bluuebunny/crossref_metadata_2025 huggingface dataset:
Zur Synthese von Derivaten des Pyrido[1,2‐a]azepins
Authors: Maicr Wolfram , Eberbach Wolfgang , Fritz Hans
Published: December 2004

On the Synthesis of Derivatives of Pyrido[1,2‐a]lazepine

The conjugated pyridinium ylides of type 4 undergo 8π ‐electrocyclisation affording the heterobicyclic allenes 5 which, in the presence of H2O and H2O2, are subsequently transformed into the substituted pyrido[1,2‐a]‐azepinones 6, a novel heterocyclic system. The chemical behaviour of 6a,b is dominated by cleavage of the lactam bond and subsequent reaction to pyridinc and isoquinoline derivatives, respectively; e.g. reaction of 6a, b with 4,5‐dihydro‐4‐methyl‐3H‐1,2,4‐triazole‐3,5‐dione affords the cycloadducts 11 and 12 almost quantitatively. The successful transformation of 6b into 15 is described, the latter being the first representative of the presently unknown, potentially antiaromatic pyrido[1,2‐a]azepine (1).
Crystal Structures of the Lithium Cryptates of Two Macrobicyclic Ligands Containing Pyridine, Bipyridine, and Biisoquinoline Units
Authors: Cesario Michèle , Guilhem Jean , Pascard Claudine , Anklam Elke , Lehn Jean‐Marie , Pietraszkiewicz Marek
Published: December 2004

The crystal structures of the LiBr complexes of two macrobicyclic ligands, [py.py.bpy]1 and [py.biqi.biqi] 2, have been determined. They confirm the cryptate nature of these species. In 3, the Li+ cation is bound in the shallow cavity of a ligand of flattened shape, whereas in 4, it is buried inside the macrobicycle and surrounded by its three bridges.
Quantitative Formation in Water of [Ru(CO)(H2O)5]2+ from Hexaaquaruthenium(II) and Carbon Monoxide
Authors: Laurenczy Gábor , Helm Lothar , Ludi Andreas , Merbach André E.
Published: December 2004

High‐pressure 17O‐ and 13C‐NMR show that [Ru(H2O)6]2+ reacts quantitatively with carbon monoxide (50 bar) in water to form [Ru(CO)(H2O)5]2+.
The Synthesis of 1,2,4‐Trioxan‐5‐ones
Authors: Jefford Charles W. , Currie Jane , Richardson Geoffrey D. , Rossier Jean‐Claude
Published: December 2004

Several 3,6‐substituted 1,2,4‐trioxan‐5‐ones have been prepared in good yield by condensing aldehydes and ketones with trimethylsilyl α‐[(trimethylsilyl)peroxy]alkanoates in the presence of trimethylsilyl trifluoromethane sulfonate as catalyst.
Lagotoside: A New Phenylpropanoid Glycoside from Lagotis stolonifera
Authors: Çaliş İhsan , Taşdemir Deniz , Wright Anthony D. , Sticher Otto
Published: December 2004

From the aerial parts of Lagotis stolonifera (Scrophulariaccae), a new phenylpropanoid glycoside, lagotoside (8), and the three known glycosides ehrenoside (5), verbascoside (= acteoside; 6), and plantamajoside (7) were isolated, together with the four known iridoid glucosides aucubin (1), catalpol(2), globularin (4), and lythantosalin (3). The structure of the new compound 8 was elucidated on the basis of chemical and spectral data as 2‐(3‐hy‐droxy‐4‐methoxyphenyl)ethyl O‐[α‐L‐arabinopyranosyl‐(1 → 2)]‐O‐[α‐L‐rhamnopyranosyl‐(1 → 3)]‐4‐O‐feruloyl‐β‐D‐glucopyranoside.
Madeiranes, a New Class of Pentacyclic Triterpenes: D‐Friedo‐madeir‐14‐en‐3β‐ol and ‐3‐one, D:C‐Friedomadeir‐7‐en‐3β‐ol and ‐3‐one
Authors: Ferreira Maria‐José Umbelino , Lobo Ana M. , O'Mahoney Caroline A. , Williams David J. , Wyler Hugo
Published: December 2004

The pentacyclic triterpenes 1a, b and 2a, b are isolated from extracts of Euphorbia mellifera Arr. Their structures, determined by X‐ray analysis, are supported in part by MS, NMR, and IR data. The skeleton differs from that of representatives of the lupane or hopane class by (i) an unusual arrangement of the substituents in the cyclopentane ring E, the Me group being on C(19) and the i‐Pr group in an angular position on C(17), and (ii) a cis D/E ring junction. We propose to name Die unknown pentacyclic parent ‘madeirane’, thus 1a, b and 2a, b are D‐friedomadeir‐14‐ene and D:C‐friedomadeir‐7‐ene derivatives. The biosynthesis of these compounds may be rationalized via ‘spirodammaranol’‐cation intermediates 5 and 5′ and madeiranol cation 6, as outlined in Scheme 2.
On the Stability of the Schiff Bases of Pyridoxal 5′‐Phosphate with Polypeptides Containing L‐Lysine
Authors: del Vado M. Angeles García , Echevarria Gerardo R. , Blanco Francisco García , Blanco José G. Santos , Vallejo José Laynez , de Paz José L. García
Published: December 2004

We determined the apparent equilibrium constant of formation, KpH, of the Schiff bases of pyridoxal 5′‐phosphate (PLP) and poly‐ and copolymers containing L‐lysine, as a function of pH at 25° and a constant ionic strength of 0.1 M. The KpH values obtained at acidic and neutral pH were larger that those reported for Schiff bases of PLP and hexylamine. We determined calorimetrically ΔH of formation of Schiff bases of PLP and poly(L‐lysine) (−4.5′kcal/mol), and PLP and hexylamine (−3.4 kcal/mol) at pH 7.00. Semi‐empirical theoretical calculations (INDO and AMI methods) of a model compound of Schiff base of PLP and polypeptide containing L‐lysine show the capability of specific interactions between groups of PLP and the peptide skeleton.
Stereochemical Studies on Homoterpene Biosynthesis in Higher Plants; Mechanistic, Phylogenetic, and Ecological Aspects
Authors: Gäbler Andreas , Boland Wilhelm , Preiss Ute , Simon Helmut
Published: December 2004

