) m OR 5 (where n is an integer having a value of from 1 to 3 and m is an integer having a value of from 1 to 5), -CX(CHR 5 ) n X or CH 2 R 8 (where R 8 is a hydrophilic R 5 group) or SR 10 or S0 2 R 10 where R 10 is a group R 5 or an alkyl group optionally substituted by one or more, especially two or three groups COOR 5 , OCOR 5 , CHO, CN, CONR 5 2 , NR 5 COR 5 , NR 5 2 , S0 2 NR 5 2 , OR 5 , P0 3 2' , SOR 5 , S0 2 R 5 , S0 3 M\ COOM 1 , or - (0(CH 2 ) n ) m 0R 5 . Especially preferred as solubilizing groups M are groups of formula C(H) 3 _ n (CH 2 0H) n , R 9 , COR 9 , SR 9 , SOR 9 , S0 2 R 9 , C0N(R 9 ) 2 , NR 9 2 , NHR 9 and CONHR 9 [where R 9 may represent a hydroxylated alkyl group such as a group CH 2 0H CH 2 0H I I - C-CH 2 0H , - C-OH -CH 2 CH 2 0H , - CH 2 0H I I CH 2 OH CH 2 OH CH 3 -CH 2 CHOHCH 2 OH, -C-OH, -0-CH 2 CH 2 OH, -CH 2 OCH 2 CH 2 OH, COOH CH 2 OH, CH 2 OH, CH 2 OH CH-, CH, CH 2 OH • CH ■ COH ■ C-OH or ■ C-CH 3 I CH 2 OH, CH 2 OH, CH- 3/ CH,OH (although any R 9 group attached to a sulphur, nitrogen or oxygen atom is preferably not hydroxylated at the a carbon) ] , and groups of formula S0 2 R 12 or SR 12 where R 12 is a group CH 2 COOR 13 , CH(COOR 13 ) 2 , CH 2 CONHR 9 , CH 2 C0NR 9 2 , CR 5 (C0OR 13 ) 2 , CH(CN)C0 2 R 13 , (CH 2 ) n S0 3 " M 1 , (CH 2 ) n COR 13 , CH(COR 9 )CH 2 COR 9 and CH(R 5 )COR 9 where n, M 1 and R 5 are as earlier defined and R 13 is a hydrogen atom, an alkyl group or a group M 1 or R 9 . In the radical compounds of formula I any alkyl or alkenyl moiety preferably contains up to 6, especially preferably up to 4, carbon atoms and generally it is preferred for each of the three aryl monomers of the triaryl structure to be identical. In groups Ar 3 according to the invention, group X is preferably selected from oxygen or sulphur atoms or S0 2 groups. Within each Ar 3 moiety, preferably two and especially preferably all four X groups are sulphur atoms or S(0) n groups, preferably sulphur atoms or S0 2 groups. Suitable Ar 3 groups thus include for example those wherein the central aromatic ring carries fused rings of formula Preferred Ar 3 groups include : χά Rl χ : Within such preferred Ar 3 groups, R 7 is preferably a hydrogen atom or an optionally hydroxylated alkyl group, preferably a hydroxyC 1 . 3 -alkyl, especially a hydroxymethyl group. In the radical compounds of formula I preferred identities for the group R 1 include: -H -SCH 2 COO-Na+ -S0 2 R 2 -SR 2 -SCH 2 COOCH 2 CH 3 -S0 2 C (R 2 ) 2 CH 2 CHOHCH 2 OH -S0 2 NR 2 2 -S0 2 CH 2 CON(R 2 ) 2 -S0 2 -C- (CH 2 CH 2 OH) 2 COOCH 2 CH 3 -S0 2 C- (CH 2 CH 2 OH) 2 CH 2 OH -C- (CH 2 CH 2 OH) 3 -S0 2 -C(H) (COOCH 2 CH 3 ) 2 -CH 2 CON(CH 2 CH 2 OH) 2 -COOH -C0 2 Me -C0 2 Et (where R 2 is H or optionally hydroxylated alkyl eg. CH 2 CH 2 OH CH 2 CHOHCH 2 OH CH 3 0 ^ X 2^X13 CH 2 (CHOH) 4 CH 2 OH) Preferred radical structures of formula I include radical compounds of formulae la, lb, Ic and Id: where R 11 and R 12 are selected from the list of- preferred R 1 identities indicated above. Preferably R 11 and R 12 are selected from H, SCH 3 , SCH 2 C0 2 CH 2 CH 3 , SCH 2 COOH, S0 2 N(CH 3 )CH 2 (CHOH) 4 CH 2 OH, S0 2 NH 2 , S0 2 NCH 2 CH 2 OH and S0 2 NCH 2 CHOHCH 2 OH, and particularly preferably R 11 and R 12 are identical. Further aspects of the invention provide use of a compound of formula I for the manufacture of a contrast medium for use in OMRI and a method of magnetic resonance investigation of a sample, said method comprising introducing into said sample an inert carbon free radical, exposing said sample to a first radiation of a frequency selected to excite electron spin transitions in said free radical, exposing said sample to a second radiation of a frequency selected to excite nuclear spin transitions in selected nuclei in said sample, detecting free induction decay signals from said sample, and, optionally, generating an image or dynamic flow data from said detected signals. Viewed from a still further aspect, the invention also provides a magnetic resonance imaging contrast medium composition comprising a radical compound of formula I together with at least one pharmacologically acceptable carrier or excipient. For in vivo imaging, the radical compound should of course preferably be a physiologically tolerable radical, or one presented in a physiologically tolerable, e.g. encapsulated, form. . Viewed from a yet still further aspect the invention provides a method of magnetic resonance imaging wherein there is introduced into a human or non-human, preferably mammalian, subject an effective amount of a magnetic resonance signal amplifying agent and wherein an image of at least a part of said subject is generated, the improvement comprising introducing as said amplifying agent a radical according to the invention. Detailed Description The novel triarylmethyl radicals of the invention have the advantages of the beneficial properties of stability at physiological pH, long half lives (at least one minute, and preferably at least one hour) , long relaxation times, and surprisingly good relaxivity. Particularly advantageously, the novel radical compounds of the invention exhibit surprising stability when compared with corresponding compounds lacking Ar 3 groups as defined above. Stability is of paramount importance when considering suitability of radical compounds for use as MRI contrast agents, and thus the radical compounds of the present invention represent a considerable advance in the art. Thus for example the radical compounds l - where n is 0, 1, 2 or 3 and R 1 is hydrogen, SCH 3 , SCH 2 C00CH 2 CH 3 or SCH 2 C00H have been found to be at least 10 times more stable for each increment in n, e.g. the radical with n=3 is at least 1000 times more stable than the radical corresponding to n=0. For use according to the invention free radicals which have relatively few transitions, e.g. less than 15, preferably less than 10, in their ESR spectra and radicals having narrow linewidth ESR transitions, e.g. up to 500 mG, preferably less than 150 ttiG, especially less than 60 mG and particularly less than 25mG, are especially preferred for use as OMRI contrast agents. (The linewidths referred to are conveniently the intrinsic linewidths (full width at half maximum in the absorption - 2Λ - spectrum) at ambient conditions) . Whilst low numbers of ESR transition lines are generally preferred to obtain more effective coupling of the. ESR and NMR transitions, we have found that surprisingly good coupling, and therefore enhancement of the MR signal, may also be achieved with radicals showing a large number of ESR transitions. Where the radicals have a multiplicity of ESR transitions, the hyperfine splitting constant is preferably very small. In this connection radicals having as few as possible non-zero spin nuclei, positioned as far away as possible from the paramagnetic centre are thus especially preferred. The triarylmethyl radicals may be coupled to further molecules for example to lipophilic moieties such as long chain fatty acids or to macromolecules, such as polymers, proteins, polysaccharides (e.g. dextrans) , polypeptides and polyethyleneimines. The macromolecule may be a tissue-specific biomolecule such as an antibody or a backbone polymer such as polylysine capable of carrying a number of independent radical groups which may itself be attached to a further macromolecule. Coupling to lipophilic molecules or substitution of the radical with lipophilic groups is particularly useful since it may enhance the relaxivity of the radicals in certain systems such as blood. Such lipophilic and macromolecular derivatives of the radicals of formula I and salts thereof form a further aspect of the present invention. The linkage of a compound of formula I to the further molecule may be effected by any of the conventional methods such as the carbodiimide method, the mixed anhydride procedure of Krejcarek et al. (see Biochemical and Biophysical Research Communications 7_Z: 581 (1977)), the cyclic anhydride method of Hnatowich et al. (see Science 220: 613 (1983) and elsewhere) , the backbone conjugation techniques of Meares et al. (see Anal. Biochem. 142.- 68 (1984) and elsewhere) and Schering (see EP-A-331616 (Deutsch/Schering) for example) and by the use of linker molecules as described for in US-A-5208324 (Klaveness/Nycomed) . In view of their surprisingly beneficial properties, the novel triarylmethyl radicals of the invention may also be used as conventional MRI contrast agents, as ESR contrast agents or as ESR spin labels in ESR imaging or in magnetometry. The compounds according to the invention, particularly the perdeuterated compounds (eg perdeuterated trityl and deuterated hydroxy trityl) , are especiallly useful as MR signal enhancement agents for use in OMRI imaging of oxygen concentration (oximetry) or as spin labels in spin label oximetry. Thus in a further aspect the present invention provides the use of compounds according to formula I, preferably perdeuterated compounds, in oximetry. A method of spin label oximetry is described by Lurie at al, SMRM, llth Annual Meeting 1992, Abstract No. 4101 and a method of oximetry based on OMRI is described by Grucker et al in (MRM, :219-225 (1995) ) . The radical compounds of formula I may be prepared from their non-radical precursor compounds by conventional radical generation methods. Suitable non-radical precursor compounds include the corresponding triaryl - X - methanes, triaryl methyl halides and triaryl methanols, and derivatives, e.g. ethers, of the triaryl methanols. Thus in a further aspect the invention provides a process for the preparation of the radical compounds of formula I which comprises subjecting a radical precursor therefor to a radical generation step and optionally subsequently modifying the substitution on the aryl moieties, e.g. by oxidation or reduction. By such modification for example sulphide substituents, (e.g. -SCH 3 or -SCH 2 COOEt) may be oxidized to the corresponding sulphones so avoiding problems of acidic hydrogens prior to radical formulation. Similarly lipophilic substituents (such as -SCH 2 COOEt) may be reduced to corresponding hydrophilic substituents (e.g. -SCH 2 CH 2 OH) . Thus by way of illustration the radical-precursor can be represented by formula II (Ar 1 ) 3 CLv (II) where (Ar 1 ) 3 C is as previously defined and Lv is a group displaceable to produce a radical. Formula II embraces formulae such as (Ar^ a COH (III) (Ar 1 ) 3 CHal (IV) (Ar 1 ) 3 CH (V) (Ar 1 ) 3 CCOOH (VI) (Ar 1 ) 3 C.CO.O.O.CO.C(Ar 1 ) 3 (VII) (Ar 1 ) 3 C.NN C(Ar x ) 3 (VIII) (Where Hal represents halogen, e.g. Br or Cl) . - 2-f - Thus for example radical compounds of formula I may conveniently be prepared from corresponding triaryl methyl halides by reduction with a metal catalyst, such as copper, zinc or silver, or by electrolytic reaction on an electrode or by photochemical reaction in the presence of a chlorine radical scavenger, e.g. an olefin. Alternatively, the radicals may be prepared from the ' corresponding triaryl methanes by reaction with a base, e.g. in the presence of sodium hydride followed by a reaction with an oxidant, e.g. iodine in the presence of oxygen or a quinone such as chloranil, following for example the method described in US-A-3347941. Another method to prepare the radicals is to react triarylmethanes with other, less stable radicals such as tert-butoxyl radicals. The latter radicals are generated in situ via thermolysis or photolysis of an appropriate precursor, such as a peroxide or an azo compound. A further example of a method by which radical preparation may be effected is reaction of the corresponding triaryl methanols in the presence of an acid to form a carbonium ion followed by reduction to the free radical in the presence of a suitable reducing agent, such as metal ions e.g. Cr 2+ , Fe 2+ , or by electrochemical reduction. The carbon free radicals may also be generated by a comproportionation reaction between cations and anions of a corresponding radical precursor. In such a reaction an electron is exchanged between the anion and the cation, and two radicals are generated. Triaryl ethyl radicals may thus be prepared by mixing together a triarylmethyl radical precursor cation with a corresponding anion. Triarylmethyl radicals may also be prepared by thermolysis or photolysis or a corresponding dimeric triarylmethyl structure, for example an azobistriarylmethyl or a bis (triarylmethylcarboxylic acid) peroxide. An alternative method of preparation of triarylmethyl radicals is the electrochemical decarboxylation of a triarylmethylcarboxylate. Radicals with long half lives in aqueous solution, for example at least one hour, preferably ten days, more preferably fifty days and especially preferably at least one year are particularly desirable for use in in vivo imaging. The non-radical precursors may themselves be prepared by methods conventional in the art and a