bluuebunny/crossref_metadata_2025 huggingface dataset:
Kinetics and mechanism of base‐catalysed decomposition of tris(2,4‐pentanedionato)cobalt(III) in water and liquid ammonia
Authors: Balt S. , de Kieviet W.
Published: September 2010

The complex tris(2,4‐pentanedionato)cobalt(III) shows a base‐catalysed solvolysis reaction both in alkaline solution and in liquid ammonia. Activation parameters of the reaction in alkaline solution and in liquid ammonia are discussed and compared. A preliminary proposal for a mechanism involving pre‐dissociation of the acidic proton on the ligand is discussed.
Ring transformations and tele‐aminations in the reaction of 2‐halogeno‐ and 8‐halogeno‐1,7‐naphthyridines by potassium amide in liquid ammonia
Authors: van der Plas H.C. , Wózniak M. , van Veldhuizen A.
Published: September 2010

2‐X‐1,7‐Naphthyridines (X = Br, Cl) give a variety of products on treatment with potassium amide, namely 2‐amino‐1,7‐naphthyridine [formed by an SN(AE) process], 4‐amino‐ and 8‐amino‐1,7‐naphthyridine (obtained by a tele‐amination[5N(AE)t]) and a ring transformed product, 2‐methyl‐4‐amino‐1,3,7‐triazanaphthalene. By 1H‐NMR spectroscopy of a solution of 2‐chloro‐1,7‐naphthyridine and its 8‐deutero‐ and 6,8‐dideutero derivatives in KNH2/NH3 good evidence has been obtained that at −40° 2‐chloro‐1,7‐naphthyridine is completely converted into a mixture of three 1 : 1 adducts formed by addition of the amide ion at C(8), C(6) and C(4). 8‐X‐1,7‐Naphthyridines (X = Cl, Br) give as sole reaction products 8‐amino‐1,7‐naphthyridine [5N(AE)] and 2‐amino‐1,7‐naphthyridine [5N(AE)t]. By 1H‐NMR spectroscopy strong evidence has been obtained that in 8‐X‐1,7‐naphthyridines the adduct formation occurs exclusively at C(2). The mechanism of the tele‐amination is discussed.
Stereospecific synthesis of mono‐ and di‐substituted 1‐alkenyl sulfides, phosphines and tin compounds.: A new application of substituted vinylcopper(I) intermediates. Part IV
Authors: Westmijze H. , Meijer J. , Vermeer P.
Published: September 2010

A new stereospecific synthesis for substituted 1‐alkenyl sulfides, phosphines and tin compounds RR′C=CHX (X = −SMe, −PPh2, −SnPh3) is presented, starting from stereospecifically generated vinylcuprates [RR′C=CHCu(Br, I or R)]MgX′ (X′ = Cl or Br) and MeSSO2Me, Cl−PPh2 and Cl−SnPPh3, respectively, using tetrahydrofuran as solvent.

Iodovinylcuprates [RR′CCHCu−I]MgX′ are obtained by selective methylation of the R‐radical in the mixed homocuprates [RR′CCHCu−R]MgX′ with 1 equivalent of Me−I at −50°.
C‐Nitroso compounds. Part XXX. The mechanism of photolysis of chloro nitroso compounds; irradiation of 2‐chloro‐2‐nitrosoadamantane in methanol
Authors: Kayen A. H. M. , de Boer Th. J.
Published: September 2010

The photolysis of gem‐chloronitrosoalkanes 1 in methanol has been studied with the model compound 2‐chloro‐2‐nitrosoadamantane (AdClNO) 1a. The mechanism is outlined in Scheme 1, and involves initial C‐NO bond cleavage. The generated chloroalkyl radicals 2 are captured by unchanged starting material giving the key intermediate, a 1,1′‐dichloro nitroxide 3. Although this process is probably reversible in inert solvents, nitroxide 3 is irreversibly removed in alcohols by conversion into a N‐(1‐chloroalkyl)nitrone 4, which is solvolysed to an equimolar mixture of an acetal and an oxime. During photolysis no ESR signals are observed. Nevertheless, the transient existence of chloro nitroxides 3 is supported by the detection of di‐sec‐alkylnitroxides 5, which are formed when a photolyzed solution of a chloro nitroso compound 1 is treated with tri‐n‐butylstannane (reaction [7]). The mechanism is further supported by photolysis of AdClNO in the presence of nitrosobenzene in aprotic solvents and isolation of the relatively stable hydrochloride of the nitrone: phenyladamantylideneamine oxide 6a.
A new synthesis of 3‐alken‐1‐ynes and 1‐methoxy‐1,3‐butadienes from cumulenic ethers and organocopper(I) compounds
Authors: Kleijn H. , Meijer J. , Westmijze H. , Vermeer P.
Published: September 2010

Cumulenic ethers of the type R′R″C=C=C=CHOMe 1 (R′, R″ = H, H; H, Me or Me, Me) are readily convertible into 3‐alken‐1‐ynes R′R″C=CR−C≡C−H 2 (R = alkyl or Ph) upon treatment with Grignard reagents, RMgX, in the presence of small amounts of CuBr. Interaction of substrates 1 with homocuprates [RCuR]MgX results generally in mixtures of 2 and 1‐methoxy‐1,3‐butadienes R′R″C=CRCH=CHOMe 3, whereas the adducts 3 are formed exclusively if heterocuprates [RCuBr]M (M = Li or MgX) are used.
Unexpected formation and crystal structure of 3‐(4‐toluenesulfonyl)‐2‐(4‐toluenesulfonylimino)‐1,3‐thiazetidine
Authors: Selling H.A. , Verweij A.B. , Piepenbroek A. , Kiers C. Th.
Published: September 2010

Reaction of N‐(4‐toluenesulfonyl)dithiocarbamic acid with methylene iodide, in the presence of base in DMSO solution has not yielded the expected (4‐toluenesulfonyl)‐1,3‐dithietane. Instead in low yield 3‐(4‐toluenesulfonyl)‐2‐(4‐toluenesulfonylimino)‐1,3‐thiazetidine has been isolated. The structure of the compound is deduced from elemental analysis and spectral data and confirmed by X‐ray diffraction.
Flash vapour‐phase pyrolysis of thiophene 1,1‐dioxides
Authors: van Tilborg W.J.M. , Plomp R.
Published: September 2010