In higher plants, the two homotcrpenes 4,8‐dimethylnona‐l,3,7‐triene (1) and 4,8,12‐trimethyltrideca‐1,3,7,11‐teiracnc (2) originate from nerolidol (3) or geranyllinalool (4) by anoxidative cleavage of their C‐skele‐lons. The reaction proceeds with exclusive loss of H8–C(5) of 3 and formal production of a C4 fragment. The site specificity of the enzyme(s) is identical for all of the hitherto examined plant families (Agavaceae, Asclepiadaceae, Asteraceae, Leguminosae, Magnoliaceae, and Saxifragaceae). The enzyme tolerates a wide range of structural modifications at the polar head of 3. Instead of 3, also gcranylacclone 12 and the secondary alcohol 13 can be cleaved to the homoterpene 1 and as yet unidentified carbonyl fragments. The CC bonds within the aliphatic chain of 3 seem to be essential for the oxidative bond cleavage as well as for recognition and embedding of the substrate into the active center of the enzyme(s). The feed‐induced biosynthesis of 1 and 2 in leaves of the Lima bean Phaseolus lunatm infested with the spider mite Tetranychus urticae probably requires a preceding release of nerolidol (3) or geranyllinalool (4) from phylogenic glycosides prior to the fragmentation reaction. The microbial reduction of the trienoic acids 6 and 6ais the key stop for the synthesis of deuterium labelled nerolidol (3RS,5R)‐and (3RS,5S)‐9.
3J(H,H) Coupling Constants as a Simple Criterion for π Delocalization of Pentafulvenes and Pentafulvalenes
Authors: Neuenschwander Markus , Bönzli Peter
Published: December 2004

A simple criterion for estimating the extent of π delocalization in the five‐membered ring of pentafulvenes and pentafulvalenes is described. It is based on the fact that changes of bond lengths (induced by exocyclic substituents R1‐R2 of 1) are reflected by systematic changes of 3J(H,H) values, so that linear correlations of σ vs, 3J(H,H)are obtained. Plots of that type (Fig. 1) are very useful for determining the extent of π delocalization of various pentafulvalenes 2–5 (Fig. 3) which show a very similar behavior to pentafulvenes. In principle, these plots could additionally be used for estimating substituent constants σ or for approximating the extent of π overlap between exocyclic substituents and the π system of pentafulvenes. Charge‐density effects of pentafulvenes and pentafulvalenes are observed by substituen‐induced shifts of the ring C‐atoms (Fig. 4).
Thiocyclosporins: Preparation, Solution and Crystal Structure, and Immunosuppressive Activity
Authors: Seebach Dieter , Ko Soo Y. , Kessler Horst , Köck Matthias , Reggelin Michael , Schmieder Peter , Walkinshaw Malcolm D. , Bölsterli J. Jakob , Bevec Dorian
Published: December 2004

The reaction of cyclosporin A (CsA) withLawesson's reagent under different conditions yields various thiocyclosporins, in which carbonyl O‐atoms and/or the hydroxy O‐atom of the MeBmt residue are replaced by an S‐atom. The position of the S‐atom is determined by NMR spectroscopy, and the conformations of the products are studied by NMR spectroscopy and X‐ray crystallography. Some of the thiocyclosporins show interesting conformational properties. Whereas one conformation strongly dominates for CsA in CDCL3, two conformers A and B, in a ratio 58:42 are found for [1ψ2, CS–NH]CsA. Extensive NMR studies including new 2D and 3D heteronuclear techniques and restrained MD calculations using ROE effects demonstrate that the major conformer A is identical to CsA, while the minor conformer B contains an additionalcispeptide bond between the Sar3and MeLeu4residues. [4ψ5, CSNH;7ψ8, CS–NH]CsA exhibits a conformation very similar to crystalline CsA. However, the D‐Ala8NH, MeLeu6Co γ‐turn H‐bond is not present in this dithio analogue. Also different is the MeBmt1side‐chain conformation, the dithio conformation showing a strong MeBmt1OH, Sar3COH‐bond. Immunosuppressive activities of thiocyclosporins are measured in IL‐2 and IL‐8 reporter gene assays. Their activities are discussed in relation to their conformations.
Convenient Synthesis of 2‐Azido‐2‐deoxy‐aldoses by Diazo Transfer
Authors: Vasella Andrea , Witzig Christian , Chiara Jose‐Luis , Martin‐Lomas Manuel
Published: December 2004

Diazo transfer from trifluoromethanesulfonyl azide (TfN3) to 2‐amino‐2‐deoxy‐glycoses constitutes a high‐yielding, simple procedure for the preparation of partially protected or unprotected 2‐azido‐2‐deoxy‐aldoses. Thus, the D‐allosamine derivative 2 gave 93% of 3, while diazo transfer to D‐glucosamine, D‐mannosamine, and D‐galactosamine, followed by acetylation, yielded the azides 5, 7, and 9 in yields of 74–91, 65, and 70%, respectively.
Warum Pentose‐ und nicht Hexose‐Nucleinsäuren?? Teil I. Einleitung und Problemstellung, Konformationsanalyse für Oligonucleotid‐Ketten aus 2′,3′‐Dideoxyglucopyranosyl‐Bausteinen (‘Homo‐DNS’) sowie Betrachtungen zur Konformation von A‐ und B‐DNS
Authors: Eschenmoser Albert , Dobler Max
Published: December 2004

Why Pentose and Not Hexose Nucleic Acids? Part I.
Introduction to the Problem, Conformational Analysis of Oligonucleotide Single Strands Containing 2′,3′‐Dideoxyglucopyranosyl Building Blocks (‘Homo‐DNA’), and Reflections on the Conformation of A‐ and B‐DNA

Summary in collaboration with Prof. Dr. C.E. Wintner, Haverford College, Haverford, PA 19041‐1392; academic guest, ETH, March and June/July, 1991.

Chemical rationalization of the structure of a biomolecule can be sought through consideration of two criteria: first, the relationship between the structure and its biological function; and second, the structure's potential for constitutional self assembly. The latter criterion convers the judgment, by chemical reasoning, of the chance of its preformation, that is, a synthetic event which must have been undergone by any molecular structure in order to have been selected (or to have selected itself) to become a biomolecule. One way to further the task of rationalizing a biomolecule's structure by experimental means is the design, synthesis, and study of structural alternatives which might have become biomolecules on the basis of either criterion, but which do not, in fact, appear in Nature today.