number of suitable methods are described in WO-A-91/12024. The documents referred to herein are incorporated herein by reference. Thus for example the following reaction schemes may be used: SCHEME 1: HCl Acetone -Z7 - SCHEME 2: 2. (EtOfeCO - 22 - SCHEME 3: l.n-BuLi 2. (EtO).CO - 2\ - SCHEME 4: 1. n-BuLi 2. (EtO) CO 3o - SCHEME S: Perdeuterated trityl may be prepared by the method described for the preparation of its non-deuterated analogue in Examples b5 to 0 below but with the use of - 3\ - acetone-d 6 instead of acetone in the initial ketalisation step (described in Example 2 of WO-A-91/12024) . Deuterated hydroxy trityl is prepared generally by successive steps of fused ring formation and deuterative reduction followed by analogous steps to those described for the preparation of the non-deuterated analogue in Examples 73 to 77 below. For use in OMRI, the radical compounds of formula I ' are conveniently formulated into contrast media together with conventional pharmaceutical carriers or excipients. Contrast media manufactured or used according to this invention may contain, besides the inert free radicals (or the non-radical precursor where radical formation is to be effected immediately before administration) , formulation aids such as are conventional for therapeutic and diagnostic compositions in human or veterinary medicine. Thus the media may for example include solubilizing agents, emulsifiers, viscosity enhancers, buffers, etc. The media may be in forms suitable for parenteral (e.g. intravenous) or enteral (e.g. oral) application, for example for application directly into body cavities having external voidance ducts (such as the gastrointestinal tract, the bladder and the uterus), or for injection or infusion into the cardiovascular system. However solutions, suspensions and dispersions in physiological tolerable media will generally be preferred. Free radicals which are relatively unstable or insoluble in the sample environment may be encapsulated, e.g. in gastric juice resistant capsules containing a medium in which they are stable. Alternatively, the radical may be presented as an encapsulated freeze dried powder in a soluble capsule. Such formulations might - 3.7. - conveniently be dissolved shortly before in vivo use. For use in in vivo diagnostic imaging, the medium, which preferably will be substantially isotonic, may conveniently be administered at a concentration sufficient to yield a 1 micromolar to 10 mM concentration of the free radical in the imaging zone; however the precise concentration and dosage will of course depend upon a range of factors such as toxicity, the organ targetting ability of the contrast agent, and the administration route. The optimum concentration for the free radical represents a balance between various factors. In general, optimum concentrations would in most cases lie in the range 0.1 to lOOmM, especially 0.2 to lOmM, more especially 0.5 to 5mM. Compositions for intravenous administration would preferably contain the free radical in concentrations of 10 to lOOOmM especially 50 to 500 mM. For ionic materials, the concentration will particularly preferably be in the range 50 to 200mM, especially 130 to l70mM and for non-ionic materials 200 to 400mM, especially 290 to 330mM. For imaging of the urinary tract or the renal or biliary system however, compositions may perhaps be used having concentrations of for example 10 to lOOmM for ionic or 20 to 200mM for non-ionic materials. Moreover for bolus injection the concentration may conveniently be 0.1 to lOOmM, preferably 5 to 25mM, especially preferably 6 to 15mM. The present invention will now be further illustrated by the following non-limiting Examples (percentages, parts and ratios are by weight and temperatures are in degrees Celsius unless otherwise stated) . - S5 ' Example 1 2.2,6.6-Tetramethylbenzori,2-d:4,5-d'1-bis(l,3)dioxole-4- carboxylic acid 2,2,6,6-Tetramethylbenzo[l,2-d:4,5-d']-bis(1,3)dioxole (10.0 g, 45.0 mmol; prepared according to WO-91/12024) was dissolved in dry THF (200 mL) under an argon atmosphere. The solution was cooled to -20 °C and n-butyllithium (20.0 mL, 50.0 mmol) in hexane was added. After attaining ambient temperature, the reaction mixture was transferred onto solid carbon dioxide (150 g) and allowed to stand overnight. Water (200 mL) was added and pH was adjusted to 10 using 2M aqueous NaOH. After washing with ether, the aqueous phase was acidified wit "2M hydrochloric acid to - 3<<- - pH 2 and extracted with ether (2*300 L) . The organic phases were dried (Na 2 S0 4 ) and evaporated to give the pure product. Yield: 10.7 g (89 %) . IH NMR (CDC13, 300 MHz) S: 6.50 (S, IH) , 1.71 (s, 12H) . 13 C NMR (CDC1 3 75 MHz) δ : 165.1, 140.9, 140.8, 119.8, 98.9, 97.3, 25.6. Example 2 2.2.6.6-Tetramethylbenzof1.2-d:4.5-d'1-bis(1.3)dioxole-4- carboxylic acid methyl ester 2,2,6,6-Tetramethylbenzo[1,2-d:4,5-d']-bis(1,3)dioxole- 4-carboxylic acid (10.0 g, 38.0 mmol) was dissolved in dry DMF (100 mL) . Potassium carbonate (15.2 g, 110.0 mmol) was added and the reaction was heated to 55 °C for 30 min. After cooling to ambient temperature, methyl iodide (15.6 g, 110.0 mmol) was added and the solution was stirred overnight. The precipitate was filtered off and the solution was evaporated. The residue was dissolved in saturated aqueous NaHC0 3 and ether. The aqueous layer was discarded and the organic phase was dried (Na 2 S0 4 ) , filtered and evaporated to give 9.4 g (88 %) of the pure product. 'H NMR (CDC1 3 300 MHz) S : 6.44 (ε, IH) , 3.85 (s, 3H) , 1.65 (s, 12H) . 13 C NMR (CDCI 3 75 MHz) S: 163.4, 140.8, 140.6, 119.0, 99.9, 99.4, 51.9, 25.6. Example 3 Bis-(2.2.6.6-tetramethylbenzof1.2-d:4.5-d' 1-bis(1,3)- dithiole-4-yl)-mono-(2.2.6.6-tetramethyl- benzor1.2-d:4.5-d']-bis(1.3)dioxole-4-yl )methanol 2,2,6,6-Tetramethylbenzo[1,2-d:4,5-d' ]bis(1,3)dithiole (2.86 g, 10 mmol; prepared according to WO-91/12024) was dissolved in anhydrous THF (75 mL) and cooled to -70 °C. n-Butyllithium (4.4 mL, 2.5 M in hexane) was added. The reaction mixture was allowed to reach ambient temperature. 4-Methoxycarbonyl-2,2,6,6-tetramethylbenzo- [l,2-d:4,5-d')-bis-(l,3)-dioxole (1.4 g, 5 mmol) was added as a solid. After 1 hour, the mixture was quenched with saturated aqueous NaH 2 P0 4 . The aqueous phase was discarded and the organic layer evaporated. The residue was dissolved in dichloromethane, washed with water and dried (Na 2 S0 4 ) . The product was purified by column chromatography (dichloromethane:heptane, 1:1) giving 1.8 g (44%) of pure product. Η NMR (CDCl j , 300 MHz) δ : 7.10 (broad s, 2 H, ArH) , 6.39 (s, 1 H, ArH), 4.79 (s, 1 H, OH) , 1.82-1.56 (m, 24 H, CH 3 ) , 1.53 (s, 6 H, CH 3 ) , 1.46 (s, 6 H, CH 3 ) . Example 4 Bis- (2 .2,6.6-tetramethylbenzo|'1.2~d:4,5-d' 1-bis(1.3) dithiole-4-yl)-mono-(2.2.6.6-tetramethylbenzof1.2-d:4.5- d'1-bis(1.3)dioxole-4-yl)methyl Bis-(2,2,6,6-tetramethylbenzo[1,2-d:4,5-d' ]-bis(1,3) dithiole-4-yl)-mono-(2,2,6,6-tetramethylbenzo[1,2-d:4,5- d']-bis(1,3)dioxole-4-yl)methanol (15.0 mg, 0.018 mmol) was dissolved in dichloromethane (0.8 mL) and BF 3 .Et 2 0 (6.6 μL, 0.054 mmol) was added. After a few minutes, tin(II) chloride (10 g, 0.054 mmol) was added together with acetonitrile (0.4 L) . After 5 min, the mixture was poured into dichloromethane (50 mL) and was washed with water (2*50 mL) . The organic phase was dried (MgS0 4 ) , filtered and evaporated. The product was not purified, but was analyzed by ESR. ESR (THF, 200 G) triplet, a H =2.5 G, line width 400 mG. Example 5 Bis-(8-ethoxycarbonylmethylthio-2.2,6.6-tetramethylbenzo r1.2-d:4.5-d'1-bis(1.3)dithiole-4-ylX mono-(8-ethoxycarbonylmethylthio-2,2,6,6-tetramethylbenz oT1,2-d:4,5-d']-bis(l,3)dioxole-4-yl)methanol All reaction steps until the addition of ethyl bromoacetate were performed under an argon atmosphere using degassed solvents. Bis-(2,2,6, 6-tetramethylbenzo- [l,2-d:4,5-d']-bis(l,3) dithiol-4-yl) -mono- (2 ,2,6,6- tetramethylbenzo[ 1, 2-d: 4 , 5-d' ] bis ( 1 , 3 ) ioxole-4- yl)methanol (0.399 g, 0.486 mmol) was dissolved in dry benzene (9.0 mL) . t-Butyllithium (2.0 L, 1.5 M in pentane) was added followed by tetramethyl ethylendiamine (TMEDA) (0.447 mL, 3.0 mmol). The mixture was stirred for 5 min and then treated with ultrasound for 30 min. S8 (0.100 g, 3.12 mmol) was added and the ultrasound treatment was continued for 2 h. The reaction was quenched by addition of aqueous 0.2M KOH (50.0 mL) . After washing with benzene (40 mL) , the aqueous phase was collected, ether (60 L) was added and the aqueous phase was acidified using 2M hydrochloric acid. The organic phase was separated, filtered and evaporated. The residue was dissolved in degassed acetonitrile (40 mL) . Ethyl bromoacetate (0.55 L, 5.0 mmol) and potassium carbonate (1.0 g, 7.2 mmol) were added. The mixture was stirred overnight. The reaction was filtered and concentrated to 10 mL. A mixture of ether (50 mL) and aqueous NaH 2 P0 4 (50 mL) was added. The organic phase was collected, dried (MgS0 4 ) and evaporated. The product was purified by preparative HPLC. Yield 83.2 mg (15 %) . 'H NMR (CDC1 3 , 300 MHZ) δ : 4.71 (IH, s) , 4.09-4.20 (6H, m) , 3.53-3.65 (6H, m) , 1.51-1.80 (36H, TO), 1.24-1.32 (9H, m) . Example 6 Bis-(8-ethoxycarbonylmethylthio-2.2.6.6-tetramethylbenzo ri,2-d:4,5-d , 1-bis(l.3 dithiol-4-yl)-mono-f8- ethoxycarbonylmethylthio-2.2.6.6-tetramethylbenzori.2-d: 4,5-d'1- bis(1.3)dioxole-4-yl)methyl Bis-(8-ethoxycarbonylmethylthio-2,2,6,6-tetramethylbenzo [1,2-d:4,5-d' ]-bis(1,3)dithiole-4-yl)-mono-(8-ethoxycarb onylmethylthio-2,2,6,6-tetramethylbenzo[1, 2-d:4,5-d' ]- bis(1,3)dioxol-4-yl)methanol (15 mg, 0.013 mmol) was dissolved in dichloromethane (0.8 mL) and BF 3 -Et 2 0 (8.0 μL, 0.065 mmol) was added. Tin(II) chloride (12.3 mg, 0.065 mmol) and acetonitrile (0.4 mL) was added to this mixture. After aqueous workup and evaporation, the product was purified by preparative HPLC. " - Bis(8-ethoxycarbonyl-2,2,6,6- tetra(hydroxymethyl)benzo[1,2-d:4,5-d' ]bis(1,3)dithiol- 4-yl) -mono(8-ethoxycarbonyl-2,2,6,6- tetramethylbenzo[l,2-d:4,5-d' ]-bis(1,3)dioxol-4- yl))methyl (316 mg, 0.275 mmol) was dissolved in a mixture of IM aqueous NaOH (3 mL) , water (1.5 mL) and ethanol (3 mL) . The solution was stirred at ambient temperature for 15 min, the ethanol was removed by evaporation, and the residue was stirred at ambient temperature for additional 2 hours. After evaporation to near dryness, the pure acid (240 mg, 82 %) was isolated by preparative HPLC followed by lyophilization. The acid was converted into the corresponding sodium salt by the addition of water (50 mL) followed by adjustment of the pH to 7 with IM aqueous NaOH and lyophilization. ESR (3.4 mM in H 2 0, 200 G) : singlet, linewidth 120 mG. Overhauser enhancement (aqueous solution as above) : 164 at 5 W microwave power. Stability measurements: Half life in water without exclusion of air: 120 h. - ILO - Yield : 13 mg ( 85% ) . ESR (THF, 200 G) :triplet, a H =100 mG, linewidth 73 mG. Overhauser enhancement (THF, 2.1 mM): 195 at 2.13 W microwave power. Stability measurements: Half life in acetonitrile without exclusion of air: 2700 h. Example 7 yl) ethyl Bis-(8-ethoxycarbonylmethylthio-2,2,6,6-tetramethylbenzo [1 ,2-d:4,5-d' ]-bis(1,3)dithiole-4-yl)-mono-(8-ethoxycarb onylmethylthio-2,2,6,6-tetramethylbenzo[l,2-d:4,5-d' ]-bi s(l,3)dioxole-4-yl)methyl (9 mg, 0.008 mmol) was dissolved in ethanol (25 mL) . Water (5 mL) and IM NaOH (48μL, 0.048 mmol) was added and the reaction was stirred for 1 h at 35°C. The solution was evaporated. The product was - l+l - purified by preparative HPLC. The collected fractions was evaporated, dissolved in water and ether. The ether layer was evaporated and the residue was dissolved in water and IM NaOH (24 μL, 0.024 mmol). The solution was lyophilized. Yield: 7 mg (75%) . ESR (water, 5 mM, 200 G) : singlet, linewidth 73 mG. Overhauser enhancement (water, 5 mM) : 179 at 1.1 W. microwave power. Stability measurements: Half life in water without exclusion of air: 60 h. Example 8 Bis-(8-sodium carboxylate (2H 2 )methylthio-2.2.6.6-tetramethylbenzor 1.2-d: 4.5-d' 1- bis(1.3) dithiole-4-yl) -mono-(8-sodium carboxylate ( 2 H ? )methylthio-2.2.6.6-tetramethylbenzof 1.2-d: 4.5-d' 1- bisfl.3) dioxole-4-yl) methyl This radical was prepared using the procedure described in Example 7 from bis-(8-ethoxycarbonylmethylthio- 2,2,6, 6-tetramethy lbenzo[l, 2-d: 4, 5-d' ] -bis (1, 3) dithiol e- 4-yl) -mono- ( 8-ethoxycarbonylmethylthio- ,2,6,6- tetramethy 1 -benzo [ 1 , 2-d : 4 , 5-d ' ] -bis ( 1 , 3 ) dioxole-4-yl) methyl (9 mg, 0.008 mmol) in methanol-dj (25 mL) , D 2 0 (5 mL) and IM NaOD (48 μL, 0.048 mmol). The product was purified by preparative HPLC and the pure product was lyophilized. Yield: 7 mg (75%) . ESR (H 2 0, 200 G) : singlet, linewidth 86 mG. Overhauser enhancement (H 2 0) : 47 at 10 W microwave power. Example 9 Bis- (8-ethoxycarbonyl-2.2.6.6-tetramethylbenzor 1.2-d: 4.5 -d' 1 -bis ( 1.3 ) dithiole-4-yl) -mono- (8-ethoxycarbonyl- 2.2.6.6 -tetramethv Ibenzo 1.2-d: 4.5-d' 1 -bis (1.3)dioxol- 4-yl) methanol Bis-(2,2, 6, 6-tetramethylbenzo[l,2-d:4,5-d']- biε(1,3)dithiole-4-yl)-mono-(2,2,6,6-tetramethyIbenzof1, 2-d:4,5-d' ]-bis(1,3)dioxol-4-yl)methanol (0.50 g, 0.61 mmol) was dissolved in dry benzene (6.0 mL) under an atmosphere of argon. t-Butyllitium (2.44 mL, 1.5 M in pentane) and TMEDA (0.545 mL, 3.66 mmol) were added. The reaction mixture was subjected to ultrasound for 25 min. and was then slowly added to a solution of diethyl carbonate (7.2 mL, 59.4 mmol) in dry benzene (16 mL) . After stirring for 1.5 h, aqueous NaH 2 P0 4 (50 mL) was added. The organic layer was separated, washed with water, dried (Na 2 S0 4 ) and evaporated. After purification by preparative HPLC 130.0 mg (21 %)) of the pure product was obtained. Η NMR (CDC1 3 , 300 MHz) δ : 4.98 (s, IH) , 4.28-4.37 (m, 6H) , 1.48-1.79 (m, 36H) , 1.46 (t, 6H, J 7.0 Hz) , 1.38 (t, 3H, J 7.0 Hz) . 