Flash vapour‐phase pyrolysis1of 2,5‐ or 2,4‐dialkyl‐, 2,5‐diphenyl‐ and dibenzothiophene 1,1‐dioxides at temperatures above 800°C/10−2mm Hg results in the formation of the corresponding furans. Thiophenes are obtained as co‐products if stabilized cyclobutadiene‐type intermediates can occur, especially at lower temperatures (<700°C) and/or higher pressure (10−1mm Hg). Both tetraphenylthiophene 1,1‐dioxide and tetraphenylcyclopentadienone yield diphenylacetylene as the main product. Benzothiete is obtained from the pyrolysis of benzothiophene 1,1‐dioxide. A general reaction scheme, starting from one common intermediate, is proposed which interrelates the formation of the aforementioned products. On the basis of this scheme the seemingly unrelated results reported in the literature can be easily explained.
A13C‐NMR study of squalene. Part II. Functionalized squalene‐like compounds
Authors: van Dommelen Marianne E. , van de Ven Leo J. M. , Buck Henk M. , de Haan Jan W.
Published: September 2010

13C‐NMR chemical shifts of substituted squalenes and model compounds with a heteroatom in δ‐position to a double bond show that a steric interaction exists between the hetero‐atom and the double bond. This points to the presence of “precoiled” conformations in the hetero‐substituted squalenes and also in squalene itself. The nature of the interaction remains veiled, but it appears to be a general feature as is shown in a series of 2‐ and 3‐substituted thiophenes with a double bond in the side chain.
Reverse‐phase high‐performance liquid chromatography of protected peptides in the sequential synthesis of secretin and analogues
Authors: Bakkum J. T. M. , Beyerman H. C. , Hoogerhout P. , Olieman C. , Voskamp D.
Published: September 2010

The synthesis of the gastrointestinal hormone secretin, a heptacosapeptide, and analogues by the Repetitive Excess Mixed Anhydride (REMA) method was monitored by reverse‐phase high‐performance liquid chromatography (HPLC). All intermediate protected peptides were thus purified on a micro‐preparative scale (100‐200 μg, ≥ 20 nmole).

The method allowed the separation of the protected diastereoisomeric [L‐Ala4]‐ and [D‐Ala4]‐des‐His1‐secretin.

An aspartyl‐glycyl rearrangement could be detected in the protected pentacosa‐, hexacosa‐, and heptacosa‐peptide (secretin).
Pyrromethene‐BF2 complexes (4,4′‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacenes). Synthesis and luminescence properties
Authors: de Wael E. Vos , Pardoen J. A. , van Koeveringe J. A. , Lugtenburg J.
Published: September 2010

BF2 complexes of alkylated pyrromethenes were prepared by reacting the corresponding pyrromethenes with BF3·OEt2 under basic conditions. A striking aspect of these complexes is a pronounced fluorescence in organic solvents, even at room temperature, with fluorescence quantum yields between 0.3‐0.8. The efficiency and wavelength maximum of the fluorescence seems to be correlated with the substitution pattern at the 2 and 6 position. In none of these systems could phosphorescence be detected.
Evaluation of potentiometric data of weak polyelectrolytes taking account of nearest‐neighbour interaction
Authors: van Treslong C. J. Bloys
Published: September 2010

For a number of weak polyelectrolytes potentiometric titration data are interpreted on the basis of the dependence of the pK on the degree of ionization σ by considering the separate contributions for a) the mean electrostatic potential field, b) interactions between neighbouring functional groups, c) the distribution of the functional groups over the macromolecule, d) specific conformational aspects and branching. For the various polyelectrolytes this procedure leads at low ionic strength to a satisfactory picture of the charging process. However, on increasing the ionic strength definite discrepancies between the two weak polybases with strong neighbour interactions, poly(ethylene‐imine) and poly(vinylamine), and the other polyelectrolytes are found. The very strong dependence of pK on σ in these two cases even at high ionic strength can only be explained by concluding that the electrostatic potential at the site of reaction in these molecules is much higher than expected on the basis of additivity of the various specific effects.
Distribution of counterions in solutions of weak polyelectrolytes A study to the effects of neighbour interactions between charged sites and the structure of the macromolecule
Authors: van Treslong C. J. Bloys , Moonen P.
Published: September 2010

Counterion distributing coefficients obtained from isopiestic and electromotive force experiments are presented for solutions of poly(acrylic acid), poly(vinylamine) and poly(ethyleneimine) with and without added salt. The observations lead to the conclusion that a number of factors determining the dependence of the dissociation equilibrium of the functional groups on the degree of ionization, do not affect or scarcely affect the counterion condensation. The very strong counterion binding by the branched poly(ethyleneimine) in comparison with linear polyelectrolytes indicates that the macromolecular structure is of great importance. The values found for the counterion distribution coefficient in salt‐free solutions of both linear polyelectrolytes are compared with values calculated from the theories of Lifson‐Katchalsky and Manning. The “additivity rules” for polyelectrolyte‐salt mixtures as formulated by Lifson‐Katchalsky are not obeyed.
Preparation, properties and X‐ray structure of trimethylantimony(V). bis(N,N‐dimethyldithiocarbamate)
Authors: Cras J. A. , Willemse J.
Published: September 2010

The preparation of an antimony(V) dithiocarbamato compound Me3Sb(Me2dtc)2 is reported. The X‐ray structure of this compound revealed an orthorhombic cell, space group Cmc21, with a = 7.448(4), b = 18.071(7) and c = 12.475(9) Å, which contains 4 formula units. The atomic parameters were refined to a conventional R value of 0.06. The structure can be described as a slightly deformed trigonal bipyramid, with the three methyl groups in equatorial position. Except for two of these methyl groups all non‐hydrogen atoms are positioned in the mirror plane Oyz. The stabilisation of the oxidation state + 5, which is very uncommon for antimony sulfur compounds is due to the σ‐electron‐donating methyl groups.
Furo‐ and thieno[2,3‐b]pyrazines. Part 1. Synthesis of 2‐substituted derivatives
Authors: Timmermans P. B. M. W. M. , Kruse C. G. , van der Gen A.
Published: September 2010

A method is described for the preparation of 2‐substituted furo[2,3‐b]pyrazines (1) and thieno[2,3‐b]pyrazines (2) by ring closure of the 1,4‐diketones 7 (X = Y = O) and 1,4‐oxothiones 7 (X,Y = O,S). The requisite (thio)carbonyl compounds were obtained by reaction of esters and thiono esters with lithium salts of dianions, prepared from 3‐methyl‐2‐pyrazone (3, X = O) and 3‐methyl‐2‐pyrazinethione (3, X = S), respectively. The scope of these condensation and cyclisation reactions is discussed.
A 13C‐NMR study on the phenyllithium‐diazine adducts and the corresponding dihydrodiazines
Authors: van der Stoel R. E. , van der Plas H. C.
Published: September 2010