In the formation of sugar phosphates from glycolaldehyde phosphate under basic conditions, straightforward and selective formation of rac‐hexose 2,4,6‐triphosphates is observed in the absence of formaldehyde, while rac‐pentose 2,4‐diphosphates are dominant, when (0.5 equiv.) formaldehyde is present [1]. This and other observations indicate that hexose sugars should be regarded to have had a chance of preformation comparable with that of pentose sugars. Why, then, did Nature choose pentoses and not hexoses as the sugar building blocks of nucleic acids? The reason must be functional; it must be the case that pentose nucleic acids are biologically superior to potential hexose alternatives. To the extent that biological function is a consequence of molecular structure and reactivity, the origin of this superiority should be decipherable through chemical experiment, that is, through synthesis of hexose nucleic acids, systematic study of their chemical properties, and comparison of these properties with those of their natural counterparts. This has been the object of the present investigation, initiated in 1986. The paper introduces a series of papers which will describe the results of a model study, namely, the synthesis, pairing properties and structure of homo‐DNA oligonucleotides.
The Synthesis of a Novel Epoxycyclohexane from the Fungus Eutypa lata (Pers: F.) TUL
Authors: Defrancq Eric , Gordon John , Brodard André , Tabacchi Raffaele
Published: December 2004

The synthesis of the novel (1′R*,2′S*,3′R*, 4′S*,5′R*)‐1‐(3′,4′‐epxoy‐2′, 5′‐dihydroxycyclohexyl)‐3‐methyl‐but‐2‐enone (2), recently isolated from the culture medium of the fungus Eutypa lata, is described.
On the unusual propensity by the red seaweed Laurencia microcladia of II Rogiolo to form C15 oxepanes: Isolation of rogioloxepane A, B, C, and their likely biogenetic acyclic precursor, prerogioloxepane
Authors: Guella Graziano , Mancini Ines , Chiasera Giuseppe , Pietra Francesco
Published: December 2004

It is shown here that the red seaweed Laurencia microcladia KÜTZNING, collected in the Mediterranean off the torrent Il rogiolo, contains the C15 acetogenin rogioloxepane A ( = (+)‐(2R,7R)‐2‐(1‐bromopropyl)‐7‐[(Z)‐1‐chlorohex‐3‐en‐5‐ynyl]‐2,3,6,7‐tetrahydrooxepin; (+)‐3) besides its epoxy derivative rogioloxepane B ((+)‐4) and the rogioloxepane C ((+)‐5) derived from (+)‐4 by epoxide opening. Co‐occurring is an acyclic C15 acetogenin, perrogioloxepane ( = (7R,4Z,9Z,12Z)‐6‐chloropentadeca‐3,9,12‐trien‐1‐yn‐7‐ol; 9), which exists preferentially in a folded conformation as required to give, by intramolecular cyclization, rogioloxepane A (3TC) in its favored twist‐chair form. Molecular‐mechanics calculations support this view and help also to assign the relative configurations at the side chains, whereas the configurations at the seven‐membered ring are established via NMR spectroscopy and chemical correlations. The absolute configuration is established by Mosher's NMR method applied to the MTPA esters of (+)‐5 or 9.
Synthesis and Enzyme‐Inhibition Studies of Phenylsemicarbazones derived from D‐glucono‐1,5‐lactone and 2‐acetamido‐2‐deoxy‐D‐glucono‐1,5‐lactone
Authors: Wolk Diane R. , Vasella Andrea , Schweikart Fritz , Peter Martin G.
Published: December 2004

The carbohydrate‐derived lactone phenylsemicarbazones 3 and 4 were prepared from 5 and 8 (Scheme). Treatment with 4‐phenylsemicarbazide gave 6 and 7 (77:23) and 9 and 19 (76:24), respectively. Oxidation of 6 and 9 by CrO3–pyridine to 11 and 13, followed by deprotection, yeilded 3 and 4. The structure of 3 was established by X‐ray analysis. Enzyme‐inhibition studies using revealed that 3 is a competitive inhibitor with Ki = 23 μm. The activity of 4 was examined using N‐acetylglucosaminidase from bovine kidney, Aspergillus niger, and Artemia salina. compound 4 was found to be a competitive inhibitor of all three enzymes with Ki values of 0.13, 6.0, and 0.71 μm and KM/Ki values of 6910, 45, and 465, respectively.
Radical Anions of Cyclic Azoalkanes: An ESR, ENDOR, and TRIPLE‐Resonance Study
Authors: Ess Caspar H. , Gerson Fabian , Adam Waldemar
Published: December 2004

Radical anions of ten monocyclic and bicyclic azoalkanes containing the azo group in (Z)‐conformation, have been fully characterized by their hyperfine data with the use of ESR, ENDOR, and general‐TRIPLE‐resonance spectroscopy. These azoalkanes are represented by 3,3,5,5‐tetramethyl‐1‐pyrazline (1), 2,3‐diazabicyclo[2.2.1]hept‐2‐ene (4), and 2,3‐diazabicyclo[2.2.2]oct‐2‐ene (9), as well as by their derivatives 2, 3, 5–8, and 10. For all radical anions \documentclass{article}\pagestyle{empty}\begin{document}$1^{- \atop \dot{}}-10^{- \atop \dot{}}$\end{document}, the 14N‐coupling constant, aN, is in the range of +0.83 to +0.97 mT; this finding indicates that the spin population is essentially restricted to the π system of the azo group. The 14N‐hyperfine anisotropy largely affects the width of ESR lines, particularly at low temperatures. Substantial coupling constants of 7Li‐, 23K‐, and 133Cs‐nuclei point to a close association of the radical anions with their alkakimetal counterions. With the exception of 39K, these nuclei give rise to readily observable ENDOR signals which appear along with those stemming from protons. The prominent hyperfine features of \documentclass{article}\pagestyle{empty}\begin{document}$1^{- \atop \dot{}}-10^{- \atop \dot{}}$\end{document} are discussed.
Reduction of Aldehydes and Ketones by Transition Metal Hydrides. Part 2. Reaction of trans,trans‐[WH(CO)2(NO)(PMe3)2] with pyridine, functionalized aldehydes, and ketones
Authors: Van Der Zeijden Adolphus A. H. , Berke Heinz
Published: December 2004