13 C NMR (CDCI 3 , 75 MHZ) δ : 166.2, 166.0, 162.9, 141.9, 141.6, 141.2, 140.8, 140.4, 140.0, 136.6, 134.5, 129.9, 128.5, 128.1, 127.8, 127.2, 120.3, 118.9, 111.9, 101.1, 80.6, 62.1, 61.0, 60.3, 60.2, 59.8, 59.2, 34.4, 34.3, 33.5, 28.8, 28.1, 27.0, 26.9, 26.5, 25.8. Example 10 Bis- (8-ethoxycarbonyl-2.2.6.6-tetramethylbenzor 1.2- d:4.5-d' 1-bisd.3)dithiole-4-yl)-mono-(8-ethoxycarbonyl- 2.2.6.6-tetramethylbenzori.2-d:4.5-d' ]-bis(1.3)dioxol-4- yl)methyl - ( H+ - Bis- (8-ethoxycarbonyl-2,2,6, 6-tetramethyIbenzof1,2- d:4,5-d']-bis(l,3)dithiol-4-yl)-mono-(8-ethoxycarbonyl- 2,2,6,6-tetramethylbenzo[1,2-d:4,5-d']-bis(1,3)dioxol-4- yl)methanol (520 mg, 0.501 mmol) was dissolved in dry degassed dichloromethane (15 mL) together with tin(II) chloride (95 mg, 0.501 mmol) and acetonitrile (5 mL) . BF 3 -Et 2 0 (70 μL, 0.557 mmol) was added and the solution was stirred for 20 min. After addition of dichloromethane (80 mL) and washing with degassed water (80 mL) , the organic layer was separated, dried (MgS0 4 ) , filtered and evaporated. The product was purified by preparative HPLC. Yield: 110 mg (22%) . ESR (THF, 200 G) singlet, line width 325 mG. Overhauser enhancement (THF, 2.1 mM) : 156 at 4 W microwave power. Stability measurements: Half life in acetonitrile without exclusion of air: 2000 h. Example 11 Bis-(8-potassium carboxylate-2.2.6.6-tetramethylbenzo ri.2-d:4.5-d' 1-bisfl.3)dithiol-4-yl)-mono-(8-potassium carboxylate-2.2.6.6-tetramethylbenzori.2-d:4.5-d'1-bis (1.3)dioxol-4-y1)methyl Bis- (8-ethoxycarbonyl-2 ,2,6, 6-tetramethyIbenzof1,2- d:4,5-d']-bis(l,3)dithiol-4-yl)-mono-(8-ethoxycarbonyl- 2,2,6,6-tetramethylbenzo[l,2-d:4,5-d']-bis(l,3)dioxol-4- yl)methyl (132 mg, 0.129 mmol) was dissolved in ethanol (10 mL) . Aqueous potassium hydroxide (5 mL, 1.0M) was added and the reaction mixture was stirred at 50 °C overnight. After evaporation of the ethanol, the mixture was stirred for 1 h at 50 °C and was then acidified with 2M hydrochloric acid. The aqueous phase was extracted with ether. The organic phase was separated, dried (MgS0 4 ) filtered and evaporated. The product was purified by preparative HPLC. The fractions were evaporated and water was added. The aqueous layer was extracted with ether. The organic layer was separated, dried (MgS0 4 ) , filtered and evaporated. The product was dissolved by adding water and IM KOH (0.387 mL, 0.387 mmol). The solution was lyophilized. Yield: 101 mg (75%) . ESR (H 2 0, 200 G) : singlet, line width 105 mG. Overhauser enhancement (H 2 0, 6.9 mM) : 219 at 0.012 W microwave power. Example 12 Bis- ( 8-N , N-dimethylcarboxamide-2.2 , 6.6 - tetramethy Ibenzof 1.2-d: 4.5-d' ] -bis (1.3 ) dithiol-4-yl) - mono- ( 8-N . N-dimethylcarboxamide-2.2.6.6 - tetramethylbenzo F 1.2-d : 4.5-d' 1 -bis ( 1.3 ) dioxol-4- yl) methanol - 1*6 - Bis- (2 ,2,6, 6-tetramethyIbenzof 1, 2-d: 4 , 5-d' ] -bis (1,3) dithiol-4-yl) -mono-(2 ,2,6, 6-tetramethyIbenzof 1 , 2-d:4 ,5- d']bis(l,3)dioxol-4-yl)methanol (0.292 g, 0.356 mmol) was dissolved in dry benzene (7.0 mL) in an argon atmosphere. t-Butyllithium (1.45 mL, 1.5M in pentane) and TMEDA (0.325 mL, 2.18 mmol) were added. The solution was subjected to ultrasound for 30 min After cooling to 10 °C, the reaction mixture was added to a solution of N,N-dimethyl carbamoylchloride (1.4 mL, 15.0 mmol) and t-butyllithium (0.25 mL, 1.5 M in pentane) in dry benzene (20 mL) . The mixture was stirred for 17 h and quenched with aqueous NaH 2 P0 4 . The organic layer was separated, washed with water, dried (Na 2 S0 4 ) and evaporated. After purification by preparative HPLC 10.0 mg (3 %) of the pure product was ob¬ tained. 'H NMR (CDC1 3 300 MHz) δ : 4.80 (s, IH) , 2.97-3.12 ( , 18H) , 1.44-1.80 (m, 36H) . - Example 13 Bis-(8-dimethylaminocarbonyl-2,2,6,6-tetramethylbenzo[1, 2-d:4,5-d' ]-bis(1,3)dithiol-4-yl)-mono-(8-dimethylamino- carbony1-2,2,6,6-tetramethylbenzo[l,2-d:4,5-d']-bis(1,3) dioxol-4-yl)methanol (3 mg, 0.003 mmol) was dissolved in degassed acetonitrile (2 mL) under an atmosphere of argon. Tin(II) chloride (3 mg, 0.016 mmol) and BF 3 -Et 2 0 (20 μL, 0.16 mmol) were added sequentially. The reaction mixture was stirred for 2 min and then transferred to a separatory funnel containing degassed ether (20 mL) and water (20 mL) . The organic layer was separated, dried (MgS0 4 ) , filtered and evaporated. No further purification was done. The yield was not determined. ESR (diethyl ether, 200 G) : singlet, line width 535 G. Overhauser enhancement (diethyl ether) : 18 at 4 W microwave power. Example 14 4-Methoxycarbonyl-(2.2.6.6-tetramethylbenzof1.2-d:4.5- d'1-bis(1.3)dithiole) 2,2,6, 6-Tetramethylbenzo[l,2-d:4, 5-d' ]-bis(l, 3) dithiole (2.0 g, 6.98 mmol) was dissolved in dry ether (50.0 mL) in a dry, argon filled reaction flask. n-Butyllithium (3.07 mL, 2.5M in hexane) was added and the reaction mixture was stirred for 30 min. The solution was poured onto solid carbon dioxide and, after stirring overnight, the ether was filtered off. The solid residue was washed with chloroform and dried under vacuum. The crude product was mixed with potassium carbonate (0.97 g, 6.98 mmol) in dry DMF (20.0 mL) and was stirred for 1.5 h at 60 °C. After cooling to ambient temperature, methyl iodide (0.435 mL, 6.98 mmol) was added. The stirring was continued overnight. The mixture was filtered, the solution evaporated and the residue was dissolved in dichloromethane and water. The organic phase was washed with water (2*30 mL) , dried (Na 2 S0 4 ) and evaporated to give after additional drying under vacuum a bright green-yellow crystalline pure product. Yield 1.56 g (65 %) . Η NMR (CDC1 3 , 300 MHz) <S: 7.14 (s, IH) , 3.94 (s, 3H) , 1.85 (s, 12 H) . 13C NMR (CDCI 3 , 75 MHz) δ: 140.1, 136.5, 118.2, 118.0, 117.6, 52.4, 25.6. < 9 - Example 15 Bis-(2.2.6.6-tetramethylbenzori .2-d: 4.5-d' ~ ) • bis(l.3)dioxol-4-yl) -mono- (2.2.6.6-tetramethylbenzof 1.2- d : 4.5-d ' 1 -bis (1.3) dithiole-4-yl ) methanol 2,2,6, 6-Tetramethylbenzo[l, 2-d: 4 , 5-d' ]-bis (1, 3) dioxole (5.15 g, 23.2 mmol) was dissolved in dry ether (40.0 mL) in a dried, argon filled reaction vessel. The solution was cooled to 0 °C and n-butyllithium (9.29 mL, 2.5M in hexane) was added. After stirring for 15 min at ambient temperature, the mixture was cooled to 0 °C and 4-methoxycarbonyl-(2 ,2,6, 6-tetramethylbenzo[1, 2-d: 4 , 5- d']-bis(l, 3)dithiole) (4.0 g, 11.6 mmol) was added portionwise. The reaction was stirred at ambient temperature overnight. The voluminous precipitate was dissolved by addition of water (70 mL) and ether (50 mL) . The organic layer was separated and the aqueous phase was washed once more with ether. The organic phase was dried (Na 2 S0 4 ) and evaporated to give a semisolid residue. The product was recrystallized from acetonitrile. Yield of pure product was 5.26 g (60%) . Η NMR (CDC1 3 , 300 MHz) δ : 7.06 (s, IH) , 6.30 (s, 2H) , 4.48 (s, IH, OH) , 1.29-1.86 (m, 36 H) . 13C NMR (CDCI 3 , 75 MHz) δ: 140.1, 139.6, 139.4, 136.5, 367 - 5o - 136.2, 120.2, 118.0, 117.6, 82.4, 25.8, 25.6, 25.2. MS (thermospray) m/z: 756 (M+) . Example 16 Bis- (8-ethoxycarbonylmethylthio-2.2.6.6-tetramethylbenzo fl.2-d:4.5-d' " |-bis(l,3) dioxol-4-yl) -mono- f 8-ethoxγ- carbonylmethylthio-2.2.6.6-tetramethylbenzof 1.2-d: 4.5- d' 1 -bis (1.3) dithiol-4-γl) methanol Bis-(2,2,6,6-tetramethylbenzo[l,2-d:4,5-d']-bis(l,3) dioxo1-4-yl)-mono-(2,2,6,6-tetramethylbenzo[l,2-d:4,5- d']-bis(l,3)dithiol-4-yl)methanol (0.37 g, 0.489 mmol) was dissolved in dry THF (40 mL) under an atmosphere of argon. The solution was cooled to -20 °C, n-butyllithium (0.78 mL, 2.5M in hexane) was added and the solution was stirred for 1 h. S 8 (0.050 g, 1.47 mmol) was added, the cooling bath was removed and the reaction was allowed to reach ambient temperature overnight. The reaction was again cooled to -20 °C and was treated with n-butyllithium (2.34 L, 2.5M in hexane) for 1 h. S 8 (0.15 g, 4.41 mmol) was added, the cooling bath was removed and the reaction was stirred overnight at ambient temperature. A third time the reaction was cooled to -20°C, n-butyllithium (0.78 mL, l. Q fi mmol) was added, stirred for 1 h and S 8 (0.050 g, 1.47 mmol) was added. The cooling bath was removed. After stirring for four hours the reaction mixture was quenched by the addition of a degassed ether/aqueous NaH 2 P0 4 mixture. The organic phase was separated and the aqueous phase was washed once with ether. The organic phases were dried (Na 2 S0 4 ) and evaporated to give a semi solid residue. Acetonitrile (50 mL) , potassium carbonate (1.081 g, 7.82 mmol) and ethyl bromoacetate (0.868 mL, 7.82 mmol) were added. The mixture was stirred overnight, filtered and evaporated. After preparative HPLC 54.3 mg (10%) of the pure product was obtained. 'H NMR (CDC1 3 , 300 MHz) δ : 4.38 (ε, IH, OH) , 4.05-4.20 (m, 6H) , 3.59 (s, 4H) , 3.53 (s, 2H) , 1.34-1.75 (m, 36H) , 1.16- 1.33 (m, 9H) . 13 C NMR (CDCI 3 , 75 MHz) δ : 168.8, 144.5, 136.2, 135.0, 119.5, 118.9, 117.7, 110.0, 97.9, 76.0, 61.2, 60.4, 35.5, 25.5, 14.1. Example 17 Bis-(8-ethoxycarbonylmethylthio-2.2.6.6-tetramethylbenzo T1.2-d:4.5-d' 1-bis(1.3)dioxol-4-yl) -mono-(8- ethoxycarbonylmethylthio-2.2.6.6-tetramethylbenzor1.2- d:4.5-d' 1-bis(1.3)dithiol-4-yl) ethyl S2 - Bis- ( 8-ethoxycarbonylmethylthio-2 ,2,6, 6-tetramethylbenzo [l,2-d:4,5-d' ]-bis(l,3)dioxol-4-yl)-mono-(8- ethoxycarbonylmethylthio-2 ,2,6, 6-tetramethylbenzo [1,2- d:4,5-d']-bis(l,3)dithiol-4-yl)methanol (12.3 mg, 0.011 mmol) was dissolved in dry, degassed acetonitrile (2 mL) under an argon atmosphere. BF 3 -Et 2 0 (4.7 μL, 0.022 mmol) was added with stirring. Tin(II) chloride (5 mg, 0.022 mmol) was added followed by amalgamated zinc (2 mg, 0.030 mmol) . The reaction was added to a mixture of degassed dichloromethane (40 mL) and water (30 mL) . The organic layer was separated, dried (Na 2 S0 4 ) and evaporated. The radical was purified by preparative HPLC. The yield was not determined. ESR (THF, 200 G) : singlet, linewidth 222 mG. Overhauser enhancement (THF, 5.7 mM) : 232 at 1.5 W microwave power. Stability measurementε: Half life in acetonitrile without exclusion of air: 100 h. - S 3 - Example 18 Bis-(8-sodium carboxylate methylthio-2.2.6.6-tetramethylbenzori.2-d:4.5-d' bis(1.3)dioxol-4-yl)-mono-(8-sodium carboxylate methylthio-2.2.6.6-tetramethylbenzofl.2-d:4,5-d'] bis(1.3)dithiol-4-yl)methyl Θ Bis-(8-ethoxycarbonylmethylthio-2,2,6,6-tetramethylbenzo [1,2-d:4,5-d' ]-bis(1,3)dioxol-4-yl)-mono-(8-ethoxycarbonyl methylthio-2,2,6,6-tetramethylbenzo[l,2-d:4,5-d']-bis(l, 3)dithiol-4-yl)methyl (9 mg, 0.009 mmol) was dissolved in dry, degassed acetonitrile (1.5 mL) . Aqueous potassium hydroxide (54 μL, l.OM) was added and the reaction was stirred for lh. Tris buffer (2 mL, pH 8) was added and the solution was concentrated to almost dryness. The sample was prepared for Overhauser and ESR analysis by adding tris buffer until the total volume was 2 mL. ESR (H 2 0, 0.16 mM, 200 G) : singlet, linewidth 236 mG. Overhauser enhancement (H 2 0, 0.159 mM) 130 at 32.4 mW microwave power. - S4 - Example 19 Bis-(8-carboxy-2.2.6.6-tetramethylbenzor1.2-d:4.5-d'1- bis(1.3)dioxol-4-yl)-mono-(8-carboxy-2.2.6.6- tetramethylbenzor1.2-d:4.5-d'1-bis(1.3)dithiol-4- yl)methanol Dry TMEDA (1.21 mL, 8.04 mmol) and t-butyllithium (5.36 mL, 1.5 M in pentane) were dissolved in dry cyclohexane (12 mL) at 0 °C. Bis-(2, 2, 6, 6-tetramethylbenzo[1,2- d:4 , 5-d' ]-bis(1,3)dioxol-4-yl) -mono-(2,2,6, 6-tetramethyl benzo[1,2-d:4,5-d' ]-bis(1,3)dithiol-4-yl)methanol (0.608 g, 0.804 mmol) was then added at ambient temperature as a solid. After 20 min, solid carbon dioxide was added followed by dry ether (50 mL) . After stirring for 17 h, the reaction mixture was filtered and the precipitate was dried. The acid was purified by preparative HPLC. Yield 0.285 g (40%) . Η NMR (CDC1 3 , 300 MHz) δ: 1.42-1.77 (m, 36H) . I3 C NMR (CDC1 3 , 75 MHz) δ: 165.0, 140.3, 137.8, 136.1, 119.8, 118.1, 113.3, 100.2, 25.7. Example 20 Bis-(8-methoxycarbonyl-2.2.6.6-tetramethylbenzof1.2- d:4.5-d")-bis(1.3)dioxol-4-yl)-mono-fδ-methoxycarbonyl- 2.2.6.6-tetramethylbenzor1.2-d:4.5-d'1-bis(1.3)dithiol- 4-yl)methanol The tricarboxylic acid was prepared as described in Example 19, however, after treatment with carbon dioxide overnight, the reaction mixture was filtered and the precipitate was transferred to a reaction flask and mixed with potassium carbonate (0.222 g, 1.61 mmol) in DMF (15 mL) . After stirring at ambient temperature for 30 min methyl iodide (0.228 g, 1.61 mmol) was added and the reaction was stirred overnight. To the mixture was added hydrochloric acid (45 mL, 0.25M) and ether (45 mL) . The ether phase was separated and the aqueouε phase was ex¬ tracted with ether (2*70 mL) . The collected organic phases were washed with slightly acidic water, (5*60 L, 1 L of 2M hydrochloric acid had been added) , dried (Na 2 S0 4 ) and evaporated. After preparative HPLC, 0.22 g (30 %) of the pure ester was obtained. - Sb - Η NMR (CDCI 3 , 300 MHz) δ : 4.62 (s, IH) , 3.94 (s, 3H) , 3.89 (s, 6H) , 1.37-1.76 (m, 36H) . 