Pyrimidine, 4‐phenylpyrimidine, pyridazine, 3‐phenylpyridazine and pyrazine were treated with phenyllithium in ether, yielding adducts resulting from attack of phenyllithium on the various positions of the heteroaromatic ring. With pyrimidine addition took place mainly at C(4), with pyridazine at C(3). By using TMEDA, addition at C(2) in 4‐phenylpyrimidine and at C(4) in pyridazine was strongly promoted. The structure of the adducts was studied by NMR spectroscopy. The charge distribution pattern in the C(4)‐adduct of pyrimidine (2a) and in both the C(3)‐adduct (10a) and C(4)‐adduct (12a) of pyridazine was determined by comparing the carbon chemical shifts of these compounds with those of the corresponding dihydrodiazines obtained by hydrolysis of the adducts. C(5) in 2a was found to carry little negative charge. C(4) in 10a was shown to have a considerable amount of charge while the electron density at C(6) in 10a and both C(3) and C(5) in 12a was moderate.
Préparation et propriétés de cyclobutanethiones
Authors: Paquer D. , Vazeux M. , Leriverend P.
Published: September 2010

Five substituted cyclobutanethiones were prepared from the corresponding cyclobutanones by sulfurization with hydrogen sulfide, in the presence of hydrogen chloride at 0°C. Spectroscopic data (UV, MS, 1H and 13C NMR) of these new compounds were determined and discussed.
The synthesis of δ‐substituted δ‐lactones
Authors: Peterse A. J. G. M. , de Groot Ae. , van Leeuwen P. M. , Penners N. H. G. , Koning B. H.
Published: September 2010

The addition of LiCH2COOEt to (n‐butylthiomethylene)cyclohexanones (1) gives after hydrolysis the ethyl 2‐formylcyclohex‐1‐eneacetates 4. With organolithium reagents these formyl esters can be cyclised to the 1‐substituted hexahydro‐3H‐2‐benzopyran‐3‐ones 5. After saponification of 4 an acid‐catalysed cyclisation gives the tetrahydro‐3H‐2‐benzopyran‐3‐ones 12.
Derivatives of meta‐di‐tert‐butylbenzene. Part V. The dissociation constants of 2‐halogeno‐3,5‐di‐tert‐butylanilines and the rates of deacetylation of the corresponding acetanilides
Authors: de Koning Adrianus Jan
Published: September 2010

The dissociation constants of 2‐chloro‐, 2‐bromo‐ and 2‐iodo‐3,5‐di‐tert‐butylanilines have been compared with those of 2‐chloro‐, 2‐bromo‐ and 2‐iodoaniline. The influence of the ortho‐halogen atom on the basicity of the anilines is not significantly affected by the buttressing tert‐butyl group. The rates of deacetylation of the corresponding acetanilides are discussed. It appears that the effective size of the ortho‐substituent markedly influences the reaction rate.
Furo‐ and thieno[2,3‐b]pyrazines. Part 2. Chemical properties of 2‐substituted derivatives
Authors: Kruse C. G. , Timmermans P. B. M. W. M. , van der Laken C. , van der Gen A.
Published: September 2010

The reactivity of 2‐substituted furo[2,3‐b]pyrazines (1a‐c) and thieno[2,3‐b]pyrazines (2a,b) towards several electrophilic reagents is described.

Although the furan (thiophene) ring is strongly deactivated by the annelated pyrazine ring, 3‐halogenated derivatives could be obtained in excellent yields. By contrast, nitration occurred mainly in the phenyl side‐chain. Reaction with peracids easily provided the N7‐monoxides and, in the case of the thienopyrazines, also the N4, N7‐dioxide as well as a N7,S,S‐trioxide could be obtained. A mechanism is suggested for the formation of both 5‐ and 6‐chlorinated products from reaction of N7‐monoxides with phosphoryl chloride. The 3‐lithio derivative of 1a underwent ring opening upon protonation at −60°C to provide the pyrazone‐substituted acetylene 25a.
The nature of the SS bonds in different compounds. Part III. The crystal and molecular structures of S‐p‐tolyl p‐toluenethiosulfinate and S,S‐diphenyl dithiosulfite
Authors: Kiers C. Th , Vos Aafje
Published: September 2010

The crystal structures of S‐p‐tolyl p‐toluenethiosulfinate (1) and S,S‐diphenyl dithiosulfite (2) have been determined by X‐ray diffraction. Observed (average) values for the bonds involving a sulfur atoms are for (1): S‐S = 2.108(2), S‐O = 1.457(5), S‐C = 1.776(5) Å, and for (2): S‐S = 2.132(2), S‐O = 1.476(4), S‐C = 1.775(4) Å.
On the calculation and the use of gas chromatographic retention indices of perfluoro‐dideutero alkylated peptide derivatives
Authors: van de Graaf B. , Schuyl P. J. W. , Beyerman H. C. , Knol‐Kalkman A.
Published: September 2010

The calculation of gas chromatographic retention indices of the title compounds according to Nau and Biemann can be simplified if more accurate retention index increments assigned to the amino acid residues are used. The increments can be calculated effectively from experimental retention indices using standard least squares procedures. It is proposed to implement such a procedure in a computer program for the identification of the peptide derivatives.
Cannabis XVII: Pyrolysis of cannabidiol. Structure elucidation of two pyrolytic conversion products
Authors: Spronck H. J. W. , Salemink C. A.
Published: September 2010

Pyrolysis of cannabidiol in a nitrogen atmosphere, which was shown to be a good model for the smoking process, yields a mixture of compounds. Next to the previously described major rearrangement product 2 we have now isolated and identified two more compounds formed by rearrangement processes: 2‐[2‐(p‐tolyl)propyl]olivetol (3) and 4‐(3,4‐trans‐p‐mentha‐1,8‐dien‐3‐yl)olivetol (1b).
Potential carcinostatics: Synthesis and properties of potential inhibitors of the adenylosuccinate synthetase and adenylosuccinate lyase systems
Authors: Wanner M. J. , Hageman J. J. M. , Koomen G. J. , Pandit U. K.
Published: September 2010