The reaction of trans,trans‐[WH(CO)2(NO)(PMe3)2] (1) with (pyridin‐2‐yl)‐substituted aldehydes and ketones, (pyridin‐2‐yl)C(O)R where R = H, Me, Ph, pyridin‐2‐yl, and with 6‐methylpyridine‐2‐carbaldehyde was studied. In all cases, facile insertion of the CO bond into the WH bond was observed, with rapid subsequent extrusionof a coordinated CO ligand affording O,N‐bidentate coordinated tungsten alkoxides. Only in case of pyridine‐2‐carbaldehyde and di(pyridin‐2‐yl) ketone, the initial n1 O‐bonded insertion product could be observed as unstable intemediates by low‐temperature NMR.
Synthesis of 3‐Adenyl‐ and 3‐Thyminylcyclobutane‐1,1‐dimethanols and Their Homo‐octameric Phosphodiesters
Authors: Henlin Jean‐Michel , Rink Hans , Spieser Erich , Baschang Gerhard
Published: December 2004

Starting from 3‐(benzyloxy)cyclobutane‐1,1‐dimethanol, 3‐thyminyl‐ and 3‐adenylcyclobutane‐1,1‐dimethanol were synthesized by direct introduction of the heterocycles (Scheme 1). The mono‐O‐substituted methoxytrityl derivatives were separately converted to octameric phosphodiesters on an aminomethyl‐polystyrene carrier by the phosphotriester method. These oligomers of carba‐nucleosides were prepared in order to study their annealing behaviour towards ribo‐ and deoxyribo‐nucleic acids as well as their potential for homologous hybridization.
Isolation of Toxic and Potentially Toxic Sesqui‐ and Monoterpenes from the Tropical Green Seaweed Caulerpa taxifolia Which Has Invaded the Region of Cap Martin and Monaco
Authors: Guerriero Antonio , Meinesz Alexandre , D'Ambrosio Michele , Pietra Francesco
Published: December 2004

The green seaweed Caulerpa taxifolia (VAHL) C. AGARDH (Caulerpales), which, after its recent accidental introduction, is growing in the region of Cap Martin much more vigorously than in the tropics, is shown to contain the known sesquiterpenic toxins caulerpenyne (1) – in larger amounts than in tropical Caulerpales – and oxytoxin 1 (2). Novel, potentially toxic products isolated in small amounts from this seaweed include the sesquiterpenes taxifolial A ( = (5E)‐6,10‐dimethyl‐2‐[(E)2‐oxoethylidene]undeca‐5,9‐dien‐7‐ yne‐1,3‐diyl diacetate; 3), taxifolial B (= (1E,6E,10E)‐3‐[(Z)‐acetoxymethylidene]‐7, 11‐dimethyl‐12‐oxododeca‐1,6,10‐trien‐8‐yne‐1,4‐diyl diacetate; 4), 10,11‐epoxycaulerpenyne ( = (1E,6E)‐3‐[(Z)‐acetoxymethylidene]‐10,11‐epoxy‐7, 11‐dimethyldodeca‐1,6‐dien‐8‐yne‐1,4‐diyl diacetate; 1:1 diastereoisomer mixture; 5), and taxifolial C ( = (2Z,6E)‐3‐formyl‐7,11‐dimethyldodeca‐2,6,10‐trien‐8‐yne‐1,1, 4‐triyl triacetate; 6), besides, as the first example of a monoterpene from the Caulerpales, taxifolial D ( = (2Z)‐3,7‐dimethylocta‐2, 6‐dien‐4‐ynal; 7).
Nitrostyrene Derivatives of Adenosine 5′‐Glutarates as Selective Inhibitors of the Epidermal Growth Factor Receptor Protein Tyrosine Kinase
Authors: Peterli Stefan , Stumpf Regina , Schweizer Marc , Séquin Urs , Mett Helmut , Traxler Peter
Published: December 2004

The syntheses and biological activities of some nitrostyrene derivatives of adenosine 5′‐glutarates, a novel class of selective, bi‐substrate‐type inhibitors of the EGF receptor protein tyrosine kinase with IC50 values around 1 μM. Only marginal inhibition of the tyrosine kinases v‐able and c‐src and of the serine/threonine kinase PKC was observed. Compounds 8, 9, 11, and 12 – lacking the adenosine moiety – were ten times less active than the most potent derivatives, whereas 17 – lacking the nitrostyryl part – showed no inhibitory activity at all. Most of the compounds showed potent antiproliferative activity against an EGF‐dependent mouse keratinocyte cell line.
Synthesis of Dipeptides via Addition of N‐(Nitroacetyl)amino‐Acid Derivatives to Michael Acceptors: Scope, Stereoselectivity, and Absolute Configuration
Authors: Thomas Achamma , Manjunatha Sulur G. , Rajappa Srinivasachari
Published: December 2004

Michael addition of N‐nitroacetyl derivatives 1 of proline esters using KF under phase‐transfer catalysis resulted in the formation of adducts 3–9 with chemical yields ranging from 40–90% (Scheme). Stereoselectivity of up to 51% was obtained on addition of benzyl N‐(nitroacetyl)‐L‐prolinate (1a). The absolute configuration at the newly created chiral centre was established in the case of 9 by carrying out a reductive acylation and comparing the product 10 with an authentic sample of ethyl N‐(O5‐ethyl N2‐acetyl‐L‐glutam‐1‐yl)‐L‐prolinate (L, L‐10).
Synthese von (6R, all‐E)‐Neoxanthin und verwandten Allen‐Carotinoiden
Authors: Baumeler Andreas , Eugster Conrad Hans
Published: December 2004

Synthesis of (6R, all‐E)‐Neoxanthin and Related Allenic Carotenoids

We present the first synthesis of enantiomerically pure neoxanthin (1) by a Wittig‐Horner condensation between the ylide from the novel diethyl 12′‐apo‐15, 15′‐didehydroviolaxanthin‐12′‐phosphonate (35) and the allenic C15‐aldehyde 31 (Scheme 4) via the crystalline 15, 15′‐didehydroneoxanthin (36; 70% yield). After partial hydrogenation of the triple bond of 36 and isomerisation of the (15Z)‐intermediate 37, neoxanthin (1) was obtained in good yield. Similar syntheses gave (15Z, 9′Z)‐neoxanthin (45; Scheme 5) and (9Z)‐15, 15′‐didehydroneoxanthin (47; Scheme 6). Comparison of the physical data of synthetic 1 with those of a freshly isolated sample of neoxanthin from the flowers of Trollius europaeus confirmed their identity. The unusually low melting point of 1 is caused by a very easy thermal isomerisation into a mixture of the neochromes 4 and 5 (Scheme 1). Such a thermal rearrangement is not observed with 15, 15′‐didehydroneoxanthin (36). To explain this, we assume a zwitterionic excited state of the allenic group that induces the rearrangement of the violaxanthin end group into the furanoid epoxide (Scheme 7).
Reaktionen des 4‐Bromo‐1,6‐methano[10]annulen‐3‐carbaldehyds: Synthese von 3,4‐heteroanellierten 1,6‐Methano[10]annulenen
Authors: Neidlein Richard , Schröder Günter
Published: December 2004