13 C NMR (CDCI 3 , 75 MHz) δ: 166.6, 163.5, 140.3, 137.8, 136.3, 120.0, 119.5, 118.2, 113.3, 99.8, 82.4, 52.2, 52.0, 25.8. MS (ther ospray) m/z: 931 (M+) . Example 21 Bis- (8-methoxycarbonyl-2.2.6.6-tetramethylbenzof 1.2- d:4.5-d' l-bis(l.3)dioxol-4-yl) -mono-(8-methoxycarbonyl- 2.2.6.6-tetramethylbenzof1.2-d:4.5-d']-bis(l.3)dithiol-4- yl)methane Bis-(8-methoxycarbonyl-2, 2,6, 6-tetramethyIbenzof1,2- d:4, 5-d' ]-bis(l, 3)dioxol-4-yl) -mono-(8-methoxycarbonyl- 2,2,6, 6-tetramethyIbenzofl,2-d:4 , 5-d' ]-bis(l, 3)dithiol- 4-yl)methanol (5 mg, 0.005 mmol) was dissolved in dry, degassed acetonitrile under an argon atmosphere. BF 3 -Et 2 0 (2.0 μL, 0.011 mmol) and tin(II) chloride (3.0 mg, 0.016 mmol) were added. Amalgamated zinc (1.0 mg, 0.015 mmol) 6/39367 - 57 - was added. The product was purified by flash chromatography (ether/heptane 5:1). Due to the small amount used the yield was not determined. ! H NMR (CDC1 3 , 300 MHz) δ : 5.33 (s, IH) , 3.92 (s, 6H) , 3.25 (S, 3H) , 1.16-1.37 (m, 36H) . MS (thermospray) m/z: 914 (M+) . Example 22 Bis-(8-methoxycarbonyl-2.2.6.6-tetramethylbenzor1.2- d:4.5-d']-bis(1.3)dioxol-4-y1) -mono-(8-methoxycarbony1- 2.2.6.6-tetramethyIbenzof1.2-d:4.5-d'1-bis(l.3)dithiol- 4- yl)methyl Procedure A: Bis-(8-methoxycarbonyl-2,2,6, 6-tetramethylbenzo- [1,2-d:4,5-d']-bis(1,3)dioxol-4-yl)-mono-(8- methoxycarbonyl-2,2,6,6-tetramethyIbenzof1,2-d:4,5-d']- bis(l,3)dithiol-4-yl)methane (5.0 mg, 0.5 mmol) was dissolved in a 1/1 mixture of dry, degassed DMSO and acetonitrile (1.0 mL) under argon atmosphere. Potassium t-butoxide (1.2 mg, 10.6 mmol) was added. The solution was added to a mixture of degassed ether (50 mL) and water (40 mL, pH 2) .The ether layer was separated and evaporated. ESR (acetonitrile, 200 G) multiplet, linewidth 65 mG. /39367 - 58 - Overhauser enhancement (acetonitrile): 38 at 1.1 W microwave power. Procedure B: Bis-(8-methoxycarbonyl-2 ,2,6, 6-tetramethylbenzo[1, 2- d:4,5-d' ]-bis(l, 3)dioxol-4-yl) -mono-(8-methoxycarbonyl- 2,2,6, 6-tetramethylbenzo[1,2-d:4,5-d' ]-bis(1,3)dithiol- 4-yl)methanol (30 mg, 0.032 mmol) was dissolved in dried and degassed acetonitrile (2.5 mL) in a dry and argon filled reaction vessel. BF 3 -Et 2 0 (13.0 μL, 0.064 mmol) was added. A sample from the reaction mixture showed a large Overhauser effect, thus a radical had been formed. The solution was transferred to separatory funnel containing degassed dichloromethane (25 mL) and water (70 mL) . The dark green organic layer was separated, dried with Na 2 S0 4 and evaporated. The yield was not determined. Overhauser enhancement (acetonitrile, 12.9 mM) : 230 at 9 W microwave power. Example 23 Bis-(8-potassium carboxylate-2.2.6,6-tetramethylbenzof1, 2- :4 , 5-d' " |- bisfl,3)dioxol-4-yl)-mono-(8-potassiumcarboxylate- 2,2,6, 6-tetramethyIbenzof1,2-d:4 ,5-d' l-bis(l , 3)dithiol- 4-yl) ethyl The radical was synthesized as described in Example 22B from bis- (8-carboxy-2 ,2,6, 6-tetramethylbenzo[ 1,2- d:4,5-d' ]-bis(l, 3)dioxol-4-yl) -mono-(8-carboxy-2,2,6,6- tetramethyIbenzof l,2-d:4,5-d']-bis(l,3)dithiole-4- yl)methanol (59.2 mg, 0.067 mmol) and BF 3 -Et 2 0 (26.0 μL, 0.13 mmol) in acetonitrile (2.0 mL) and dichloromethane (0.5 mL) . The yield was not determined. ESR (H 2 0, 200 G) : singlet, linewidth 60 mG. Overhauser enhancement (H 2 0, pH 9 buffer) : 60 at 5 W microwave power. Example 24 2.2.6.6-Tetrakis-( 2 H ? -methyl)benzor1.2-d:4.5- d'lbiεf1.3)dithiole This compound was prepared from benzene-1,2,4,5- tetrathiole (20.0 g, 97 mmol), acetone-d 6 (48 mL, 0.6b mol) and HBF 4 (16.8 mL, 54 % in ether, 0.123 mol) using the procedure described for the corresponding protio compound in WO-91/12024. Yield 20.9 g (72 %) . Η NMR (CDC1 3 , 300 MHz) δ: 7.02 (s, 2H) . Example 25 Tris-(2.2.6.6-tetrakis-( 2 H ? -methyl)-benzor1.2- d:4 ,5-d' Ibis(1.3)dithiole)methanol 2,2,6, 6-Tetrakis-( 2 H 3 -methyl) -benzof1,2-d:4,5-d']bis(1,3) dithiole (19.7 g, 66 mmol) was dissolved in dry diethyl ether (400 mL) under an atmosphere of argon. n-BuLi (41.2 mL, 1.6M in hexane) was added and the reaction mixture was stirred at ambient temperature for 2 h. Diethyl carbonate (2.40 mL, 19.8 mmol) was then added dropwise over a period of 30 minutes. After stirring overnight, ether (200 mL) and saturated aqueous NaH 2 P0 4 (100 L) were added. The organic phase was dried (MgS0 4 ) and evaporated and the residue was recrystallized from THF containing 5 % acetonitrile. Yield: 10.8 g (53 %) . Η NMR (CDClj, 300 MHz) δ : 7.17 (s, 3H) , 6.23 (s, OH, IH) . 13 C NMR (CDCI 3 , 75 MHz) δ : 138.8, 137.5, 136.8, 135.6, 133.1, 131.7, 126.6, 124.7, 116.4, 83.1, 64.5, 61.5, 35.2, 30.2, 29.5, 20.7, 12.3. Example 26 Tris-(8-ethoxycarbonyl-2.2.6.6-tetrakis-( 2 H ? -methyl) - benzof1.2-d:4.5-d' 1bis(1.3)dithiole)methanol Tris- (2 , 2 , 6, 6-tetrakis- ( 2 H 3 -methyl) -benzo[ 1 , 2-d: 4 , 5- d' ]bis(1,3)dithiole)methanol (10.8 g, 11.7 mmol) was dissolved in dry benzene (140 mL) together with TMEDA (17.6 mL, 118 mmol). t-BuLi (79 L, 1.5M in pentane) was then added and the reaction mixture was stirred at ambient temperature for 35 min. The nearly homogeneous solution was then transferred into a solution of diethyl pyrocarbonate (90 mL, 611 mmol) in benzene (76 mL) . After 2 h, a saturated aqueous solution of NaH 2 P0 4 was added and the mixture was stirred for 10 min. The organic phase was dried (MgS0 4 ) , evaporated and the product was recrystal¬ lized from acetonitrile. Yield: 5.30 g (40%) . Η NMR (CDCI 3 , 300 MHz) δ: 6.78 (s IH, OH), 4.43 (m, 6H, CH 2 ) , 1.46 (m, 9H, CH 3 ) . 13 C NMR (CDCI 3 , 75 MHz) <S: 166.2, 141.8, 140.3, 139.2, 134.0, 121.3, 84.3, 62.3, 60.9, 60.8, 33.8, 31.8, 29.2, 28.6, 14.3. Example 27 Tris- (8-ethoxycarbonyl-2.2.6, 6-tetrakis- (2H 3 -methyl) benzof1.2-d:4 , 5-d' Ibis(1, 3)dithiole)methyl Tris-(8-ethoxycarbonyl-2,2,6, 6-tetrakis-( 2 H 3 -methyl) - benzo[1,2-d:4,5-d']bis(1,3)dithiole)methanol (5.33 g, 4.6 mmol) was dissolved in CH 2 C1 2 (40 mL) and a solution of trifluoromethane-sulfonic acid (1.5 mL, 17 mmol) in CH 2 C1 2 (5 mL) was then added. After stirring for 7 min, a solution of tin(II) chloride (1.74 g, 9.2 mmol) in THF (6 mL) was added and the mixture was stirred for another 10 min. A saturated aqueous solution of NaH 2 P0 4 was added and, after stirring for a few minutes, the organic phase was separated, dried (MgS0 4 ) and evaporated. The radical was not purified, HPLC indicated 80 % pure product. Yield: 4.72 g (91 %) . ESR (diethyl ether, 200 G) : singlet, linewidth 180 mG. Overhauser enhancement (diethyl ether) : 192 at 5 W microwave power. Stability measurements: Half life in acetonitrile without exclusion of air: >30000 h. Example 28 Tris-(8-carboxyl-2.2,6,6-tetrakis-( 2 H-methyl)-benzor1.2- d:4.5-d' Ibisfl.3)dithiole) ethyl sodium salt - 63 - Tris-(8-ethoxycarbonyl-2,2,6,6-tetrakis-( 2 H 3 -methyl) - benzo[l,2-d:4,5-d']bis(l,3)dithiole)methyl (4.72 g, 4.21 mmol) was dissolved in dioxane (82 mL) and IM KOH (41 mL) was added. The solution was stirred at 50 °C for 2 h and then evaporated. Water (50 mL) was added and stirring was continued at 50 °C for another hour. The aqueous solution was acidified with 2M HCl and extracted with ether (2*150 mL) . The organic phases were dried (MgS0 4 ) and evaporated. The product was purified by preparative HPLC. The combined fractions were then concentrated and partitioned between diethyl ether and water. The organic phase was extracted with water and a sufficient amount of aqueous IM NaOH for the product to dissolve in the aqueous phase. After lyophilization, 3.5 g (80 %) of the pure radical was obtained. ESR (0.94 mM in H 2 0 buffered to pH 7, 200 G) : singlet,linewidth 74 mG. Overhauser enhancement (aqueous solution as above) : 71 at 5 W microwave power. Stability measurements: Half life in water without exclusion of air: 5400 h. Example 29 1,3-Dihydroxypropane-2-one 1,3-diacetate l,2-dihydroxypropane-2-one 1,3-diacetate was prepared using the procedure described in the literature (Bentley and McCrae Org. Chem. 3_5 2082 (1970)). 1,3- Dihydroxyacetone (60 g) was dissolved in pyridine (200 mL) . The solution was cooled to 0 °C, acetic anhydride (200 mL) was added and the reaction mixture was stirred at ambient temperature for 2 h. The pyridine, acetic acid and acetic anhydride were evaporated in vacuum. The residue was dissolved in ethyl acetate (400 mL) , washed with IM HCl (2*100 L) and water (100 mL) . The solution was dried (Na 2 S0 4 ) and evaporated. The crude product was recrystallized from ligroin. Yield: 63 g (56 %) . *H NMR (CDCI 3 , 300 MHz) δ: 4.75 (s, 4H, CH 2 ) , 2.17 (s, 6H, CH 3 ) . Example 30 2.2.6.6-Tetra(acetoxymethyl)benzori.2-d:4.5- d' 1bis(1.3)dithiole The reaction was performed under argon atmosphere using deoxygenated solvents. 1,2,4,5-Benzotetrathiole (4.9 g, 24 mmol) and 1,3-dihydroxypropane-2-one 1,3-diacetate (10.4 g, 60 mmol) were mixed in toluene (250 mL) . The mixture was cooled to 0 °C and HBF 4 (10.7 mL, 54% in diethyl ether) was added and the reaction mixture was stirred at 0 °C for 2 h. The organic solution was decanted and the residue was extracted with toluene (3*50 mL) . The combined organic phases were evaporated and the crude product was purified by chromatography (neutral alumina, ethyl acetate:heptane 1:1). Yield: 3.5 g (28 %) . Η NMR (CDC1 3 , 300 MHz) δ: 6.93 (s, 2H, ArH), 4.50 (s, 8H, CH2) , 2.09 (s, 12H, CH3). Example 31 2.2.6.6-Tetra(hvdroxymethyl)benzof1.2-d:4.5-d' Ibis(1.3) dithiole 2,2,6,6-Tetra(acetoxymethyl)benzof1,2-d:4,5- d' ]bis(1,3)dithiole (1.8 g, 3.5 mmol) and K 2 C0 3 (1.9 g) were stirred in methanol (100 mL) for 1 h at ambient temperature. The solvent was evaporated and water (100 mL) was added. The mixture was neutralized (2M HCl) and the precipitate was collected. Yield: 1.20 g (99 %) . IH NMR (DMSO-d 6 , 300 MHz) <S: 7.09 (s, 2H, ArH), 5.55 (t, 4H, J 5.6 Hz, OH), 3.74 (d, 8H, J 5.6 Hz, CH 2 ) . 13C NMR (DMS0-d 6 , 75 MHz) δ: 134.4, 116.5, 75.7, 63.8. Example 32 2.2.6.6-Tetra(dimetylthexylsilyloxymethyl)benzor1.2- d:4.5-d' Ibis (1.3)dithiole The reaction was performed under argon atmosphere. 2,2,6,6-Tetra(hydroxymethyl)benzof1,2-d:4,5- d' ]bis(1,3)dithiole (0.8 g, 2.2 mmol) was dissolved in DMF (20 mL) . Imidazole (1.1 g, 15.8 mmol) was added and the solution was cooled to 0 °C. Dimethylthexylsilyl chloride (2.8 g, 15.8 mmol) was added dropwise (ca 2 min) . The solution was stirred for 48 hours at ambient temperature. The reaction mixture was poured into ice/water, CH 2 C1 2 (100 mL) was added and the two phases were separated. The organic phase was washed with IM HCl and water (3*100 mL) . The solution was dried (Na 2 S0 4 ) and evaporated. The product was purified by column chromatography using dichloromethane-heptane (1:9) as eluent. Yield: 1.1 g (52 %) . Η NMR (CDC1 3 , 300 MHz) <5 : 6.84 (s, 2H, ArH) , 3.94 (s, 8H, CH 2 ) , 1.62 (septet, 4H, J 6.8 Hz, CH) , 0.88 (d, 24H, J 6.8 Hz, CH 3 ) , 0.84 (s, 24H, CH 3 ) , 0.08 (s, 24H, Si(CH 3 ) 2 ) . 13 C NMR (CDCI 3 , 75 MHz) δ: 134.3, 115.8, 74.2, 65.0, 34.2, 25.1, 20.3, 18.6, -3.6. έ>7 - Example 33 yl) ) methanol The reaction was performed under argon atmosphere. 2,2,6,6-Tetra(di etylthexylsilyloxymethyl)benzof1,2-d:4,5- d']biε (1,3)dithiole (7.0 g, 7.6 mmol) was dissolved in dry THF (50 mL) . The solution was cooled to -70 °C. n- Butyllithium (5.0 mL, 1.6M in hexane) was added and the temperature was allowed to attain ambient temperature and was stirred for 1 h. The solvent was evaporated in vacuum at ambient temperature and diethyl ether (20 mL) was added . Then , 4 - e t h o xy c a r b o n y 1 - 2 , 2 , 6 , 6 - tetramethylbenzo[1,2-d:4,5-d' ]-biε(1,3)dioxole (0.8 g, 2.9 mmol) waε added in one portion and the reaction mixture was stirred at ambient temperature for 12 h. The mixture was poured into a NaH 2 P0 4 solution, the phases were separated and the aqueous phase was extracted with diethyl ether (2*100 mL) . The organic phases were dried (Na 2 S0 4 ) and evaporated. The residue was purified by preparative HPLC. Yield: 3.7 g (62 %) . Η NMR (CDC1 3 , 300 MHz) <S : 6.80 (s, 2H, ArH) , 6.26 (s, IH, ArH), 4.95 (S, IH, OH) , 3.8 (br m, 16H, CH 2 ) , 1.5 (br m, 20H, CH 3 +CH) , 0.9 (d, 48H, CH 3 ) , 0.7 (s, 48H, CH 3 ) , 0.2 ( 2 s, 48H, Si(CH3) 2 ) . 