Treatment of 6‐chloro‐9‐(tetrahydro‐2‐pyranyl)purine (8) with 2 equivalents of methylene triphenylphosphorane (−30°) yields the corresponding purinyl ylide 9. Reaction of the latter with ethyl glyoxylate gives the expected acrylic ester derivative 10. Ethyl (dimethylsulfuranylidene)acetate adds to the acrylic ester moiety to form a mixture of isomeric purinyl substituted cyclopropanedicarboxylic esters (11a and 12a). The ylide 9 reacts with cis‐dimethyl epoxysuccinate (13) to give the oxaphospholane derivative 14, which undergoes ring opening (DMSO, 80°) to 6‐[2,3‐(diethoxycarbonyl)‐1‐propenyl]‐9‐(tetrahydro‐2‐pyranyl)purine (15). Hydrogenation (H2/Pd) of 15 leads to the corresponding propyl derivative 17; while its treatment with base (NaH/DME) produces the tricyclic product 16. The significance of purinyl derivatives 16 and 17 (R = H) as potential inhibitors in the biosynthesis of adenosine monophosphate (AMP, 3) is discussed.
The chemistry of small ring compounds. Part 41: Methylenethiirane, an isomer of cyclopropanethione
Authors: Jongejan E. , Buys Th. S. V. , Steinberg H. , de Boer Th. J.
Published: September 2010

The cyclic carbonate 3, prepared from bis(1‐hydroxycyclopropyl) sulfide and phosgene, has been thermolysed at 500° and 0.5 mm, yielding methylenethiirane (4), an isomer of cyclopropanethione. The structure of 4 is supported by mass spectrometry, 1H‐ and 13C‐NMR and infrared (in vapour phase and in solution). Attempted reactions with methyl fluorosulfate, diazomethane, triphenylphosphine and 1,3‐diphenylisobenzofuran all lead to polymeric material of uncertain structure.
1,3,4,6‐Tetraketones. Part XVIII. Reaction of unsymmetrical 1,3,4,6‐tetraketones with lead tetraacetate: Oxidative cyclization and rearrangement into dehydroacetic acid analogues
Authors: Poje M. , Gašpert B. , Balenović K.
Published: September 2010

Oxidation of l‐phenylheptane‐1,3,4,6‐tetrone (5) with lead tetraacetate gave the dehydroacetic acid analogues 6 and 7 in 3 : 1 ratio and no traces of dehydroacetic or dehydrobenzoylacetic acids (4a and 4b). Oxidation of an equimolar mixture of the symmetrical 1,3,4,6‐tetraketones 1a and 1b with lead tetra‐acetate gave only the compounds 4a and 4b. These results can be rationalized in terms of a mechanism involving the cyclic intermediates 9 and 10 in the oxidation of 1,3,4,6‐tetraketones with lead tetraacetate.
A Model synthesis of Ring B of Warburganal and Muzigadial
Authors: Peterse A. J. G. M. , Roskam J. H. , de Groot Ae.
Published: September 2010

The synthesis of 1,2‐diformyl‐6,6‐dimethyl‐2‐cyclohexen‐1‐o1 from 2,2‐dimethylcyclohexanone is described.
A convenient synthesis of L‐Nτ‐methylhistidine and L‐Nτ‐ethylhistidine (Imidazole chemistry, Part IV)
Authors: Noordam A. , Maat L. , Beyerman H. C.
Published: September 2010

Interaction of L‐histidine methyl ester (2) with N,N′‐carbonyldiimidazole gave near‐quantitative ring closure between the Nα and the Nπ function to form (7S)‐5,6,7,8‐tetrahydro‐7‐(methoxycarbonyl)‐5‐oxoimidazo[1,5‐c]pyrimidine (3). Alkylation of 3, e.g. with an alkyl halide, trialkyloxonium tetrafluoroborate or dialkyl sulfate, is then only possible at the “Nτ function” to form (7S)‐2‐alkyl‐5,6,7,8‐tetrahydro‐7‐(methoxycarbonyl)‐5‐oxoimidazo[1,5‐c]pyrimidinium salt (4). Hydrolysis of 4, aided by hydrochloric acid, afforded L‐Nτ‐alkylhistidines in high yield. The L‐Nτ‐methylhistidine (5a) was found to be identical with the costly natural amino acid and optically pure.
Addition of aldehydes to cyclooctyne under the influence of boron trifluoride. 2‐Alkylidenecyclooctanones
Authors: Verboom W. , Bos H. J. T.
Published: September 2010

Addition of aliphatic and aromatic aldehydes 4 to cyclooctyne 5 under the influence of boron trifluoride produces (E)‐2‐alkylidenecyclooctanones 6 in good yields.
Protonation as a driving force for pentacoordination of phosphorus
Authors: van Aken D. , Castelijns A. M. C. F. , Buck H. M. , Verkade J. G.
Published: September 2010

CNDO/2 calculations are reported on four caged phosphorus compounds, Y‐P(OCH2CH2)3X (X = N, CH; Y = O, OH+, H+). Structures with and without a transannular bond between P and X were taken into account; the influence of this bond on the stability of the compounds was examined. For X = N and Y = H+ the lowest‐energy structure corresponds with the geometry found by X‐ray analysis. If X = CH, formation of a transannular bond proves to be unlikely, as expected. The calculations predict the formation of a P‐N bond upon protonation of the phosphate (X = N, Y = O). The net atomic charges in the protonated phosphite were used to calculate its NMR coupling constant, 1JPH, which correlates well with the value found experimentally.
Biomimetic polyene cyclizations. Synthesis of racemic 19‐norprogesterone
Authors: Johnson William S. , Huffman William F. , Boots Sharon G.
Published: September 2010

The aim of this study was to apply biomimetic cyclization strategy to a total synthesis of racemic 19‐norprogesterone (8). Treatment of enone 1 with lithium aluminium hydride in ether afforded trienynol 2 which was cyclized with trifluoroacetic acid in methylene chloride containing ethylene carbonate to give ketone 4 as a 76/24 mixture of the 17β/17α epimers in 52% yield. Conversion of 4 to DL‐19‐norprogesterone (8) was effected in 68% yield by ozonolysis (to give 6) followed by cyclodehydration.
A convenient synthesis of human calcitonin mainly via the repetitive excess mixed anhydride method
Authors: Hirt J. , Kranenburg P. , Beyerman H.C.
Published: September 2010

The synthesis is described of the sequence containing 32 amino acid residues proposed for human calcitonin (HCT), presumably a thyroid hormone (Fig. 1)4.