Reactions of 4‐Bromo‐1,6‐methano[10]annulene‐3‐carbaldehyde: Syntheses of 3,4‐Heteroanellated 1,6‐Methano[10]annulenes

Similarly to β‐chlorovinyl ketones, the title compound 1 reacts with dinucleophiles to give the 3,4‐heteroanellated 1,6‐methano[10]annulenes 2–4, 6, and 12–16, containing thiophene, pyrazole, pyrimidine, and thiazepine rings (Schemes 2 and 3). On reaction of 1 with tert‐butylhydrazin, an unusual Michael‐addition‐dehydrogenation sequence, resulting in 2,3‐anellation of a pyrazole ring (→5), is observed. Synthetic as well as some mechanistic aspects are briefly discussed.
Saponins with an Unusual Secoursene Skeleton from Sesamum alatum THONN.
Authors: Potterat Olivier , Hostettmann Kurt , Stoeckli‐Evans Helen , Saadou Mahamane
Published: December 2004

Three new saponins, alatoside A–C (4a, 5, and 6a, resp.), with a 18,19‐secours‐12‐ene skeleton were isolated from the aerial parts of Sesamum alatum THONN. (Pedaliaceae) by preparative liquid chromatography. Their structures were elucidated by spectroscopic methods, including X‐ray diffraction analysis, and by chemical degradation (acidic and enzymatic hydrolyses). In addition, verbascoside (1) and two cyclohexylethanol derivatives, rengyol (2a) and isorengyol (3a), were isolated and identified.
Diels‐Alder Reactions of [2.2] Paracyclophan‐1‐ene and [2.2] Paracyclophane‐1,9‐diene with 3,6‐Disubstituted 1,2,4,5‐Tetrazines
Authors: de Meijere Armin , König Burkhard
Published: December 2004

[2.2] Paracyclophan‐1‐ene (1) and [2.2] paracyclophane‐1,9‐diene (6) apparently act as dienophiles with inverse electron demand and smoothly react with dimethyl 1,2,4,5‐tetrazine‐3,6‐dicarboxylate (2a) at room temperature forming dihydropyridazine adducts, which are dehydrogenated to the pyridazino‐anellated [2.2] paracyclophanes 5a and 8a, respectively. The molecular structure of 5a is determined by X‐ray crystal‐structure analysis. Under more rigorous conditions, phenyl‐substituted derivatives 5b and 8b are obtained from 1 and 6, respectively, with 3,6‐diphenyl‐1,2,4,5‐tetrazine. Compounds 1 and 6 are less reactive dienophiles than other strained cyclic olefins as shown by kinetic measurements.
Coenzyme F430 from Methanogenic Bacteria: Mechanistic studies on the reductive cleavage of sulfonium ions catalyzed by F430 pentamethyl ester
Authors: Lin Shu‐Kun , Jaun Bernhard
Published: December 2004

Mechanistic questions regarding the reductive cleavage of sulfonium ions by the NiI form of coenzyme F430 pentamethyl ester (F430M) were addressed in a series of kinetic studies and isotope labeling experiments. In neat DMF, methane formation from dialkyl(methyl)sulfonium ions consistently showed a delay time of ca. 1 h. In the presence of excess propanethiol, no delay was observed and methane formation followed pseudo‐first‐order kinetics with a logarithmic dependence of the initial rate on the concentration of propanethiol. From the temperature dependence of the reaction rate, an estimate for the activation parameters of ΔH# = 49 kJ mol−1 and (apparent) ΔS# = –114 J K−1 mol−1 was derived. The observation of deuterium incorporation into methane from (CH3)2CHOD, but not from (CH3)2CDOH, indicates that the fourth H‐entity is introduced into CH4 as a proton, and that free CH3 radicals are not involved. In contrast to the reaction with the homogeneous one‐electron reductant sodium naphthalide, the F430M‐catalyzed reduction of mixed dialkyl(methyl)sulfonium ions showed a pronounced selectivity for the cleavage of MeS over that of alkyl‐S (alkyl ≠ Me) bonds. Mechanisms that are consistent with these results, as well as possible explanations for the time delay and the apparent highly negative entropy of activation, are discussed.
Glycosylidene Carbenes. Part 7. Neighbouring‐group participation by the 2‐benzyloxy group in the glycosidation of strongly acidic hydroxy compounds
Authors: Takahashi Yoshikazu , Vasella Andrea
Published: December 2004

To demonstrate the neighbouring‐group participation of the 2‐benzyloxy group in the glycosidation of phenols and of strongly acidic alcohols by the diazirine 1, we examined the glycosidation of 4‐nitrophenol, 4‐methoxyphenol, (CF3)2CHOH, MeOH, and i‐PrOH by the diazirine 11, derived from the 2‐deoxypyranose 6. Oxidation of the oximes 7 yielded (E)‐ and (Z)‐8. In solution, (E)‐8 isomerised to (Z)‐8. Similarly, the (E)‐configurated mesylate 9, prepared from 8, underwent acid‐catalysed isomerisation to (Z)‐9. Treatment of (Z)‐9 with NH3, followed by oxidation of the resulting diaziridine 10 with I2, yielded the desired diazirine 11. Glycosidation by 11 of the above mentioned hydroxy compounds yielded the glycosides 12–21. In agreement with the postulated neighbouring‐group participation, these glycosidation proceeded without, or with a very low diastereoselectivity, favouring the axial anomers.
Glycosylidene Carbenes. Part 8. Crystal Structure of a Cyclic, N‐Acetylallosamine‐Derived Alkoxy‐alkyl‐diazirine
Authors: Linden Anthony , Vasella Andrea , Witzig Christian
Published: December 2004