13 C NMR (CDC1 3 , 75 MHz) 5: 141.5, 140.3, 139.8, 139.6, 131.7, 118.6, 117.1, 108.1, 94.4, 80.0, 65.4, 34.1, 25.9, 25.0, 20.3, 18.7, -3.2. Example 34 B i s ( 8 - e t h o x v c a r b o n v l - 2 i 2 , 6 , 6_ tetra (hvdroxymethyl ) benzo r 1.2-d : .5-d ' 1 bis (1.3) dithiol- 4-yl) -mono (8-ethoxycarbonyl-2.2.6.6-tetramethylbenzo T 1.2- d:4.5-d * 1-bis(l.3) dioxol-4-yl) ) methanol Bis(2 , 2 , 6, 6-tetra (dimetylthexylsilyloxymethyl)benzof1,2- d:4 , 5-d' ]bis (l , 3 ) dithiol-4-yl) -mono (2 , 2 , 6 , 6- tetramethylbenzo[l,2-d:4,5-d , ]-bis(l,3)dioxol-4- yl))methanol (3.2 g, 1.54 mmol) was dissolved in heptane (12.8 mL) and dry benzene (10.7 L) together with TMEDA (3.2 mL, 21.6 mmol) under an atmosphere of argon. The solution was cooled to -22 °C and t-BuLi (14.4 mL, 1.5M in pentane) was added. After stirring for 3 h at -22 °C, the reaction mixture was transferred into a solution of diethyl pyrocarbonate (12.8 mL, 87 mmol) in heptane (23 mL) and dry benzene (23 mL) which was kept at -22 °C. The reaction mixture was then allowed to attain ambient temperature. After stirring for an additional hour, a saturated aqueous solution of NaH 2 P0 4 (40 mL) was added. The mixture was stirred for one hour, the organic phase was separated, washed with water (2*100 mL) and acetonitrile (2*100 mL) . The heptane/benzene phase was evaporated and then dissolved in THF (25 mL) . A solution of Bu 4 NF in THF (20 mL, 20 mmol) was added and the mixture was stirred overnight. After evaporation of the solvent, the residue was partitioned between water (300 mL) and ethyl acetate (300 mL) . The organic phase was washed with water (2*100 mL) , dried (Na 2 S0 4 ) and evaporated. Purification by preparative HPLC gave 400 mg (22 %) pure product. Η NMR (CDC1 3 , 300 MHz) δ: 5.78-5.92 (m, 6H) , 5.03-5.52 (m, 24H) , 2.98-3.21 (m, 12H) , 2.90 (t, 6H, J 7.0 Hz) , 2.84 (t, 3H, J 6.9 HZ) . Example 35 B i s ( 8 - e t h o x y c a r b o n v l - 2 . 2 . 6 . 6 - tetra (hvdroxymethyl) benzof 1.2-d: 4.5-d' Ibis (1.3)dithiol-4- yl) -mono(8-ethoxycarbonyl-2.2.6.6-tetr amethy Ibenzof 1.2- d:4.5-d' 1 -bis (1.3) dioxol-4-yl) )methyl Bis(8-ethoxycarbonyl-2,2,6,6- tetra(hydroxymethyl)benzo[1,2-d:4 ,5-d' ]bis (1,3)dithiol- 4-yl)-mono(8-ethoxycarbony1-2,2,6,6- tetramethylbenzo[1,2-d:4,5-d' ]-bis(1,3)dioxol-4- yl))methanol (294 mg, 0.25 mmol) was dissolved in acetonitrile (70 L) under an atmosphere of argon. After cooling to 0 °C, trifluoromethane sulfonic acid (190 μL, 2.2 mmol) was added. After stirring for 3 min, tin(II) chloride (48 mg, 0.25 mmol) dissolved in acetonitrile (7 mL) was added. After 1 min, a saturated aqueous solution of NaH 2 P0 4 (50 mL) was added. The aqueous phase was washed with acetonitrile (2*50 mL) , the combined organic phases were dried (Na 2 S0 4 ) and evaporated. Purification by preparative HPLC gave 176 mg (61 %) of the pure product. ESR (H 2 0, 200 G) : singlet, linewidth 433 mG. Example 36 Bis (8-carboxy-2 ,2,6, 6-tetra (hydroxymethyl) benzof 1, 2- d:4 , 5-d ' Ibis (1 , 3)dithiol-4-yl) -mono(8-carboxy-2 ,2,6,6- tetra ethyIbenzof1 , 2-d:4 , 5-d' 1-bisfl, 3)dioxo1-4- yl))methyl sodium salt - 71 - Bis(8-ethoxycarbonyl-2,2,6,6- tetra(hydroxymethyl)benzo[1,2-d:4,5-d* ]bis(1,3)dithiol- 4-yl)-mono(8-ethoxycarbonyl-2,2,6,6- tetramethylbenzo[1,2-d:4,5-d' ]-bis(1,3)dioxol-4- yl))methyl (316 mg, 0.275 mmol) was dissolved in a mixture of IM aqueous NaOH (3 mL) , water (1.5 mL) and ethanol (3 L) . The solution was stirred at ambient temperature for 15 min, the ethanol was removed by evaporation, and the residue was stirred at ambient temperature for additional 2 hours. After evaporation to near dryness, the pure acid (240 mg, 82 %) was isolated by preparative HPLC followed by lyophilization. The acid was converted into the corresponding sodium salt by the addition of water (50 mL) followed by adjustment of the pH to 7 with IM aqueous NaOH and lyophilization. ESR (3.4 mM in H 2 0, 200 G) : singlet, linewidth 120 G. Overhauser enhancement (aqueous solution as above) : 164 at 5 W microwave power. Stability measurements: Half life in water without exclusion of air: 120 h. Example 37 2.2.6.6-Tetramethylbenzor1.2-d:5.4-d' 1bis(1.3)oxathiole 2,6-Dioxo-benzo[l,2-d:5,4-d' ]bis(l, 3)oxathiole (1.0 g, 4.4 mmol), prepared according to the literature procedure (Fiedler, H. Berichte 9_5_, 1771 (1962)) was suspended in dry methanol (30 mL) and a solution of sodium methoxide in methanol (prepared from 20 mL methanol and 2.2 mmol sodium) was then added over a period of 15 minutes. After stirring for 15 minutes, the mixture was poured onto diethyl ether (50 mL) and 1 M aqueous HCl (25 mL) . The aqueous phase was extracted twice with ether and the combined organic phases were dried (MgS0 4 ) and evaporated. The residue (0.60 g) was dissolved in dry acetonitrile (40 mL) containing acetone (6 mL) and BF 3 .Et 2 0 (4 L) was then added. After stirring for 20 minutes, water (100 mL) and dichloromethane (50 mL) were added. The aqueous phase was extracted twice with dichloromethane and the combined organic phases were dried (MgS0 4 ) and evaporated. The brownish residue was passed through a short silica column using ethyl acetate:cyclohexane (1:5) as the eluent to give 0.30 g of a yellow solid which was further purified by preparative HPLC. Yield 0.25 g (23 %) . Η NMR (CDCl j , 300 MHz) δ : 6.75 (s, IH) , 6.35 (s, IH) , 1.80 (s, 12H, CH 3 ) . Example 38 8-Trimethylsilyl-2.2.6.6-tetramethylbenzof1.2-d:5.4-d' ~ |- bis(1.3)oxathiole The reaction was performed under an argon atmosphere using deoxygenated solvents. 2,2,6,6- tetramethylbenzofl,2-d:5,4-d']-bis(1,3)oxathiole (6.0 g, 23.6 mmol) was dissolved in dry THF (120 mL) . The mixture was cooled on an ice-bath and n-butyllithium (10.8 mL, 2.5M in hexane) was added dropwise over 10 min. After 15 min, chlorotrimethylsilane (6.0 mL, 47.2 mmol) was added dropwise over 5 min. After another 15 min, the reaction mixture was quenched with dietyl ether/aq. NaHC0 3 , the aqueous layer was extracted with ether and the combined organic layers were washed with water and dried (Na 2 S0 4 ) . After evaporation and chromatography (silica gel, chloroform) essentially pure product (6.3 g, 92%) could be collected. l H NMR (CDC1 3 , 300 MHz) δ: 6.80 (s, IH, ArH), 1.79 (s, 12H, CH3), 0.28 (s, 9H, SiCH3). Example 39 Tris-(8-trimethylsily1-2.2,6.6-tetramethylbenzof1,2- d:5,4-d' " )-bis(1,3)oxathiol-4-yl)methanol The reaction was performed under an argon atmosphere using deoxygenated solvents. 8-Trimethylsilyl-2 , 2 , 6, 6- tetramethy1-benzof1,2-d:5,4-d' ]-bis(1,3) oxathiole (14.2 g, 43.5 mmol) was dissolved in dry THF (300 mL) . The mixture was cooled using to -70 °C and n-butyllithium (59.0 mmol, 2.5M in hexane) was added dropwise over 10 min and the mixture was then left to attain ambient temperature over 90 min. Neat diethyl carbonate (1.75 mL, 14.5 mmol) was added dropwise over 60 min and the mixture was then stirred overnight. The reaction mixture was then quenched with diethyl ether/aq. NaH 2 P0 4 , the aqueous layer was extracted with ether and the combined organic layers were washed twice with water, dried (Na 2 S0 4 ) , evaporated, chromatographed (silica gel; dichloro-methane:heptane 1:2) and finally triturated with ethanol to give the pure product as colorless crystals. Yield: 8.4 g (58%) . Η NMR (CDC1 3 , 300 MHz) δ : 4.43 (s, IH, OH), 1.78-1.55 (m, 36H, CH 3 ) , 0.28 (s, 27H, SiCH 3 ) . 13C NMR (CDCI 3 , 75 MHz) δ : 158.8, 128.5, 120.7, 119.5, 116.1, 105.7, 95.6, 94.3, 81.5, 30.9, 30.5, 29.1, 28.9, 0.5. MS (electrospray) m/z: 1005 (M+H) . Example 40 Tris- (2.2.6.6-tetramethylbenzof 1.2-d: 5.4-d' 1 bis(1.3)oxathiol-4-yl) ethanol Tris-(8-trimethylsilyl-2,2,6,6-tetramethylbenzo[1,2-d:5,4- d']-bis(1,3)oxathiol-4-yl)methanol (0.62 g, 0.62 mmol) was dissolved in acetonitrile (150 L) . Sodium iodide (0.75 g, 6.0 mmol) and chlorotrimethylsilane (0.65 g, 6.0 mmol) was added in one portion. The mixture was stirred for 20 min and then poured onto diethyl ether/aq. NaHC0 3 . The aqueous layer was extracted with ether and the combined organic layers were washed twice with water, dried (Na 2 S0 4 ) , evaporated, chromatographed (silica gel; dichloromethane) and finally triturated with ethyl acetate to yield the product as slightly brown crystals. Yield: 0.44 g (87%) . •H NMR (CDC1 3 , 300 MHz) δ : 6.42 (s, 3H, ArH), 4.59 (s, IH, OH), 1.82-1.62 (m, 36H, CH3) . 13C NMR (CDCI 3 , 75 MHz) δ 154.4, 154.3, 127.1, 121.2, 120.4, 97.1,96.6, 96.0, 81.1, 30.9, 30.4, 29.1 MS (electrospray) m/z: 788 (M+) . Example 4 : Tris- ( 8-ethoxycarbonyl-2.2.6.6-tetramethv lbenzo f 1.2-d: 5.4- yl) methanol The reaction was performed under an argon atmosphere using deoxygenatedsolvents. Tris-(2,2,6,6-tetramethylbenzo[1,2- d:5,4-d']bis(1,3) -oxathiol-4-yl)methanol (0.40 g, 0.50 mmol) was suspended in dry diethyl ether (80 mL) . The mixture was stirred and n-butyllithium (1.5 mL, 2.5M in hexane) was added dropwise over 10 min. After 10 min, the temperature was lowered to -78 °C and neat diethyl carbonate (5.25 g, 44.5 mmol) was added in one portion. After 10 min, the cooling bath was removed and after 90 min the reaction mixture was poured onto diethyl ether/aq. NaH 2 P0 4 . The aqueous layer was extracted with ether and the combined organic layers were washed twice with water, dried (Na 2 S0 4 ) and evaporated. The resulting yellow solid was purified by preparative HPLC. Yield: 0.20 g (39%) . Η NMR (CDCI 3 , 300 MHz) δ : 4.76 (s, IH, OH), 4.86 (dq, 6H, CH2), 1.85-1.65 (m, 36H, CH 3 ) , 1.37 (s, 6H, CH 3 ) MS (electrospray) m/z: 1004 (M+) . Example 42 Tris- (8-ethoxycarbonyl-2.2 .6.6-tetramethylbenzo f 1.2-d: 5.4- d ' 1 -bis ( 1.3 ) oxathiol-4-y 1 ) ethyl The reaction was performed under an argon atmosphere using deoxygenated solvents. Tris-(8-ethoxycarbonyl-2,2,6,6- tetramethyl-benzo fl,2-d:5,4-d']-bis(l,3) oxathiol-4- yl)methanol (0.10 g, 0.10 mmol) was dissolved in dichloromethane (20 mL) and stirred with BF 3 -Et 2 0 (0.10 mL, 0.80 mmol) for 48 min. A solution of tin(II) chloride (32 mg, 0.17 mmol) in THF (4 mL) was added and after 3 min, the mixture was poured onto water/ dichloromethane. The organic layer was dried (Na 2 S0 4 ) and evaporated. Purification by preparative HPLC gave the pure radical. Yield: 50 mg (50 %) . ESR (diethyl ether,200 G) : 9 equidistant lines, a H = 85 mG, linewidth 40 mG. Stability measurements: Half life in acetonitrile without exclusion of air: 5800 h. Example 43 Tris-(8-ethoxycarbonyl-3 ,3.5.5.-tetraoxo-2.2.6.6- tetramethyl-benzof1.2-d: 5, 4-d' 1-bis(1, 3) oxathiol-4- yl) ethyl Tris-(8-ethoxycarbonyl-2,2,6,6-tetramethylbenzo[1,2-d:5,4- d']-bis(1,3)oxathiol-4-yl)methyl (0.050 g, 0.050 mmol) was dissolved in a premixed solution of trifluoroacetic acid (3 mL) , acetic acid (3 mL) , acetic anhydride (3 mL) and H 2 0 2 (1 L, 35 % aqueous solution) and was left under an argon atmosphere for 80 h. The mixture was poured onto a saturated aqueous solution of NaCl and dichloromethane. The organic layer was washed with sat. NaCl and evaporated. After preparative HPLC, 0.008 g (16%) of the pure radical was isolated. MS (electrospray) m/z: 1202 (M+Na)+, 1180 (M+H)+. ESR: (H20, 200 G) : singlet, linewidth 120 mG. Overhauser enhancement (H 2 0) : 221 with 10 W microwave power. Stability measurements: Half life in acetonitrile without exclusion of air: 800 h. Example 44 Tris-(8-carboxy-2.2,6, 6-tetramethylbenzo-f1.2-d:5,4-d' 1 bis-(l .3) oxathiol-4-yl)methanol The reaction was performed under an argon atmosphere using deoxygenatedsolvents. Tris-(2,2,6, 6-tetramethylbenzo[1,2- d:5,4-d ' ]bis(l,3)-oxathiol-4-yl)methanol (5.45 g, 53.5 mmol) was suspended in dry diethyl ether (800 mL) . The mixture was stirred and n-butyllithium (33.5 mL, 2.5M in hexane) was added dropwise over 10 min. After 1 hour, the temperature was lowered to -78 °C and the mixture was rapidly transferred to a flask containing a large excess of solid carbon dioxide. The mixture was allowed to reach ambient temperature and was then poured onto water. The organic layer was removed and discarded and the aqueous layer acidified (pH 0) and extracted three times with ether. The combined organic layers were dried (Na 2 S0 4 ) and evaporated. The resulting yellow solid was purified by preparative HPLC. Yield: 2.6 g (42 %) . Η NMR (DMF-d 6 , 300 MHz) δ : 4.71 (s, IH, OH), 1.85-1.69 (m, 36H, CH 3 ) . 13 C NMR (DMF-d 6 , 75 MHz) δ : 205.3, 152.1, 122.0, 121.4, 105.5, 98.0, 97.2, 81.0, 30.5, 29.8, 28.3, 28.2. MS (electrospray) m/z: 943 (M+Na) . Example 45 Tris-(8-chlorocarbonyl-2.2,6, 6-tetramethyIbenzof1, 2- d:5,4-d'1-bis(1.3) oxathiol-4-yl)methanol - gO - Tris-(8-carboxy-2,2,6,6-tetramethylbenzo-[1,2-d:5,4-d' ]- bis-(1,3)oxathiol-4-yl)methanol (0.92 g, 1.0 mmol) was dissolved in thionyl chloride (8 mL) and one drop of dimethylformamide was added. After 1 h, the mixture was placed on a rotary evaporator and evaporated with benzene (5*10 mL) . A close to quantitative yield of orange crystals, which were not further purified, was obtained. Η NMR (CDC1 3 , 300 MHz) δ : 4.70 (s, IH, OH), 1.86-1.67 (m, 36H, CH 3 ) . 