Our strategy employed the coupling of four fragments: 1–7 (with closed disulfide bridge), 8‐10, 11‐19 and 20‐32 via the azide or dicyclohexylcarbodiimide (DCCI) method. With the exception of two DCCI couplings (Asn3 and Asn17), these fragments were prepared exclusively by stepwise synthesis via the repetitive excess mixed anhydride (REMA) méthod (Fig. 2).

Removal in one operation of all protecting groups with cold concentrated hydrochloric acid and chromatographic purification afforded HCT with full hypocalcemic potency (Table 1).
Synthesis of 1,7‐difunctionalized cycloheptatrienes. Complications from rapid [1,7] sigmatropic shifts of unhindered alkylthio groups
Authors: Cavazza M. , Morganti G. , Pietra F.
Published: September 2010

4,5‐Benzotropone and 1,2‐ethanedithiol gave the expected dithioketal 8 with boron trifluoride etherate in methanol. In contrast, tropone, under similar conditions, gave the bicyclic 1,7‐disubstituted cycloheptatrienes 2, 3 and 4 on reaction with, respectively, 1,2‐ethanedithiol, 1,3‐propanedithiol, and o‐benzenedithiol. Related, monocyclic 1,7‐difunctionalized cycloheptatrienes 10–13 and 15a‐d were obtained from sodium thiolates and methoxy‐ or 1‐(alkylthio)tropenylium salts in dichloromethane‐ethanol at − 50°C. 1,7‐Bis(alkylthio)‐substituted cycloheptatrienes with unhindered alkylthio groups showed a rapid interconversion of the alkylthio groups at the C(1) and C(7) carbons (15a ⇄ 16).
Photochemistry and photophysics of a polycyclic azoalkane. Solvent and temperature effects
Authors: Turro Nicholas J. , Ramamurthy V.
Published: September 2010

Direct photochemical excitation of the polycyclic azoalkane 1 yields a mixture of (CH)6 isomers as major products at room temperature and the 1,2‐diaza‐cyclooctatetraene 2 as the major product at − 78°C. The ratio of (CH)6 isomers to 2 is strongly dependent on solvent. For example, at room temperature the ratio (CH)6/2 is 92/8 in pentane and 38/62 in dimethyl sulfoxide. Triplet photosensitized excitation of 1 yields 2 as the major product. The triplet reaction is insensitive to temperature and solvent. These results are consistent with reaction of S1(1) to yield predominately (CH)6 isomers via a thermally activated nitrogen extrusion, whereas T1(1) yields predominantly 2 via a temperature and solvent independent rearrangement. Quantitative measurement of photochemical and photoluminescence parameters reveals that the solvent effect on the product distribution in the direct photolysis of 1 is due to a combination of the influence of solvent on the rate of reaction from S1 and the rate of intersystem crossing from S1.
Aromatic lead(IV) compounds XI. 207Pb as a probe for proton assignment in PMR spectra of aryllead(IV) compounds
Authors: de Vos D. , van Barneveld W.A.A. , van Beelen D.C. , Wolters J.
Published: September 2010

A method of assigning PMR spectra of aryllead(IV) compounds by means of the broadly separated signals of satellites associated with the occurrence of the magnetic 207Pb species in all natural lead, is developed and thoroughly verified for series of phenyllead(IV) compounds. The utility of the method is demonstrated by the analysis of the PMR spectra of several series of substituted aryllead(IV) compounds. For this purpose 44 compounds, of which 27 are new, have been synthesized.
A novel synthetic approach to mitomycins. One step synthesis of a 5,10‐dioxo‐1H‐pyrrolo[1,2‐a]benz[f]indole
Authors: Geevers J. , Visser G.W. , Reinhoudt D.N.
Published: September 2010

A new method for the synthesis of pyrrolo[1,2‐a]indoles, starting from enamines via 1,5‐dipolar cyclization is described. The method is considered to be useful for the synthesis of mitomycin type of compounds.
Gas‐liquid chromatography/mass spectrometry identification of previtamin D3 and vitamin D3 in the skin of vitamin D‐deficient rats irradiated with ultraviolet light
Authors: Okano T. , Mizuno K. , Matsuyama N. , Nobuhara N. , Kobayashi T.
Published: September 2010

The existence of previtamin D3 and vitamin D3 in the skin of vitamin D‐deficient rats irradiated with ultraviolet (UV) light has been physico‐chemically confirmed by gas‐liquid chromatography/mass spectrometry (GC/MS). The lipid extracts obtained from the UV irradiated rats' skins were purified to isolate the previtamin D3 and vitamin D3 fractions. When the isolated fractions were individually applied to GC/MS equipped with units of electron impact (EI) and chemical ionization (CI) as energy sources and mass fragmentography, the existence of previtamin D3 and vitamin D3 in the corresponding fractions was clearly identified. The experimental results showed that the endogenous 7‐dehydrocholesterol (7‐DHC) in the skin of the rat was photochemically converted to previtamin D3 in the same manner as in a test tube by UV irradiation and then the in vivo generated previtamin might be biothermally isomerized to vitamin D3 at unknown places before the physiological effects would be apparent.
Ether cleavage of isomeric 2,3‐(dimethoxycarbonyl)‐7‐oxabicyclo[2.2.1]heptanes
Authors: Eggelte T. A. , de Koning H. , Huisman H. O.
Published: September 2010

The ether cleavage of the three isomeric 2,3‐(dimethoxycarbonyl)‐7‐oxabicyclo[2.2.1]‐heptanes with acetic anhydride under influence of BF3 etherate or FeCl3 has been studied. A complex mixture of products was obtained, resulting from elimination, migration and substitution reactions. The results are interpreted in terms of reactions occurring via an acyloxonium ion and not via a ringopened carbenium ion intermediate.
Zur photoinduzierten Spaltung α,β‐ungesättigter γ,δ‐Epoxy‐Ester II: Der Einfluss einer Methyliden‐Gruppe in ϵ‐Stellung
Authors: Frei B. , Schweizer W. B. , Wolf H. R. , Jeger O.
Published: September 2010

Photoinduced cleavage of α,β‐unsaturated γ,δ‐epoxyester II: the effect of a methylidene group in ϵ‐position. On direct excitation (γ = 254 nm, pentane) 4 undergoes mainly (60%) cyclo‐elimination to give the isomeric products 7–13 and to a smaller extent (7%) 4 yields the Z‐isomer 6. On sensitization by acetone (γ > 280 nm) 4 gives the isomeric cyclobutanone 16 (22%) or isomerizes to 6 (13%).