The crystal structure of 2‐acetamido‐1,5‐anhydro‐1‐azi‐3‐O‐benzyl‐4,6‐O‐benzylidene‐1,2‐ dideoxy‐D‐allitol (6) is reported, and the structural parameters of the diazirine moiety are compared with those of other free diazirines for which structural information has been determined or calculated previously. Lengthening of the NN bond and shortening of the CN bonds can be observed as the substituents at the C‐atom of the diazirine ring become more electronegative and/or stronger π‐electron donors. This is consistent with descriptions of the bonding nature of diazirines and similar observations made on substituted cyclopropanes.
Synthesis and Biological Evaluation of 2‐(2‐Deoxy‐β‐D‐ribofuranosyl)pyridine‐4‐carboxamide
Authors: Joos Pieter E. , Esmans Eddy L. , Dommisse Roger A. , Bruyn André De , Balzarini Jan M. , De Clercq Eric D.
Published: December 2004

A new protected 2‐deoxy‐D‐ribose derivative, 5‐O‐[(tert‐butyl)diphenylsilyl]‐2‐deoxy‐3,4‐O‐ isopropylidene‐aldehydo‐D‐ribose (5), was synthesized starting from 2‐deoxy‐D‐ribose. This compound was coupled with 2‐lithio‐4‐(4,5‐dihydro‐4,4‐dimethyloxazol‐2‐yl)pyridine giving a D/L‐glycero‐mixture 7 of 5‐O‐[(tert‐butyl)diphenylsilyl]‐2‐deoxy‐1‐C‐[4‐(4,5 ‐dihydro‐4,4‐dimethyloxazol‐2‐yl)pyridin‐2‐yl]‐3,4‐O‐isopropylidene‐ D‐erythro‐pentitol. The mixture 7 was 1‐O‐mesylated with methanesulfonyl chloride and subsequently treated with CF3COOH/H2O and ammonia to afford the α/β‐D‐anomers 10 of 2‐(2‐deoxy‐D‐ribofuranosyl)pyridine‐4‐carboxamide. Both anomers were purified and separated by HPLC and identified by NMR and DCI‐MS. Anomer β‐D‐10 was evaluated against a series of tumor‐cell lines and a variety of viral strains. No antitumor or antiviral activity was observed.
A New General Approach to Enantiomerically Pure Cyclic and Open‐Chain (R)‐ and (S)‐α,α‐Disubstituted α‐Amino Acids
Authors: Obrecht Daniel , Spiegler Clive , Schönholzer Peter , Müller Klaus , Heimgartner Heinz , Stierli Friedrich
Published: December 2004

A wide range of cyclic and open‐chain α,α‐disubstituted α‐amino acids 1a‐p were prepared. The racemic N‐acylated α,α‐disubstituted amino acids were resolved by coupling to chiral amines 15‐18 derived from (S)‐phenylalanine to form diastereoisomers 19/20 or 21/22 that could be separated by crystallization and/or flash chromatography on silica gel (Scheme 3). Selective cleavage via the 1,3‐oxazol‐5(4H)‐ones 10a‐p gave the corresponding optically pure α,α‐disubstituted amino‐acid derivatives 11 or 12 in high yield (Scheme 3). The absolute configurations of the α,α‐disubstituted amino acids were determined from X‐ray structures of the diastereoisomers 20, 21g′, 22d.
Strukturänderungen bei der Reduktion von 9‐(Diphenylmethyliden)fluoren mit Natrium‐Metall zu Schichten (R2O)Na⊕‐verknüpfter Kohlenwasserstoff‐Dianionen
Authors: Bock Hans , Ruppert Klaus , Herdtweck Eberhardt , Herrmann Wolfgang A.
Published: December 2004

Structural Changes on Reduction of 9‐(Diphenylmethylidene)fluorene by Sodium Metal to Sheets of (R2O)Na⊕‐ Connected Hydrocarbon Dianions

The single‐crystal structure of 9‐(diphenylmethylidene)fluorene (monoclinic, space group P21/n, Z = 4) proves a CC bond length of 135 pm and a twisting of the two Ph rings out of the methylidene‐fluorene plane by 70° and 83°. Its reduction in Et2O solution by a Na mirror under Ar yields dark green crystals with a metallic luster. Their structure, determined in a N2 flow cooled to 223 K (monoclinic, space group C2/c, Z = 4), reveals sheets of hydrocarbon dianions, connected by sodium(diethylether) links. The Na centers are sixfold coordinated as half‐sandwiches to the fluorene benzo rings and threefold allylic to the Ph rings. In the dianion of 9‐(diphenyl‐methylidene)fluorene, the central CC bond is stretched by 13 pm to 148 pm length, and the two molecular cyanine‐like halves are twisted by 51° relative to each other. Both the contact‐ion multiple structures as well as the structural changes on twofold reduction are discussed by way of comparison with literature examples and based on semiempirical calculations.
Syntheses of Novel Isopenam and Isocephem Antibiotics. Preparation of a retinamido derivative of a highly strained β‐lactam as potent anticancer agent
Authors: Hakimelahi Gholam H. , Shiao Min‐Jen , Hwu Jih Ru , Davari Hady
Published: December 2004

Syntheses of the cis‐configurated isopenam 9 (Scheme 1), isocephem 14 (Scheme 2), and isocephem 19 (Scheme 3) are described. The key step in the preparation of 14 and 19 involved a Pummerer‐type rearrangement of the corresponding sulfoxides 12 and 18. These β‐lactams were found to possess biological activity against several pathogenic microorganisms in vitro. The electronic activation of the lactam moiety of 19 remarkably enhanced its biological activity. A retinoic moiety was attached to 19 via an amino linker. The resultant retinamido‐β‐lactam 21 showed significant cytostatic activity in tracheal organ cultures obtained from vitamin‐A‐deficient hamsters.
Synthesis, Isolation, and NMR‐Spectroscopic Characterization of Fourteen (Z)‐Isomers of Lycopene and of Some Acetylenic Didehydro‐ and Tetradehydrolycopenes
Authors: Hengartner Urs , Bernhard Kurt , Meyer Karl , Englert Gerhard , Glinz Ernst
Published: December 2004