13 C NMR (CDC1 3 , 75 MHz) <S: 162.3, 151.4, 151.3, 122.1, 122.0, 109.0, 99.5, 98.5, 81.0, 31.0, 30.5, 29.2, 29.1. Example 46 Tris-( (8-(2 ,2-dimethyl-l.3-dioxiran-4-yl) -carbonyl)- 2.2.6.6-tetramethylbenzof1.2-d:5.4-d' 1-bis(1.3) oxathiol- 4-yl)methanol 400 mg Sodium was stirred with 1,2-O-isopropylidene- glycerine (5 mL) for 2 h. The resulting mixture, containing a large excess of the Na-alcoholate, was decanted from unreacted sodium and mixed with a solution of tris-(8-chlorocarbonyl-2,2,6,6-tetramethy1-benzo[1,2- d:5,4-d' ]-bis(1, 3) oxathiole-4-yl)methanol (0.10 g, 0.10 mmol) in 1,2-O-isopropylideneglycerine (5 L) . After stirring for 5 h, the mixture was poured onto dichloromethane/ water and the organic layer was dried (Na 2 S0 4 ) and evaporated. Purification by preparative HPLC gave the triester as a slightly yellow solid. 'H NMR (CDC1 3 , 300 MHz) δ: 4.79 (s, IH) , 4.42-3.90 (m, 15H) 1.85-1.65 (m, 36H) , 1.56-1.38 (m, 18H) . MS (electrospray) m/z: 1285 (M+Na) . Example 47 Tris-( (8-(2.3-dihydroxy-l-propyl) -carbonyl) -2.2.6.6- tetra-methylbenzof1.2-d:5.4-d' ]-biε(1.3) oxathiol-4- yDmethanol 22 - A solution of sodium 1,2-O-isopropylidenglyceride (prepared from 0.28 g of sodium and 1,2-0- isopropylidenglycerine (5 mL) as described in Example 46) was stirred overnight with a solution of tris-(8- chlorocarbonyl-2,2,6,6-tetramethyl-benzof1,2-d:5,4-d']- bis(l,3)oxathiol-4-yl)methanol (0.88 g, 0,88 mmol) in 1,2-O-isopropylideneglycerine (5 mL) , and the resulting crude ketal was stirred with acetonitrile (100 mL) mixed with cone. HCl (25 mL) . After 2 h, the mixture was transferred to a separatory funnel and the two layers were separated. Both layers were partitioned between dichlorometane/ water three times respectively. The aqueous layers were discarded and the combined organic layers were dried (Na 2 S0 4 ) and evaporated. Purification by preparative HPLC yielded 0.75 g (75 %) of the pure triester. Η NMR (CDC1 3 , 300 MHz) δ : 4.85 (s, IH) , 4.54-3.70 (m, 17HJ1.85-1.65 (m, 36H) . MS (electrospray) m/z: 1143 (M+Na) . € 3 - Example 48 Tris- ( (8- (2 . 3-dihvdroxy-l-propyloxy) -carbonyl) -2 , 2 .6. 6- tetra-methy lbenzo f 1 , 2-d: 5.4-d' ] -bis (1.3 ) oxathiol-4- γl) methyl The reaction was performed under an argon atmosphere using deoxygenated solvents. Tris-( (8-(2,3-dihydroxy-l- propyl)-2,2,6,6-tetramethylbenzo[1,2-d:5,4-d']- bis(1,3)oxathiole-4-yl) ethanol (0.053 g, 0.046 mmol) was dissolved in dichloromethane (6 mL) and stirred with BF 3 - Et 2 0 (0. 053 mL, 0. 42 mmol) for 10 min . A solution of tin(II) chloride (25 mg, 0.13 mmol) in THF (2 mL) was added and after 10 min, the mixture was poured onto water/ dichloromethane. The organic layer was dried (Na 2 S0 4 ) and evaporated. Purification by preparative HPLC yielded 22 mg (42 %) of the pure radical. MS (electrospray) m/z: 1142 (M+H) . ESR (H 2 0, 200 G) : singlet, linewidth 300 mG. Overhauser enhancement (H 2 0) : 56 at 10 W microwave power. Stability measurements: Half life in acetonitrile:water (1:1)without exclusion of air: 250 h. - 9 4 - Example 49 Tris-(8-di-(2-hydroxyethyl) aminocarbonyl-2.2.6.6- tetramethylbenzof1.2-d:5.4-d']-bis(1.3)oxathiol-4- yl)methanol Tris-(8-chlorocarbony1-2,2,6, 6-tetramethy1-benzo[1,2- d:5,4-d']-bis(1,3) oxathiol-4-yl)methanol (0.80 g, 0.80 mmol) was dissolved in benzene (200 mL) . A solution of of bis(2-hydroxyethyl) amine (8.0 g, 48 mmol) in water (200 L) was added and after vigorous stirring over¬ night, the mixture was transferred to a separatory funnel. The aqueous layer was removed and the remaining solid plus the benzene layer was evaporated, dissolved in methanol and passed through a short column (neutral alumina) . After elution of unwanted materials with acetonitrile, the amide was eluted with methanol. After evaporation, the product was stirred with water (50 mL) at 40 °C for 2 h and then isolated by filtration. After drying, 0.60 g (60%) of the pure amide was obtained. 'H NMR ((CD 3 ) 2 CO, 300 MHz) δ : 3.80-2.80 (m, 16H) , 1.85- 1.65 (m, 36H) . MS (electrospray) m/z: 1182 (M+H) . Example 50 Tris-(8-di-(2-hydroxyethyl) aminocarbonyl-2 ,2,6, 6-tetra¬ methyIbenzof1.2-d: 5 , 4-d' 1-bis(1.3) oxathio1-4-yl)methyl The reaction was performed under an argon atmosphere using deoxygenated solvents. Tris-(8-di-(2- hydroxyethyl) amino-carbonyl-2,2,6,6- tetramethylbenzo[l,2-d:5,4-d']-bis(1,3)oxathiol-4- yl)methanol (0.032 g, 0.027 mmol) was dissolved in dichloromethane (40 mL) and BF 3 -Et 2 0 (0.15 mL, 1.19 mmol) was added. After stirring for 20 min, a solution of 15 mg SnCl 2 (15 mg, 0.079 mmol) in THF (10 mL) was added. After stirring for an additional 5 min, the mixture was poured onto sat. NaCl/ dichloromethane. The organic layer was dried (Na 2 S0 4 ) and evaporated. Purification by preparative HPLC yielded 22 mg (69 %) of the pure radical. ESR (H 2 0, 200 G) : singlet, linewidth 600 mG. Overhauser enhancement (H 2 0) : 124 at 10 W microwave power. MS (electrospray) m/z: 1164 (M + ) . Example 51 Tris-(8-dimethylaminocarbonyl) -2,2,6,6- tetramethyIbenzof1.2-d: 5 ,4-d' 1-bis(1, 3) oxathiol-4- yl)methanol - g-6 - Tris-(8-chlorocarbonyl-2,2,6,6-tetramethyl-benzo[1,2- d:5,4-d']-bis(l,3)oxathiol-4-yl)methanol (0.50 g, 0.50 mmol) was dissolved in benzene (20 mL) . A solution of of dimethylamine (3.0 g, 67 mmol) in water (20 mL) was added and after treatment with ultrasound for lh, the mixture was transferred to a separatory funnel. The aqueous layer was extracted with benzene and the combined organic layers were dried (Na 2 S0 4 ) and evaporated. Yield: 0.52 g (100%) . 'H NMR ( (CD 3 ) 2 C0, 300 MHz) δ : 4.63 (s, IH, OH), 3.10-2.90 (m, 18H, NCH 3 ) 1.81-1.61 (m, 36H, CH 3 ) . Example 52 Tris-(8-dimethylaminocarbony1-2,2.6.6-tetra- methylbenzof1.2-d:5,4-d'l-bis(l,3)oxathiol-4-yl)methyl - tl - The reaction was performed under an argon atmosphere using deoxygenated solvents. Tris-(8- dimethylaminocarbonyl-2,2,6, 6-tetramethylbenzo[1,2- d:5,4-d']-bis(l,3)oxathiol-4-yl)methanol (0.039 g, 0.039 mmol) was dissolved in dichloromethane (10 mL) . Trifluoromethanesulphonic acid (0.050 mL, 0.57 mmol) was added and after 5 min, a solution of tin(II) chloride (15 mg, 0.079 mmol) in THF (10 mL) was added. After stirring for an additional 7 min, the mixture was poured onto water/dichloromethane. The organic layer was dried (Na 2 S0 4 ) and evaporated. Purification by preparative HPLC yielded 20 mg (49 %) of the pure radical. ESR (acetonitrile, 200 G) : singlet, linewidth 580 G. Overhauser enhancement (acetonitrile) : 120 at 10 W microwave power. MS (electrospray) m/z: 985 (M+H) . Stability measurements: Half life in acetonitrile without exclusion of air: 26 h. Example 53 Tris- (8-dimethylaminocarbonyl-3 ,3,5,5, -tetraoxo-2 ,2.6,6- tetra-methylbenzofl.2-d:5,4-d't-bis(l,3)oxathiol-4- yl)methyl - β8 - The reaction was performed under an argon atmosphere using deoxygenated solvents. Tris-(8-dimethylaminocarbonyl- 2,2,6,6-tetramethylbenzo[1,2-d:5,4-d']-bis(1,3)oxathiol-4- yl)methyl (0.020 g, 0.021 mmol) was dissolved in trifluoroacetic acid (2 mL) and H 2 0 2 (0.4 mL, 35 % in water) was added. After stirring overnight, the mixture was poured onto sat. NaCl/ dichloromethane. The organic layer was washed with sat. NaCl and evaporated. Purification by preparative HPLC gave the pure radical. Yield: 0.005 g (25 %) . ESR (H 2 0, 200 G) : singlet, linewidth 470 mG. Overhauser enhancement (H 2 0) : 185 at 10 W microwave power. MS (electrospray) m/z: 1199 (M+Na) , 1177 (M+H) . Stability measurements: Half life in acetonitrile without exclusion of air: 26 h. Example 54 Tris- (8-ethoxycarbonylmethylthio-2.2.6.6-tetra- ethylbenzof 1, 2-d: 5.4-d' 1 -bis (1 , 3) oxathiol -4-yl) ethanol - S9 - The reaction was performed under an argon atmosphere using deoxygenated solvents. Tris-2, 2 , 6, 6- tetramethylbenzof1,2-d:5,4-d' ]-bis(1,3)oxathiol-4- yl) ethanol (1.0 g, 1.24 mmol) was dissolved in THF (25 mL) . The mixture was cooled to -70 °C and n-butyllithium (5.8 mL, 1.6M in hexane) waε added dropwise over 5 min, the cooling bath was removed and the mixture was left to attain ambient temperature over 30 min. The mixture was again cooled to -78 °C and sulfur (0.24 g, 7.5 mmol) was added. The cooling bath was removed and the mixture sonicated for 5 min and then left for 1 h. The mixture was disεolved by adding 2 M NaOH (10 mL) followed by water (20 mL) and washed with diethyl ether, and then acidified with 4 M HCl and taken up in diethyl ether. After drying (Na 2 S0 4 ) and evaporation, the crude thiol was disεolved in acetonitrile (100 mL) and stirred with bromoethylacetate (1.25 mL) and K 2 C0 3 (3 g) overnight. After evaporation and addition of dichloromethane the mixture was washed twice with water, dried (Na 2 S0 4 ) and evaporated. Purification by preparative HPLC gave 0.60 g (42 %) of the triester. Η NMR (DMF-d 6 , 300 MHz) δ: 4.63 (s, IH, OH) , 4.11 (q, 6H, CH 2 ) , 3.61 (d, 6H, CH 2 ) , 1.85-1.69 (m, 36H, CH 3 ) , 1.26 (t, 9H, CHj) . lo Example 55 Tris- (8-ethoxycarbonylmethylthio-2.2.6.6-tetra- methylbenzor1.2-d:5.4-d' ]-bis(1.3)oxathiol-4-y1)methyl The reaction was performed under an argon atmosphere using deoxygenated solvents. Tris- (ethoxycarbonylmethylthio-2,2,6, 6-tetramethylbenzo[1,2- d:5,4-d' ]-bis(1, 3)oxathiol-4-yl)methanol (0.080 g, 0.070 mmol) was dissolved in dichloromethane (10 mL) . BF 3 -Et 2 0 (0.140 mL, 1.11 mmol) was added and the mixture waε stirred for 10 min. A solution of tin(II) chloride (40 mg, 0.22 mmol) in THF (4 mL) was added and after 1 h, the mixture was poured onto water/ dichloromethane. The organic layer was washed with aq. NaHC0 3 , dried (Na 2 S0 4 ) and evaporated. Purification by preparative HPLC yielded 26 mg (33 %) of the pure radical. ESR (acetonitrile, 200 G) : singlet, linewidth 269 mG. MS (electrospray) m/z: 1125 (M + ) . Example 56 Tris-(8-carboxylmethylthio-2.2,6, 6-tetra- methylbenzof1.2-d:5,4-d'1-bis(l,3) oxathiol-4-yl)methyl - T l - The reaction was performed under an argon atmosphere using deoxygenated solvents. Tris- (ethoxycarbonylmethylthio-2,2,6,6-tetramethylbenzo[1,2- d:5,4-d']-bis(l,3)oxathiol-4-yl)methyl (0.032 g, 0.028 mmol) was treated with methanol (3 mL) and 1 % aqueous KOH (1 mL) for 30 min. The mixture was poured onto 1 M NaOH/ dichloromethane, the organic layer was discarded and the aqueous layer was carefully acidified and extracted with dichloromethane. The organic layer was washed with water, dried (Na 2 S0 4 ) and evaporated. Purification by preparative HPLC yielded 0.020 g (60 %) of the triacid radical. MS (electrospray) m/z: 1041 (M+H) . ESR (H 2 0, 200 G) : singlet, linewidth 196 mG. Overhauser enhancement (H 2 0, 1.1 mM) : 156 at 1.1 W microwave power. -* ~ 12 - Example ■ 57- 2.2.6.6-Tetra(ethoxycarbonyl)benzori.2-d:4.5-d'Ibis (1.3)dithiole. The reaction was performed under argon atmosphere using deoxygenated solvents. 1,2,4, 5-Benzotetrathiole (1.50 g, 7.3 mmol) and K 2 C0 3 (4 g) were mixed with dry DMF (70 ml) and a solution of dibromodiethyl malonate (4.26 g, 14.6 mmol) in DMF (15 ml) was added. The mixture was heated to 60 β C and stirred for 65 h. After cooling to room temperature, the reaction mixture was poured into ice water and then extracted with CH 2 C1 2 (2x100 ml) . The combined organic phases were washed with water (4x50 ml) , dried (Na 2 S0 4 ) and evaporated. Yield: 3.32 g (88 %) . E NMR (CDC1 3 ) : 6.97 (s, 2H) , 4.29 (q, J=7.2 Hz, 8H) , 1.28 (t, J=7.2 Hz, 12H) . Example 58 2.2.5.6-Tetra (methoxycarbonyl) -4.8-dibromobenzofl.2-d:4.5- d'lbis (l,3)dithiole. °I3 with cπ 2 Cl 2 (3x100 ml) , the combined organic phases were washed with water (3x50 ml), dried (MgS0 4 ) and evaporated. The residue was triturated with CH 3 CN and dried. Yield: 10.1 g (72 %) . *H NMR (DMSO-d 6 ) : 4.28 (q, J=7.2 Hz, 8H) , 1.21 (t, J=7.2 Hz, 12H) . Example 59 4.8-Dibromobenzofl.2-d:4,5-d'1bis (1.3) dithiole-2.6-disoiro- (4,4- dimethyl-3,5-dioxane) . 2,2,6, 6-Tetra (methoxycarbonyl) -4, 8-dibromobenzo[1, 2-d:4, 5- d , ]bis (1, 3) dithiole (6.76 g, 10.0 mmol) was dissolved in dry THF, the solution was cooled to 0 "C and a solution of DIBAL in toluene (17.8 ml, 100 mmol) was added dropwise. The solution was heated to reflux temperature for 3 h and then allowed to cool to room temperature. Methanol (20 ml) was added dropwise followed by water (60 ml) and the pH was adjusted to 2 using aqueous 6 M HCl. The solvents, except water, were removed by evaporation and the precipitate was collected by filtration.The product was washed with water, acetonitrile, dried and then suspended in dry acetone (600 ml) . BF 3 -Et 2 0 (2.52 ml, 20 mmol) was added and the solution was stirred for 20 min. Solid K 2 C0 3 (6.0 g) was added and stirring was continued for another 5 min. After filtering through a short pad of basic alumina, the solvents were removed by evaporation, the residue was triturated with CH 2 C1 2 and dried. Yield: 1.12 g (19 %) . >H NMR (DMSO-D 6 ) : 4.15 (S, 8H) , 1.37 (S, 12H) . -M+ Example 60 4-Bromobenzof1.2-d:4,5-d'1bis (1, 3)dithiole-2.6-dispiro- (4. -dimethyl- 3.5-dioxane) . 