The structures of compounds 11, 13 and 16 were established by X‐ray analysis: 11: a = 7.114, b = 29.191, c = 7.784 Å, β = 68.33°, P21/c, Z = 4; 13: a = 10.732, b = 6.777, c = 19.254 Å, β = 105.39°, P21c, Z = 4; 16: a = 9.176, b = 11.115, e = 13.375 Å, β = 76.36°, P21/n, Z = 4.

The allene 7 and the cyclopropene 8 are products of a carbenoid species e, whereas the cyclopropenes 9 and 10 are formed via another carbenoid species d. The spirocyclic cyclopropenes 11 and 12 as well as the tricyclic cyclopropane compound 13 are shown to be photoproducts of 9 and 10.

The comparison of the results of the photolysis of 4 and 21b discloses the influence of an ϵ‐methylidene substituent on the photochemistry of the α,β‐unsaturated γ,δ‐epoxy chromophore: (a) on direct excitation selective photocleavage of the C(γ)‐O bond analogous to 2 → a is suppressed in favour of cycloelimination reactions of type 4 → d and 4 → e. In the absence of an ϵ‐methylidene substituent cycloelimination of type 4 → d does not occur; (b) on T‐sensitization photocleavage of type 2 → a is replaced by cleavage of the C(δ)‐O bond (4 → f → 16).

The photochemistry of the epoxy‐ene ester 4 on direct excitation is analogous to the photochemistry of the corresponding epoxyenone 32 on π,π* excitation. From the results of the photosensitization experiments evidence is obtained for the singlet multiplicity of the cycloelimination processes of 3 and 4.
The ring inversion in cis‐cisoid‐cis‐transoid‐cis‐perhydronaphthacene
Authors: Vanhee P. , de Pessemier F. , Anteunis M. , Tavernier D.
Published: September 2010

The barrier (ΔG≠) to ring inversion in the title compound amounts to 56.7 kJ·mol−1 at 299 K. The inversion mechanism is discussed. The barrier was measured by dynamic 13C NMR spectroscopy. The 13C shifts of the title compound have been assigned.
A new synthesis of 4‐OR‐3‐penten‐1‐ynes (C5‐fragment) as a tool for the preparation of vitamin A
Authors: Brouwer M. S. , Hulkenberg A. , Kok J. G. J. , van Moorselaar R. , Overbeek W. R. M. , Wesselman P. G. J.
Published: September 2010

In our technical synthesis of vitamin A, the intermediate β‐C18‐ketone (5) is obtained in a four‐step sequence from β‐ionone (1). In order to reduce the number of reaction steps, 1 is condensed with a C5‐fragment, leading to a convergent synthesis of 5. The title compound (8) with R = ‐CH(OC2H5)‐CH3 (8i) is prepared by the reaction of 1,3‐dichloro‐2‐propanol with ethyl vinyl ether, followed by dehydrohalogenation, substitution and isomerisation of the resulting mixed acetal (17i) in a simple one‐pot procedure by treatment with sodium acetylide. The syntheses of several C5‐synthons of type 8 with different groups R are described. The condensation product of 1 and 8i is partially hydrogenated and subsequently hydrolysed to 5 in an overall yield of 85%.
Studies on the metabolism of polyphosphates in yeast cells. I. Polyacrylamide gel electrophoresis of polyphosphates
Authors: Booij H. L. , Beekes Henny
Published: September 2010

A preliminary study was made of the usefulness of a gel electrophoresis method of polyphosphates of different chain lengths. Pre‐electrophoresis of the gels appeared to be of primary importance to get rid of substances which interfere with the detection of polyphosphates. Four detection methods have been studied, each showing its own advantages and drawbacks. Determination of the chain lengths of unknown polyphosphates is possible as there exists a linear relation between the logarithm of the chain length and the migration distance. However, added electrolytes restrict the use of this relationship. They decrease the migration velocity of the polyphosphates and alter the numerical coefficient occurring in the equation.
A new method for the determination of the dipole moment of an electronic excited state: The π of cyclohexanone and the 1La of indole
Authors: Ramos Joaquim Jose Moura , Stien Marie‐Louise , Reisse Jacques
Published: September 2010

In this work, a new method for the determination of a Franck‐Condon excited state moment in solution is described. This method is based both on the study of the solvatochromic effect and on the use of Onsager reaction field theory. The method we propose requires (1) calorimetric measurements of the heat of transfer from solvent S1 to solvent S2 for the solute in its ground state and for a homomorph; (2) the absorption frequencies of the solute in the two solvents and (3) experimental determination of the solute and homomorph ground state dipole moments.

It is easy to show that a very simple relationship exists between these various experimental quantities if the two solvents are such that the reaction field theory can be applied to describe the dipolar part of the solvent solute interactions. The use of the homomorph method permits one to take into account the variation of the cavity term from solvent to solvent and the variation of the solute solvent dispersive energy term with the nature of the solvent. Moreover, the method does not requires neither the estimation of the cavity radius nor the dangerous assumption that the dipole is centred in the cavity. This new method, which compared to older methods, must give more reliable results, is applied to the π* state of cyclohexanone and to the 1La state of indole.
Photochemistry in solution. Photoreactivity of 2,3‐dihydro‐4H‐pyran derivatives
Authors: Chaquin Patrick , Furth Bernard , Kossanyi Jean
Published: September 2010

Substituted 2‐acyl‐2,3‐dihydro‐4H‐pyrans 1–5 and 7, when irradiated at 254 nm, undergo ring contraction to form 1,2‐diacylcyclobutanes and cleavage by retro‐Diels‐Alder reaction through the common intermediate 34. Excitation at 313 nm of the carbonyl 1(n,π*) state of compounds 1–5 leads to the unsaturated acetals 8–12 while the benzoyl derivative 6 gives only polymers and the aldehyde 7 undergoes decarbonylation. The same cyclisation occurs with 4‐acetylcyclohexene 28. This reaction which leads stereoselectively to the isomer in which the substituent R is exo, originates from the 1(n,π*) singlet state as demonstrated by quenching experiments. Interaction of this chromophore with the carbon‐carbon double bond is demonstrated by comparing the olefinic compounds with that of the corresponding saturated analogues.
Radical cation states of 2,3,5,6‐tetramethylenenorbornane, 2,3,5,6‐tetramethylenebicyclo[2.2.2]octane and of related compounds
Authors: Mohraz Manijeh , Batich Christopher , Heilbronner Edgar , Vogel Pierre , Carrupt Pierre‐Alain
Published: September 2010