Eight (Z)‐isomers of lycopene were prepared by stereocontrolled syntheses and fully characterized by 1H‐NMR, 13C‐NMR, mass, and UV/VIS spectroscopy: (5Z)‐, (7Z)‐, (15Z)‐, (5Z,5′Z)‐, (7Z,7′Z)‐, (7Z,9Z)‐, (9Z,9′Z)‐, and (7Z,9Z,7′Z,9′Z)‐lycopene. Six additional (Z)‐isomers, namely (9Z)‐, (13Z)‐, (5Z,9′Z)‐, (9Z,13′Z)‐, (5Z,9Z,5′Z)‐, and (5Z,13Z,5′Z)‐lycopene, were isolated in small quantities from isomer mixtures by semiprep. HPLC and were identified by 1H‐NMR spectroscopy.
Synthesis and Structure of [4.5.5.5]Fenestranes ( = tetracyclo[5.4.1.04, 12.09, 12]dodecanes)
Authors: Hirschi Daniel , Luef Wolfgang , Gerber Peter , Keese Reinhart
Published: December 2004

The preparation and the X‐ray structure analysis of cis, trans, cis, cis‐10‐hydroxy‐[4.5.5.5]fenestrane‐1‐methanol (8b) is reported. The measured bond angles and bond lengths of the central C(C)4 fragment are better reproduced by calculations with the AM 1 than by the MNDO method.
Sydnones as Masked Hydrazines for Heterocycle Formation: Reactions of 3‐(2‐substituted phenyl)sydnones with HCl
Authors: Gelvin Christopher R. , Turnbull Kenneth
Published: December 2004

In general, reaction of 3‐(2‐substituted phenyl)sydnones with HCl gives products derived from cleavage of the sydnone ring to the corresponding hydrazine and subsequent cyclization to the side chain. In one case, 3‐(2‐aminophenyl)sydnone (43), the product obtained, l‐amino‐lH‐benzimidazole (47), apparently results from nucleophilic interception by the side chain prior to complete cleavage of the sydnone ring.
Asymmetric Synthesis of α‐Amino Acids and α‐N‐Hydroxyamino Acids from N‐Acylbornane‐10,2‐sultams: 1‐chloro‐1‐nitrosocyclohexane as a practical [NH] equivalent
Authors: Oppolzer Wolfgang , Tamura Osamu , Deerberg JöRg
Published: December 2004

Successive treatment of N‐acylsultams 3 with sodium hexamethyldisilazide, 1‐chloro‐1‐nitrosocyclohexane (1), and aq. HCl gave diastereoisomerically pure, crystalline N‐hydroxyamino‐acid derivatives 5. These were converted into various amino acids 7, N‐hydroxyamino acids 8, and an N‐Boc‐amino acid 9. (S, S)‐Isoleucine (17) and (S, S)‐2‐acetamido‐3‐phenylbutyric acid (23) were obtained from N‐crotonoylsultam 15 via 1,4‐addition of an organomagnesium or organocopper reagent followed by enolate ‘amination’ with 1.
Organometallic Derivatives of Cycloproparenes
Authors: Müller Paul , Bernardinelli Gérald , Jacquier Yvan
Published: December 2004

The tricarbonylchromium complex of cyclopropa[b]naphthalene (3a) is deprotonated at C(1) with BuLi. Quenching of the resulting anion with Mel results in a 1:1 mixture of stereoisomeric methyl derivatives 6a and 6b. Peterson olefination of the bis‐silylated complex 3b affords the complexed alkylidenecycloproparene 5 in low yield. The tricarbonylchromium complex of cyclopropa[b]anthracene (10) is accessible via bis‐hydro‐desilylation of 9. Its X‐ray structure is also reported. All attempts to synthesize a tricarbonylchromium complex of benzocyclopropene (11a) failed.
Vicinal 2H/1H Isotope Shifts for 6Li‐NMR and the Aggregation Behavior of Phenyllithium
Authors: Eppers Oswald , Günther Harald
Published: December 2004

Vicinal deuterium‐induced 6Li‐NMR isotope shifts have been found for (D5)phenyllithium. These shifts are to low field, and their magnitude varies from 7 to 19 ppb with different solvents and donor molecules. On the basis of the isotopic fingerprints observed in the 6Li‐NMR spectra of C6H56Li/C6D56Li mixtures, the aggregation behaviour of phenyllithium under different solvent conditions and in the presence of amines as donor ligands (TMEDA, PMDTA) was studied. Tetramers and dimers are observed in Et2O, dimers as well as momomers in Et2O/TMEDA and in THF, and momomers in THF/PMDTA. The hitherto unidentified species in Et2O/TMEDA was shown to be a monomer.
7‐Deazaguanosine: Synthesis of an oligorbonucleotide building block and disaggregation of the U‐G‐G‐G‐G‐U G4 structure by the modified base
Authors: Seela Frank , Mersmann Karin
Published: December 2004

Building blocks derived from 7‐deazaguanosine (c7G, 1) were prepared for solid‐phase oligoribonucleotide synthesis. Compound 1 was converted into the isobutyurl derivative 2b and the (dimethylamino)methylidene compound 3 (Scheme 1). After tritylation (→ 4a, b), silylation was studied with regard to regioselectivity. It was found that the triisopropylsilyl group in combination with the (dimethylamino)methylidene residue gave the highest 2′ ‐selectivity (→ 5e). The 2′ ‐O ‐silyl derivative 5e was reacted with PCl3 affording the 3′ ‐phosphonate 7 which was used in solid‐phase oligoribonucleotide synthesis. Oligonucleotides derived from U‐G‐G‐G‐G‐U with an increasing number of c7G residues instead of G were synthesized. Aggregation was studied by polyacrylamidegel electrophoresis and CD Spectroscopy. Disaggregation of the G4‐structure of U‐G‐G‐G‐G‐U was observed when c7G replaced G, demonstrating that guanine N(7) participates in the aggregation process.
The Conformation of Medium‐Sized Organosilicon Heterocycles: First evidence for the existence of a boat‐boat conformation in a 12H‐dibenzo[d,g][1,3,2]dioxasilocin
Authors: Fuhrer Hermann , Meier Hansrudolf , Rist Günther , Pastor Stephen D. , Shum Sai P. , Clarke Frank H.
Published: December 2004

X‐Ray Crystal‐structure analysis of 2,4,8,10‐tetrakis(1,1‐dimethylethyl)‐6,6‐dimethyl‐12H‐dibenzo[d,g][1,3,2]‐dioxasilocin showed that its eight‐membered organosilicon heterocycle adopts a boat‐boat (BB) conformation in the solid state (Figs. 2 and 3).
Synthese von selektiv N‐ funktionalisierten Polyamin‐Derivaten
Authors: Fiedler Wolfgang J. , Hesse Manfred
Published: December 2004