4, 8-Dibromobenzo[l,2-d:4,5-d']bis (1,3) dithiole-2, 6-dispiro- (4,4- dimethyl-3, 5-dioxane) (1.14 g, 1.94 mmol) was dissolved in dry THF (270 ml) under an atmosphere of argon. After cooling the solution to -45 °C, a solution of n-BuLi in hexane (2.5 M, 2.02 mmol) was added dropwise. After stirring for 5 min, methanol (3 ml) was added, the solution was allowed to attain room temperature and the solvents were evaporated. The product was purified by chromatography on silica gel using a mixture of CH 2 C1 2 and methanol (99.5:0.5) as the eluent. Yield: 0.70 g (71 %) . l ¥> NMR (CDC1 3 ) : 6.80 (s, IH) , 4.15 (s, 8H) , 1.47 (s, 12 H) . Example 61. Tris (benzofl.2-d:4.5-d' Ibis (1.3) dithiole-4-yl-2, 6-dispiro- (4, - dimethyl-3, 5-dioxane) )methanol . 4-Bromobenzo [l,2-d:4,5-d']bis(l,3)dithiole-2,6-dispiro-(4, -dimethyl- -T5- 3,5-dioxane) (0.99 g, 1.94 mmol) was suspended .in dry diethyl ether (28 ml) under an atmosphere of argon. A solution of n-BuLi (2.5 M in hexane, 1.94 mmol) was added dropwise and, after 5 min, a solution of diethyl carbonate 0.078 ml, 0.64 mmol) in diethyl ether (3 ml) was added slowly. After stirring for 18 h, ethanol (5 ml) was added and the solvent was removed by evaporation. The product was purified by chromatography on silica gel using a mixture of CHC1 3 and ethyl acetate (20:1) as the eluent. Yield: 0.65 g (76 %) . Η NMR (CDC1 3 ) : 7.16 (s, 3H) , 6.01 (s, IH) , 3.86-4.22 (m, 24H) , 1.43, 1.41, 1.37, 1.32 (4s, 36H) . EXAMPLE 62. Tris ( 8-ethoxycarbonylbenzo f l . 2-d : 4. 5-d ' I bis ( 1 , 3 ) dithiole-4-yl-2. 6- di spiro- ( 4 , 4 -dimethyl -3 , 5-dioxane ) ) methanol . Tris (benzo[1, 2-d:4, 5-d']bis (1, 3) dithiole-4-yl-2, 6-dispiro- (4, 4- dimethyl-3, 5-dioxane) )methanol (0.205 g, 0.156 mmol) was dissolved in dry benzene (12 ml) containing N,N,N' ,N'-tetramethylethylene diamine (0.33 ml, 2.18 mmol) under an atmosphere of argon. A solution of t- BuLi in pentane (1.5 M, 2.18 mmol) was added dropwise and stirring was continued for 40 min.The solution was then transferred into another flask, kept at 0 °C and containing diethylpyrocarbonate (1.3 ml, 8.82 mmol) and benzene (6 ml) . After stirring for 45 min, an aqueous NaH 2 P0 4 buffer was added, the organic phase was separated, washed with water and evaporated. The product was purified by preparative HPLC. Yield: 55 mg (23 'i ) . ' Η NMR (CDC1 : 6-68 (s, IH), 4.41-4.52 (m, 6H) , 3.86-4.21 (m, 24 H) , 1.22-1.60 (m, 45H) . ^„ EXAMPLE 63. Tris (8-ethoxycarbonyl-2, 2,6, 6-tetrahvdroxymethylbenzo f 1, 2-d: 4, 5- d'lbis (1.3) dithiole-4-yl) methanol. Tris (8-ethoxycarbonylbenzo[l,2-d:4, 5-d']bis (1, 3) dithiole-4-yl-2, 6- dispiro- (4, 4-dimethyl-3, 5-dioxane) )methanol (55 mg, 0.0359 mmol) was dissolved in a mixture of glacial acetic acid (20 ml) and water (5 ml) and the solution was stirred at room temperature for 42 h. The solvents were removed by evaporation, traces of acid were removed by addition of benzene followed by evaporation. HPLC analysis indicated >98 purity of the product. Yield: 42.4 mg (91 %) . MS (ESP " , m/e) : 1293 (M\ 68 %) , 1291 ([M-2]-, 100 %) . °π EXAMPLE 64 . Tris (8-carboxy-2.2.6, 6-tetrahvdroxymethylbenzof1, 2-d: 4, 5- d' Ibis (1.3) dithiole-4-yl)methyl sodium salt. Tris (8-ethoxycarbonyl-2, 2,6, 6-tetrahydroxymethylbenzo[1, 2-d: , 5- d']bis (1, 3)dithiole-4-yl)methanol (3.4 mg, 0.0026 mmol) was dissolved in acetonitrile (2 ml) and the solution was cooled to 0 °C. Trifluoromethanesulfonic acid (0.017 ml) was added and after 15 min, a solution of SnCl 2 (0.4 mg) in acetonitrile (1 ml) was added. After another 15 min, an aqueous NaH 2 P0 4 buffer was added and the solvents were removed by evaporation. The residue was suspended in water and the pH was adjusted to 12 using an 1 M aqueous NaOH solution. After stirring for 1 h, the solution was neutralized with 1 M aqueous HCl and the solvent was removed by evaporation. The product was purified by preparative HPLC. Yield: 2.0 mg (60 %). ESR (1.5 mM in H 2 0, 100 G) : singlet, linewidth 100 mG. Iff - KYAMP E 5 2.2.6.6-Tetramethylbenzo r 1.2-d : 4.5-d' -bis (1.3) dioxole-4- carboxylic acid 2,2,6,6-Tetramethylbenzo[1,2-d:4,5-d']-bis(1,3)dioxole (10.0 g, 45.0 mmol; prepared according to WO-91/12024) waε dissolved in dry THF (200 mL) under an argon atmosphere. The solution was cooled to -20 °C and n-butyllithium (20.0 mL, 50.0 mmol) in hexane was added. After attaining ambient temperature, the reaction mixture was transferred onto solid carbon dioxide (150 g) and allowed to stand overnight. Water (200 mL) was added and pH was adjusted to 10 using 2M aqueous NaOH. After washing with ether, the aqueous phase was acidified with 2M hydrochloric acid to pH 2 and extracted with ether (2*300 mL) . The organic phases were dried (Na 2 S0 4 ) and evaporated to give the pure product. Yield: 10.7 g (89 %) . IH NMR (CDC13, 300 MHz) δ: 6.50 (s, IH) , 1.71 (s, 12H) . 13 C NMR (CDC1 3 75 MHz) δ: 165.1, 140.9, 140.8, 119.8, 98.9, 97.3, 25.6. - J - TCΑMPLE 66 2.2.6.6-Tetramethylbenzor1.2-d;4.5-d'1-bis(1.3)dioxole-4- carboxylic acid methyl ester 2,2,6,6-Tetrameth lbenzo[1,2-d:4,5-d']-bis(1,3)dioxole- 4-carboxylic acid (10.0 g, 38.0 mmol) was dissolved in dry DMF (100 mL) . Potassium carbonate (15.2 g, 110.0 mmol) was added and the reaction was heated to 55 °C for 30 min. After cooling to ambient temperature, methyl iodide (15.6 g, 110.0 mmol) was added and the solution was stirred overnight. The precipitate was filtered off and the solution was evaporated. The residue was dissolved in saturated aqueous NaHCO 3 and ether. The aqueous layer was discarded and the organic phase was dried (Na 2 S0^) , filtered and evaporated to give 9.4 g (88 %) of the pure product. Η NMR (CDC1 3, 300 MHZ) δ : 6.44 (S, IH) , 3.85 (s, 3H) , 1.65 (s, 12H) . I3 C NMR (CDC1 3 75 MHz) δ 163.4, 140.8, 140.6, 119.0, 99.9, 99.4, 51.9, 25.6. - tcO _ EXAMPLE 67 Bis-(2.2.6.6-tetramethylbenzor1.2-d:4.5-d'1-bis(1.3 dithiole-4-yl)-mono-(2.2.6.6-tetramethyl- benzor1.2-d:4.5-d'1-bis(1.3)dioxole-4-yl)methanol 2,2,6,6-Tetramethylbenzo[1,2-d:4,5-d']bis(1,3)dithiole (2.86 g, 10 mmol; prepared according to WO-91/12024) was dissolved in anhydrous THF (75 mL) and cooled to -70 °C. n-Butyllithium (4.4 mL, 2.5 M in hexane) was added. The reaction mixture was allowed to reach ambient temperature. 4-Methoxycarbony1-2,2,6,6-tetramethylbenzo- [1,2-d:4,5-d')-bis-(1,3)-dioxole (1.4 g, 5 mmol) was added as a solid. After 1 hour, the mixture was quenched with saturated aqueous NaH 2 P0 4 . The aqueous phase was discarded and the organic layer evaporated. The residue was dissolved in dichloromethane, washed with water and dried (Na 2 S0 4 ) . The product was purified by column chromatography (dichloromethane:heptane, 1:1) giving 1.8 g (44%) of pure product. Η NMR (CDC j , 300 MHz) <5: 7.10 (broad s, 2 H, ArH), 6.39 (s, 1 H, ArH), 4.79 (s, 1 H, OH), 1.82-1.56 (m, 24 H, CH 3 ) , 1.53 (s, 6 H, CH 3 ) , 1.46 (s, 6 H, CH 3 ) . _ t c EXAMPLE 68 Bis- ( 8-ethoxycarbony 1-2 .2 .6.6-tetramethγlbenzo r 1.2-d ; 4.5 -d ' 1 -bis ( 1 . 3 ) dithiole-4-yl ) -mono- ( 8-ethoxycarbonyl- 2 .2 . 6 . 6-tetramethylbenzo f 1 . 2-d : 4 . 5-d' 1 -bis f 1 . 3 ) dioxol- 4-yl) methanol Bis-(2,2,6,6-tetramethylbenzo[l,2-d:4,5-d']- bis(1,3)dithiole-4-y1)-mono-(2,2,6,6-tetramethylbenzo[1, 2-d:4,5-d']-bis(l,3)dioxol-4-yl)methanol (0.50 g, 0.61 mmol) was dissolved in dry benzene (6.0 mL) under an atmosphere of argon. t-Butyllitium (2.44 mL, 1.5 M in pentane) and TMEDA (0.545 mL, 3.66 mmol) were added. The reaction mixture was subjected to ultrasound for 25 min. and was then slowly added to a solution of diethyl carbonate (7.2 mL, 59.4 mmol) in dry benzene (16 mL) . After stirring for 1.5 h, aqueous NaH 2 P0 4 (50 mL) was added. The organic layer was separated, washed with water, dried (Na 2 S0 4 ) and evaporated. After purification by preparative HPLC 130.0 mg (21 %) ) of the pure product was obtained. l H NMR (CDCI 3 , 300 MHZ) δ : 4.98 (s, IH) , 4.28-4.37 (m, 6H) , 1.48-1.79 (m, 36H) , 1.46 (t, 6H, J 7.0 Hz) , 1.38 (t, 3H, J 7.0 Hz) . lo\_ - l3 C NMR (CDCI 3 , 75 MHZ) : 166.2, 166.0, 162.9, 141.9, 141.6, 141.2, 140.8, 140.4, 140.0, 136.6, 134.5, 129.9, 128.5, 128.1, 127.8, 127.2, 120.3, 118.9, 111.9, 101.1, 80.6, 62.1, 61.0, 60.3, 60.2, 59.8, 59.2, 34.4, 34.3, 33.5, 28.8, 28.1, 27.0, 26.9, 26.5, 25.8. EXAMPLE <S9 Bis-(8-ethoxycarbonyl-2.2.6.6-tetramethylbenzori.2- d:4.5-d '1-bis ( 1.3 )dithiole-4-yli-mono-(8-ethoxycarbonyl- 2.2.6.6-tetramethylbenzor1.2-d:4.5-d 1-bis(1.31dioxol-4- yl)methyl Bis-(8-ethoxycarbony1-2,2,6,6-tetramethylbenzo[1,2- d:4,5-d' ]-biε(1,3)dithiol-4-yl)-mono-(8-ethoxycarbonyl- 2,2,6,6-tetramethylbenzo[1,2-d:4,5-d ' )-biε(1,3)dioxol-4- yl)methanol (520 mg, 0.501 mmol) waε diεsolved in dry degassed dichloromethane (15 mL) together with tin(II) chloride (95 mg, 0.501 mmol) and acetonitrile (5 mL) . BF 3 -Et- 2 θ (70 μL, 0.557 mmol) was added and the solution was stirred for 20 min. After addition of dichloromethane (80 mL) and washing with degassed water (80 mL) , the organic layer was separated, dried (MgS0 4 ) , filtered and evaporated. The product was purified by preparative HPLC. Yield: 110 mg (22%) . ESR (THF, 200 G) singlet, line width 325 mG. Overhauser enhancement (THF, 2.1 mM) : 156 at 4 W microwave power. .*l Stability measurements: Half life in acetonitrile without exclusion of air: 2000 h. EXAMPLE 70 Bis-(8-potassium carboxylate-2.2.6.6-tetramethylbenzo T1.2-d:4.5-d / 1-bis(1.3)dithiol-4-yl)-mono-(S-ootassium carboxylate-2.2.6.6-tetramethγlbenzor1.2-d:4.5-d'1-bis (1.3)dioxol-4-yl)methyl Bis-(8-ethoxycarbonyl-2,2,6, 6-tetramethylbenzo[1,2- d:4,5-d' ]-bis(1,3)dithiol-4-yl)-mono-(8-ethoxycarbony1- 2,2,6,6-tetramethylbenzo[1,2-d:4,5-d ']-bis(1,3)dioxol-4- yl)methyl (132 mg, 0.129 mmol) was diεεolved in ethanol (10 mL) . Aqueous potassium hydroxide (5 mL, l.OM) waε added and the reaction mixture was stirred at 50 °C overnight. After evaporation of the ethanol, the mixture was stirred for l h at 50 °C and was then acidified with 2M hydrochloric acid. The aqueous phase was extracted with ether. The organic phase waε separated, dried (MgS0 4 ) filtered and evaporated. The product waε purified by preparative HPLC. The fractionε were evaporated and water waε added. The aqueous layer was extracted with ether. The organic layer waε separated, dried (MgS0 4 ) , filtered and evaporated. The product was diεsolved by adding water and IM KOH (0.387 mL, 0.387 mmol). The solution waε lyophilized. - (Otf - Yield: 101 mg (75%) . ESR (H 2 0, 200 G) : singlet, line width 105 mG. Overhauser enhancement (H 2 0, 6.9 mM) : 219 at 0.012 microwave power. EXAMPLE -71 Benzofl.2-d:4.5-d'1bis(1.3)dithiole-2.2.6.6- tetracarboxylic acid tetraethyl ester 1,2,4,5-benzenetetrathiol (1.50g, 7.28mmol) was dissolved in dry DMF (55ml) under an atmosphere of Argon and K 2 C0 3 (4.0g) was added together with 2,2- dibromomalonate ethyl ester (4.26g, 14.6mmol) . The solution was stirred at room temperature for 16h and then at 60°C for an additional 5h. The reaction mixture was then poured into an ice-water mixture (200g-200ml) and extracted with ethyl acetate (2x250ml) . The combined organic phases were washed with water (4x100ml) dried (Na 2 S0 4 ) and evaporated. The crude product was washed sufficiently pure to be used in the next step without purification. Yield: 3.05g (80%) IH NMR (300MHz, CDC1 3 ) : 6.91 (s,2H), 4.29 (q, J=7.2Hz, 8H) , 1.28 (t, J=7.2Hz, 12H) . - ) * -r EXAMPLE 72 2 . 2 . 6 . 6-tetra (hydroxymethyl -d,) benzo fl . 2-d : 4 . 5- d ' I bis ( 1 . 3 ) dithiole A dry Soxhlet setup was provided with Benzo[1,2-d:4,5- d']bis(1,3)dithiole-2,2,6,6-tetracarboxylic acid tetraethyl ester (5.0g, 9.65mmol) in the upper compartment and a mixture of lithium aluminium deuteride (1.62g, 38.6mmol) and diethyl ether (300ml) in the lower, round-bottomed flask. The ether was heated to reflux temperature for 2Oh and the mixture was then allowed to cool. Methanol (150ml) was added dropwise by water (50ml) . The mixture was acidified with concentrated HCl (20ml) and the solvent was reduced to 50ml by evaporation in vacuum. The white solid was filtered off, washed with water (2x25ml) and dried. Yield 3.15g (91%) . IH NMR (300MHz, DMSO-d 6 ) : 7.06 (2,2H), 5.45 (br s, 4H) ( 0(0 _ EXAMPLE 73 2.2.6.6-Tetra(dimetylthexylsilyloxymethyl)benzof1.2- d:4.5-d' Ibis (1.3)dithiole The reaction waε performed under argon atmoεphere. 