The He(Iα) photoelectron spectra of 2,3,5,6‐tetramethylenebicyclo[2.2.1]heptane (1), 2,3,5,6‐tetramethylene‐7‐oxabicyclo[2.2.1]heptane (2), 2,3,5,6‐tetramethylenebicyclo[2.2.2]octane (3) and 5,6,7,8‐tetramethylenebicyclo[2.2.2]oct‐2‐ene (4) are presented. The assignments for the highest 4 or 5 π‐bands are made on the basis of LCBO and SPINDO calculations for compounds 1, 3 and 4. The results confirm the spectroscopic assignment of Klessinger et al. for the lower members of the series.

Finally, the localized orbitals derived from SPINDO results, are used to discuss the “through‐bond” and “through‐space” interaction pattern in molecules 1, 3 and 4.
Synthetic applications of 2‐chlorotetrahydrofuran: Protection of alcohols as tetrahydro‐2‐furanyl (THF) ethers
Authors: Kruse C. G. , Jonkers F. L. , Dert V. , van der Gen A.
Published: September 2010

2‐Chlorotetrahydrofuran (2‐Cl‐THF) is formed in 85% yield by reaction of sulfuryl chloride with excess THF. 2‐Cl‐THF reacts with alcohols, phenols, acids, thiols, azoles, amides and imides to give the corresponding tetrahydro‐2‐furanyl derivatives. Reaction with uracil derivatives provides N1‐ and N3‐monosubstituted and N1,N3‐disubstituted compounds. The reaction with alcohols provides a convenient general procedure for protecting the hydroxyl function. The THF group can be readily removed by acid catalyzed hydrolysis or reaction with methanol. The THF derivative of diphenylacetic acid can also be utilised for introducing the tetrahydro‐2‐furanyl moiety. THF ethers are converted into alkyl bromides by treatment with triphenylphosphine dibromide.
Vicinal carbon‐proton spin‐spin coupling constants in monosubstituted propanes
Authors: Spoormaker Tom , de Bie Marius J. A.
Published: September 2010

Analysis of single resonance 13C‐FT NMR spectra has yielded the vicinal 13C‐1H coupling constants in monosubstituted propanes for substituents from the first row of the Periodic Table. Directly bonded 13C‐1H, vicinal 1H‐1H and some directly bonded 13C‐13C coupling constants are also reported. The influence that the substituent electronegativity of the elements from the first row of the Periodic Table exerts on the vicinal 13C‐1H coupling constant is discussed qualitatively in terms of molecular orbital theory. The effects of substituents from the first row are compared with the effects of halogen substituents.
Stability constants of (hydroxy)carboxylate‐ and alditol‐calcium(II) complexes in aqueous medium as determined by a solubility method
Authors: Kieboom A. P. G. , Buurmans H. M. A. , van Leeuwen L. K. , van Benschop H. J.
Published: September 2010

Solubility measurements of calcium(II) sulfate in aqueous solutions of (hydroxy)carboxylates and alditols have yielded apparent stability constants (M−1, at 25° and 0.2–0.8 M) for the complexes of calcium(II) with d‐gluconate (23), d‐lactate (17), acetate (7), and various alditols (0–1.4).
Preparation and properties of (dimethyldithiocarbamato)iodomethylarsenic(III) and its iodine complex; Crystal structure of MeAs(Me2NCS2)I2
Authors: Beurskens Gezina , Beurskens Paul T. , Noordik J. H. , Willemse J. , Cras J. A.
Published: September 2010

The preparation of an arsenic(III) dithiocarbamato iodo compound, MeAs(Me2dtc)I, and of its iodine complex, MeAs(Me2dtc)I2, is reported. The crystal structure of the iodine complex was determined by an X‐ray analysis. The monoclinic cell, space group P21/c, with a = 12.838(3), b = 8.374(1), c = 22.944(2) Å and β = 105.44(1)°, contains eight formula units. The two symmetry independent units are arranged in a nearly perfect C‐centred lattice. The phase problem was solved by the DIRDIF procedure (direct methods, including origin fixation, applied to difference structure factors). The atomic parameters were refined to a conventional R value of 0.07.

The structure consists of I2IAsII2IAs chains, cross‐linked by IAsIAs chains to form two‐dimensional networks. The arsenic atoms are in quadrilaterally pyramidal coordination: As is slightly above the S,S,I,I‐plane; the methyl carbon forms the top of the pyramid. This methyl group breaks through the pseudo‐C‐centred translation symmetry.
A formalism for the classification and design of organic reactions III. The class of (+ ‐ )nC reactions
Authors: Arens J. F.
Published: September 2010

This paper deals with reactions involving an odd number of atoms. The bonds between the atoms alternatingly increase or decrease by one unit but one of the bonds remains unaffected. The reaction operator in terms of the formalism employed in the two previous papers thus is (+ ‐)nC in which C indicates the Constancy of the last mentioned bond.

Many synthetically important processes belong to this class, among others acid‐ or base‐induced eliminations, ditto additions, isomerisations and substitutions, dipolar cycloadditions, fragmentations and tele‐reactions.
Sulfenylated cyclic β‐keto esters and cyclic β‐diketones; synthesis and reactions with nucleophiles
Authors: de Groot Ae. , Jansen B. J. M.
Published: September 2010

Quenching the enolates of cyclic β‐diketones or 2‐oxocycloalkanecarboxylates with phenylsulfenyl chloride or methylsulfenyl chloride produces the corresponding x‐sulfenylated products. Nucleophilic attack on these x‐sulfenylated 1,3‐dicarbonyl compounds produces regiospecific sulfenylated linear dicarboxylic acids, acid esters, diesters or oxo esters suitable for further chemical transformation, this has been demonstrated in the synthesis of the queen substance of the honoy bee. In some cases recyclization of the intermediate enolates was observed to regiospecific sulfenylated cyclohexenones.
Solid compounds of dibromo(N,N‐di‐n‐butyldithiocarbamato)gold(III) with trans‐Stilbene, trans‐azobenzene and trans,trans‐1,4‐diphenyl‐1,3‐butadiene. Crystal structure determinations of [Br2AuS2CN(C4H9)2)2·C6H5(CH)2C6H5 and [Br2AuS2CN(C4H9)2]·C6H5(CH)4C6H5
Authors: Wijnhoven F. T H. H. M. , Bosman W. P. , Willemse J. , Cras J. A.
Published: September 2010