2,2,6,6-Tetra(hydroxymethyl)benzo[1,2-d:4,5- d' ]biε(1,3)dithiole (0.8 g, 2.2 mmol) was disεolved in DMF (20 mL) . Imidazole (1.1 g, 15.8 mmol) was added and the solution was cooled to 0 °C. Dimethylthexylεilyl chloride (2.8 g, 15.8 mmol) waε added dropwiεe (ca 2 min) . The solution waε εtirred for 48 hourε at ambient temperature. The reaction mixture waε poured into ice/water, CH 2 C1 2 (100 mL) waε added and the two phaεeε were separated. The organic phase was washed with IM HCl and water (3*100 mL) . The solution was dried (Na 2 S0 4 ) and evaporated. The product was purified by column chromatography using dichloromethane-heptane (1:9) as eluent. Yield: 1.1 g (52 %) . l H NMR (CDCI 3 , 300 MHz) δ : 6.84 (ε, 2H, ArH), 3.94 (ε, 8H, CH 2 ) , 1.62 (εeptet, 4H, J 6.8 Hz, CH) , 0.88 (d, 24H, J 6.8 Hz, CH j ) , 0.84 (s, 24H, CH 3 ) , 0.08 (s, 24H, Si(CH 3 ) 2 ) . 13 C NMR (CDCI 3 , 75 MHZ) δ : 134.3, 115.8, 74.2, 65.0, 34.2, 25.1, 20.3, 18.6, -3.6. ««?- EXAMPLE 74 Bis ( 2 . 2.6.6-tetra(dimetylthexylεilyloxymethyl)benzoT1.2- d ; 4.5-d ' 1biε(1.3)dithiol-4-yl) -mono (2.2.6.6- tetramethγlbenzofl.2-d:4.5-d' 1-bis(1.3)dioxol-4- yl) )methanol The reaction waε performed under argon atmosphere. 2,2,6,6-Tetra(dimetylthexylsilyloxymethyl)benzo[1,2-d:4,5- d']biε (l,3)dithiole (7.0 g, 7.6 mmol) waε diεsolved in dry THF (50 mL) . The εolution waε cooled to -70 °C. n- Butyllithium (5.0 mL, 1.6M in hexane) waε added and the temperature waε allowed to attain ambient temperature and waε stirred for 1 h. The εolvent waε evaporated in vacuum at ambient temperature and diethyl ether (20 mL) was added. Then, 4 -ethoxycarbony1- 2 , 2 , 6 , 6 - tetramethylbenzo[1,2-d:4,5-d']-biε(1,3)dioxole (0.8 g, 2.9 mmol) was added in one portion and the reaction mixture was εtirred at ambient temperature for 12 h. The mixture waε poured into a NaH 2 P0 4 solution, the phaseε were separated and the aqueous phase was extracted with diethyl ether (2*100 mL) . The organic phases were dried (Na 2 S0 4 ) and evaporated. The residue was purified by preparative HPLC. - le* - Yield: 3.7 g (62 %) . l H NMR (CDC1 3 , 300 MHZ) δ : 6.80 (s, 2H, ArH), 6.26 (s, IH, ArH), 4.95 (s, IH, OH), 3.8 (br m, 16H, CH 2 ) , 1.5 (br m, 20H, CH 3 +CH) , 0.9 (d, 48H, CH 3 ) , 0.7 (s, 48H, CH 3 ) , 0.2 ( 2 s, 48H, Si(CH3) 2 ). ,3 C NMR (CDC1 3 , 75 MHz) δ: 141.5, 140.3, 139.8, 139.6, 131.7, 118.6, 117.1, 108.1, 94.4, 80.0, 65.4, 34.1, 25.9, 25.0, 20.3, 18.7, -3.2. EXAMPLE 75 B i ε ( 8 - e t h o x v c a r b o n v l - 2 . 2 . 6 . 6 - tetra (hydroxymethyl ) benzo f 1.2-d : 4.5-d ' 1 biε (1.3) dithiol- 4-yl) -mono (8-ethoxycarbonyl-2.2.6.6-tetramethy lbenzo Tl .2- d:4.5-d' 1-biε(1.3)dioxol-4-yl) 1 methanol Biε(2,2,6,6-tetra(dimetylthexylsilyloxymethyl)benzo[1,2- d:4,5-d' ]bis.(l,3)dithiol-4-yl) -mono(2,2,6,6- tetramethylbenzo[ l,2-d:4,5-d' ]-biε(l,3)dioxol-4- yl))methanol (3.2 g, 1.54 mmol) was disεolved in heptane (12.8 mL) and dry benzene (10.7 mL) together with TMEDA (3.2 mL, 21.6 mmol) under an atmosphere of argon. The εolution waε cooled to -22 °C and t-BuLi (14.4 mL, 1.5M in pentane) waε added. After εtirring for 3 h at -22 °C, the reaction mixture was transferred into a solution of diethyl pyrocarbonate (12.8 mL, 87 mmol) in heptane (23 mL) and dry benzene (23 mL) which was kept at -22 β C. The reaction mixture waε then allowed to attain ambient temperature. After εtirring for an additional hour, a εaturated aqueouε solution of NaH 2 P0 4 (40 mL) was added. The mixture waε stirred for one hour, the organic phase waε εeparated, waεhed with water (2*100 mL) and acetonitrile (2*100 mL) . The heptane/benzene phaεe was evaporated and then diεsolved in THF (25 mL) . A εolution of Bu 4 NF in THF (20 mL, 20 mmol) waε added and the mixture waε stirred overnight. After evaporation of the εolvent, the reεidue waε partitioned between water (300 mL) and ethyl acetate (300 mL) . The organic phaεe waε waεhed with water (2*100 mL) , dried (Na 2 S0 4 ) and evaporated. Purification by preparative HPLC gave 400 mg (22 %) pure product. Η NMR (CDC1 3 , 300 MHZ) δ: 5.78-5.92 (m, 6H) , 5.03-5.52 (m, 24H) , 2.98-3.21 (m, 12H) , 2.90 (t, 6H, J 7.0 Hz) , 2.84 (t, 3H, J 6.9 HZ) . EXAMPLE 76 B i ε ( 8 - e t h o x v c a r b o n v l - 2 . 2 . 6 . 6 - tetra (hvdroxymethyl) benzof 1.2-d: 4.5-d ' 1 bis (1.3) dithiol-4- yl) -mono f 8-ethoxycarbony 1-2.2.6.6-tetramethylbenzof 1.2- d:4.5-d * 1-biε(1.31dioxol-4-vH ϊmethyl 96/39367 - Mo - Bis(8-ethoxycarbonyl-2,2,6,6- tetra(hydroxymethyl)benzo[1,2-d:4,5-d' ]bi (1,3)dithiol- 4-yl)-mono(8-ethoxycarbonyl-2,2,6,6- tetra ethylbenzo[1,2-d:4,5-d' ]-biε(1,3)dioxol-4- yl))methanol (294 mg, 0.25 mmol) was dissolved in acetonitrile (70 mL) under an atmosphere of argon. After cooling to 0 °C, trifluoromethane sulfonic acid (190 μL, 2.2 mmol) was added. After stirring for 3 min, tin(II) chloride (48 mg, 0.25 mmol) dissolved in acetonitrile (7 mL) was added. After 1 min, a saturated aqueous solution of NaH 2 P0 4 (50 L) was added. The aqueous phase was washed with acetonitrile (2*50 mL) , the combined organic phases were dried (Na 2 S0 4 ) and evaporated. Purification by preparative HPLC gave 176 mg (61 %) of the pure product. ESR (H 2 0, 200 G) : singlet, linewidth 433 mG. EXAMPLE 77 Bis(8-carboxy-2.2.6.6-tetra(hydroxymethyl)benzof1.2- d:4.5-d' lbis(1.3)dithiol-4-yl)-mono(8-carboxy-2.2.6 f 6- tetramethylbenzofl.2-d:4.5-d' ~ )-bis(l.3)dioxol-4- γl))methyl sodium salt lit Bis(8-ethoxycarbony1-2,2,6,6- tetra(hydroxymethyl)benzo[1,2-d:4,5-d' ]bis(1,3)dithiol- 4-yl)-mono(8-ethoxycarbonyl-2,2,6,6- tetramethylbenzo[1,2-d:4,5-d' ]-bis(1,3)dioxol-4- yl))methyl (316 mg, 0.275 mmol) waε disεolved in a mixture of IM aqueouε NaOH (3 mL) , water (1.5 mL) and ethanol (3 mL) . The εolution was stirred at ambient temperature for 15 min, the ethanol was removed by evaporation, and the residue was stirred at ambient temperature for additional 2 hourε. After evaporation to near dryneεε, the pure acid (240 mg, 82 %) waε isolated by preparative HPLC followed by lyophilization. The acid waε converted into the corresponding εodium salt by the addition of water (50 mL) followed by adjuεtment of the pH to 7 with IM aqueouε NaOH and lyophilization. ESR (3.4 mM in H 2 0, 200 G) : εinglet, linewidth 120 mG. Overhauser enhancement (aqueouε εolution aε above) : 164 at 5 W microwave power. Stability meaεurements: Half life in water without exclusion of air: 120 h.

Claims: - »\ 2_ - Claims ; 1 . A persistent free radical compound of formula I .CfAr 1 ) (I) (where each group Ar 1 , which may be the same or different, is an optionally substituted aromatic group, and at least one of said Ar 1 groups is a group Ar 3 of formula (wherein each X, which may be the same or different, is an oxygen or sulphur atom or a group CO or S(0) n (where n is 1 to 3) with the proviso that at least one group X is a sulphur atom or a S(0) n group; R 1 is a hydrogen atom or group of formula -M, -XM, -X-Ar 2 , or -Ar 2 where M is a water solubilising group, and Ar 2 is a 5-10 membered aromatic ring optionally substituted by a water solubilising group M; and each of the groups R 7 , which may be the same or different, is a hydrogen atom, or a hydrocarbon group or a water solubilising group M, or two groups R 7 together with the atom to which they are bound represent a carbonyl group or - Il3 - a 5 to 8 membered cycloalkylidene, mono- or di- oxacycloalkylidene, mono- or di-azacycloalkylidene or mono- or di-thiacycloalkylidene group optionally with the ring attachment carbon replaced by a silicon atom, and R 7 where it is other than hydrogen, is optionally substituted by a hydroxyl group, an optionally alkoxylated, optionally hydroxylated acyloxy or alkyl group or a water solubilising group M) ) or a perdeuterated analogue or salt thereof. 2. A compound as claimed in claim 1 wherein each Ar 1 group is an optionally substituted 5-7 membered carbocyclic or heterocyclic aromatic ring optionally carrying at least one fused carbocyclic or heterocyclic ring. 3. A compound as claimed in claim 1 where each Ar 1 group which is.not an Ar 3 group is a group Ar 1 ' of formula wherein each X, which may be the same or different, is 0, S, CO or S(0) n , and n, R 1 and R 7 are as defined in claim 1. 4. A compound as claimed in claim 3 wherein in each Ar 1 ' group X is oxygen and R 7 is hydrogen or optionally ) ■ + hydroxylated alkyl 5. A compound as claimed in claim 1 comprising at least one Ar 3 group which comprises an aromatic ring carrying at least one fused ring selected from the group consisting of wherein R 7 is hydrogen or optionally hydroxylated alkyl 6. A compound as claimed in claim 1 comprising at least one Ar 3 group selected from the group consisting of • >5 wherein R 7 is hydrogen or optionally hydroxylated alkyl and - UG - R 1 is as defined in claim 1. 7. A compound as claimed in claim 6 wherein R 1 is selected from the group consisting of hydrogen and -SCH 2 C00-Na+ -S0 2 R 2 -SR 2 -SCH 2 COOCH 2 CH 3 -S0 2 C (R 2 ) 2 CH 2 CHOHCH 2 OH -S0 2 NR 2 2 -S0 2 CH 2 CON(R 2 ) 2 -S0 2 -C- (CH 2 CH 2 OH) 2 COOCH 2 CH 3 -S0 2 C- (CH 2 CH 2 OH) 2 CH 2 OH -C- (CH 2 CH 2 OH) 3 -S0 2 -C(H) (COOCH 2 CH 3 ) 2 -CH 2 CON(CH 2 CH 2 OH) 2 -COOH -C0 2 Me -C0 2 Et (where R 2 is H or optionally hydroxylated alkyl) . 8. A compound as claimed in claim 1 of formula la, lb, Ic or Id - M - - n* - (where R 11 and R 12 are groups as defined for R 1 in claim 1) or a salt thereof. 9. A compound as claimed in claim 8 where R 11 and where present R 12 is selected from the group consisting of hydrogen, SCH 3 , SCH 2 C0 2 CH 2 CH 3 , SCH 2 COOH, S0 2 N(CH 3 )CH 2 (CHOH) 4 CH 2 OH, S0 2 NH 2 , S0 2 NCH 2 CH 2 OH and S0 2 NCH 2 CHOHCH 2 OH. 10. A magnetic resonance imaging contrast medium composition comprising a physiologically tolerable radical as defined in claim 1 together with at least one pharmacologically acceptable carrier or excipient. 11. In a method of magnetic resonance imaging wherein there is introduced into a human or non-human subject an effective amount of a magnetic resonance signal amplifying agent and wherein an image of at least a part of said subject is generated, the improvement comprising introducing as said amplifying agent a radical of formula I as defined in claim 1 or a salt thereof. 12. An ESR imaging contrast medium composition comprising a physiologically tolerable radical as defined in claim 1 together with at least one pharmacologically acceptable carrier or excipient. 13. In a method of ESR imaging wherein there is introduced into a human or non-human subject an effective amount of an ESR signal amplifying agent and wherein an image of at least a part of said subject is generated, the - tn - improvement comprising introducing as said amplifying agent a radical of formula I as defined in claim 1 or a salt thereof. 14. A radical compound as claimed in claim 1 in which each group Ar 1 is an aromatic group substituted by a solubilising group M and carrying two fused sulphur- containing heterocyclic rings each substituted by at least one solubilising groups M. 15. A radical compound as claimed in claim 14 wherein each of said Ar 1 groups is of formula of formula: (wherein R 1 represents a group of formula COOR 5 or COOM 1 (where R 5 represents a hydrogen atom or an optionally hydroxylated, optionally aminated, optionally alkoxylated, optionally carboxylated alkyl, oxo-alkyl, alkenyl or alkaryl group and M 1 is one equivalent of a physiologically tolerable cation, ; each of the groups R 7 , which may be the same or different - \ c - represents a hydxogen atom, or a hydrocarbon group such as an alkyl, hydroxyalkyl, alkoxyalkyl, alkoxycarbonyl, or carbamoyl group, and R 7 where it is other than hydrogen, is optionally substituted by a hydroxyl group, an optionally alkoxylated, optionally hydroxylated acyloxy or alkyl group; and at least one group R 7 on each fused ring is a water solubilizing group, i.e other than hydrogen and unsubstituted alkyl) or a deuterated analog, precursor or salt thereof. 16. A radical compound as claimed in claim 1 being tris (8- carboxy-2 ,2,6, 6- tetrahydroxymethy lbenzo [1 , 2-d : 4 , 5 - d']bis(l,3)dithiole-4-yl) methyl, bis- (8- sodium carboxylate -2, 2, 6, 6-tetrakis- ( 2 H 3 -methyl) -benzo [1, 2-d:4, 5- d 1 ] -bis (1, 3) dithiole-4-yl) -mono- (8-sodium carboxylate- 2,2,6, 6-tetrakis- ( 2 H 3 -methyl) -benzo [l,2-d:4,5-d'] - bis (1,3) dioxole-4-yl) methyl or bis- (8 -sodium carboxylate- 2,2,6, 6-tetrakis- (hydroxy- 2 H 2 -methyl) -benzo [1, 2-d: 4, 5-d 1 ] - bis (1, 3)dithiole-4-yl) -mono- (8-sodium carboxylate- 2, 2,6,6- tetramethylbenzo [l,2-d:4,5-d'] -bis (1 , 3 ) dioxole -4 - yl) methyl . 17. Use of a compound according to claim 1 in oximetry.

Inventors: ANDERSSON SVEN, RADNER FINN, RYDBECK ANNA, SERVIN ROLF, WISTRAND LARS-GORAN, ANDERSSON, SVEN, RADNER, FINN, RYDBECK, ANNA, SERVIN, ROLF, WISTRAND, LARS-GORAN
Applicants: ANDERSSON SVEN, COCKBAIN JULIAN R M, NYCOMED IMAGING AS, RADNER FINN, RYDBECK ANNA, SERVIN ROLF, WISTRAND LARS GORAN, ANDERSSON, SVEN, COCKBAIN, JULIAN, R., M., NYCOMED IMAGING A/S, RADNER, FINN, RYDBECK, ANNA, SERVIN, ROLF, WISTRAND, LARS-GORAN
Application year: 1995
Main classification: C07B61
IPC codes: A61K49, C07B61, C07D519, C07F7