Br2Au(Bu2dtc) forms solid compounds with trans‐stilbene, trans‐azobenzene, trans,trans‐1,4‐diphenyl‐1,3‐butadiene, tolane and dibenzyl, of formula [Br2Au(Bu2dtc)]2·organic component. The crystal structures of the title compounds are solved. No specific interactions between the dithiocarbamato compound and the organic component are observed.
Analysis of cocaine, pseudococaine, allococaine and allopseudococaine by ion‐pair reverse‐phase high‐performance liquid chromatography
Authors: Olieman C. , Maat L. , Beyerman H. C.
Published: September 2010

The title compounds can be separated by liquid chromatography on a column of octadecylsilyl‐silica with the addition to the eluent of n‐heptanesulfonate and identified by peak area measurements at different UV wavelengths.
Relation entre la configuration et la tautomérie de formyl‐hydrazino‐5 as‐triazines
Authors: Daunis J. , Follet M. , Gelize M.
Published: September 2010

L'étude de la configuration E ou Z de formyl‐hydrazino as‐triazines, établie par RMN et par le calcul théorique, permet de déterminer la structure tautomère de ces composés: de type tétrahydro‐2,3,4,5 triazine ou dihydro‐2,5 triazine selon la nature des substituants.
A note on the hydrobromination of 1‐alkenes in the presence of zeolites
Authors: Wortel Th. M. , Rozendaal S. , van Bekkum H.
Published: September 2010

The zeolite‐catalyzed hydrobromination of 1‐alkenes was investigated. Contrary to literature data, normal (Markovnikov) addition of hydrogen bromide yielding 2‐bromoalkanes was observed if peroxide impurities were removed thoroughly from the reactants. Substantial amounts of 3‐ and 4‐bromoalkanes were formed due to zeolite‐catalyzed isomerization of the alkene.
Preliminary Communications: Chemistry of 2H‐thiopyran and derivatives V. Regiospecific methylation and protonation of the di‐anion, obtained from reductive cleavage of 2H‐thiopyran
Authors: Gräting R. , Brandsma L.
Published: September 2010

Reductive cleavage of 2H‐thiopyran 1 with naphthylsodium or sodium in THF or THF/HMPT mixtures, respectively, gives the di‐anion magnified image, which is protonated and methylated by the first equivalent of methyl iodide on the terminal carbon atom.
The enhancement by visible light of the catalytic activity of Co(II)‐tetrasulphophthalocyanine on the oxidation of mercapto‐ethanol
Authors: Beelen T. P. M. , Da Costa Gomez C. O. , Kuijer M.
Published: September 2010

The rate of the selective oxidation of mercaptoethanol by dioxygen to disulphide, catalysed by Co(II)‐tetrasulphophthalocyanine, is enhanced with a factor 2 à 4 by irradiation with light in the range of 600‐700 nm. This effect can be ascribed to the conversion of dimers of Co(II)‐tetrasulphophthalocyanine to catalytically active monomers.
The role of tetraalkylammonium salts in the electro‐reduction of ketones. Part II: The reduction of acetophenone in an aprotic environment
Authors: van Tilborg W. J. M. , Smit C. J. , van Santen R. A.
Published: September 2010

A polarographic study of the electro‐reduction of acetophenone in dry acetonitrile has revealed that under strictly anhydrous conditions the carbonyl compound can be reduced only if tetraalkylammonium ions are present. The role played by these ammonium ions is discussed. It is concluded that the catalytic effect of the ammonium ions has to be related to an increased polarization of the carbonyl double bond being induced during collision of the ammonium ions with the carbonyl compound. At high ammonium ion concentrations, however, the effect of this catalysis becomes smaller due to preferential adsorption of the ammonium ions on the electrode surface, as a result of which the carbonyl compound is displaced from the direct vicinity of the electrode.
Formation of γ‐lactones by boron trifluoride‐catalysed ring closure of quinoid γ‐oxo‐α,β‐unsaturated carboxamides. Structure and reaction mechanism
Authors: Verboom W. , Bos H.J.T.
Published: September 2010

Treatment of the quinoid N,N‐dialkyl‐α‐(3,5di‐tert‐butyl‐2‐oxo‐3,5‐cyclohexadienylidene)‐α‐phenylacetamides 6 with boron trifluoride etherate generally gives rise to rearrangement into the γ‐lactones 3‐dialkylamino‐5,7‐di‐tert‐butyl‐3‐phenyl‐2,3‐dihydrobenzo[b]furan‐2‐ones 11. In the case of the carboxamide 7, N,N‐diethyl‐α‐(3,5‐di‐tert‐butyl‐6‐hydroxyphenyl)‐α‐oxoacetamide 14 is formed. The quinoid N,N‐diethyl‐α‐methylthio‐α‐(10,oxo‐9‐phenanthrenylidene)acetamide 8 is converted by boron trifluoride etherate into the γ‐lactone 3,3‐bis(methylthio)‐2,3‐dihydrophenanthro‐[9,10‐b]furan‐2‐one 22 and phenanthro[9,10‐b]furan‐2,3‐dione 23. In the presence of alcohols, intermediate carbocations are generally trapped; non‐fully aromatic phenanthrene derivatives are sometimes formed.
Chemistry of electron‐rich conjugated polyenes V. Regio‐ and stereo‐selectivity in Diels‐Alder reactions of 1‐alkoxy‐1,3‐butadienes and β‐cyanostyrenes
Authors: Broekhuis A. A. , Scheeren J. W. , Nivard R. J. F.
Published: September 2010

Effects of substituents in diene and dienophile on the regio‐ and stereo‐selectivity of Diels‐Alder reactions have been studied on cycloadditions of alkoxy‐substituted 1,3‐butadienes (1) and 2‐substituted cyanoethenes (2). The regioselectivity is always very high, and is determined by the magnitude of the orbital coefficients of the interacting carbon atoms according to the frontier orbital theory. The stereoselectivity is more determined by steric factors than by secondary orbital overlap. The results point to a transition state, in which the planes of diene and dienophile, are non‐parallel (Figure 1).