Synthesis of Selectively N‐Functionalized Polyamine Derivatives

A threefold differently protected derivative 8 of the triamine spermidine (2) has been prepared in three steps starting from propane‐1,3‐diamine (1). The protected spermidine derivative 8 was converted to its spermine analogue 12, a useful polyamine intermediate. In a convergent way, the fourfold differently protected derivative 18 of the tetraamine spermine (3) has been obtained by coupling the two different and separately prepared propane‐1,3‐diamine units 15 and 17. Spermidine derivative 19 and spermine derivative 20, both selectively protected at both primary amino groups, have been Prepared from the free polyamines 2 and 3, respectively, in a direct approach.
Template‐Assembled Synthetic Proteins (TASP). Cyclic Templates with Incorporated Turn‐Inducing Mimics
Authors: Earnest Ivan , Kalvoda Jaroslav , Francotte Eric , Rihs Grety , Fritz Hans , Blommers Marcel J. J. , Raschdorf Fritz , Sigel Christoph , Mutter Manfred
Published: December 2004

The 8‐amino‐5,6,7,8‐tetrahydronaphth‐2‐oic acid (1), 8‐(aminomethyl)‐5,6,7,8‐tetrahydronaphth‐2‐oic acid (2), and 8‐(aminomethyl)naphth‐2‐oic acid (3) were synthesized in their protected forms as turn‐inducing dipeptide mimics. Two of them (2 and 3) were incorporated into a novel type of cyclic, peptide‐based structures (see 21 and 34–36) designed as templates for the synthesis of TASP molecules.
Modification of Cyclosporin A (CS): Generation of an enolate at the sarcosine residue and reactions with electrophiles
Authors: Seebach Dieter , Beck Albert K. , Bossler Hans G. , Gerber Christian , Ko Soo Y , Murtiashaw C. William , Naef Reto , Shoda Shin‐Ichiro , Thaler Adrian , Krieger Manfred , Wenger Roland
Published: December 2004

Strong bases (lithium diisopropylamide (LDA) or BuLi) convert cyclosporin A (CS) to hexalithio derivative containing a Li alkoxide, four Li azaenolate, and one Li enolate units. The Li6 compound is solubilized in tetrahydrofuran (THF) by addition of excess LDA or LiCl. Reactions with electrophiles (alkyl halides, aldehydes, ClCO2R, CO2, (RS)2, D2O) at low temperatures give products containing new side chains in amino‐acid residue 3 of the cyclic undecapeptide (see 1–13, Schemes 1, and 2, and Figs. 1 and 2) in moderate to high yields and, with Re‐ or Si‐selectivities, depending upon the conditions of lithiation of up to 7:1, Pure CS derivatives (Scheme 2, Table 1 in the Exper. Part) can be isolated by column chromatography. N‐Alkylations or cleavage of the peptide backbone by carbonyl addition occur only at higher temperatures and/or with prolonged reaction times (see 14 and 15 in Scheme 4). Very little or no epimerization of stereogenic centers occurs under the conditions employed. Possible reasons for the feasibility of these surprizing conversions of CS are discussed (Schemes 4 and 5 and Fig. 3). For comparision, [MeAla3]CS (2b) and [D‐MeAla3]CS (2a) were also prepared by conventional peptide synthesis in solution (Schemes 6 and 7). Their 1H‐ and 13C‐NMR spectra are compared with those of CS (Table 2 in the Exper. Part).
Über Diels‐Alder‐Reaktionen des C60‐Fullerens Vorläufige Mitteilung
Authors: Krätler Bernhard , Puchberger Michael
Published: December 2004

On Diels‐Alder Reactions of the C60‐Fullerene

We report on the regioselective [4+2] cycloaddition of Buckminsterfullerene C60(1) at room temperature with 2,3‐dimethylbuta‐1,3‐diene and with the monoterpene 7‐methyl‐3‐methylideneocta‐1,6‐diene (= myrcene) and on the spectroscopic characterization of the corresponding crystalling monoadducts 2 and 3. According to these experiments, 1 acts as a reactive dienophile, which can be functionalized regioselectively under mild and controlled conditons.
Derivatisierung von PQQ und PQQ‐Triester für den Nachweis von PQQ
Authors: Martin Pierre , Winkler Tammo
Published: December 2004

Formation of derivatives of PQQ and PQQ‐Triester for the Detection of PQQ

PQQ is an organic redox/cofactor found in a class of enzymes called quinoproteins. It has been reported that materials like milk, egg yolk, vinegar, beer, serum, urine, and biological tissues contain little amounts of PQQ. To facilitate studies on PQQ, a simple detection of PQQ could be extremely useful. For this purpose, we describe in this paper some adducts of PQQ or PQQ‐triester with 1,2‐diamines, 2,4‐dihydroxyaniline, pyrrole, indole as well as an oxidation product of PQQ. UV and fluorimetric detections of these colored products show that this method is relatively sensitive. The reaction of PQQ‐triester with (MeO)3P to the phosphole adduct 13 is of great importance: the use of [32P]‐labeled trialkyl phosphites will make this reaction an extremely sensitive method to detect PQQ.
Glycosylidene Carbenes Part 12. A new synthesis and some reactions of spirooxiranes
Authors: Vasella Andrea , Dhar Preeti , Witzig Christian
Published: December 2004

The diazirine 1, upon thermolysis or photolysis in either acetone or cyclohexanone, at different concentrations, yield the spiro epoxides 2 and 3, and 4 and 5, respectively (Scheme 1). Yield of 2 and 3 depended both on the temperature and the concentration, and correlated inversely with the yield of the major by‐product, the enol‐derived glycoside 6. Other by‐product were the benzyloxglycal7 and the lactone azines 8. ZnCl2‐Promoted methanolysis of 2 under mild condition yielded mixture of the uloside 9 and 10 (1.2:1); similarly, 4 yielded 11 and 12 (1.8:1; Scheme 2). More strongly acidic conditions converted 11 into 12, evidencing that ZnCl2‐promoted methanolysis proceeds under kinetic control, which is rationalized. The diazirine 13, upon thermolysis of Photolysis in either acetone of cyclohexanone, yielded the α‐D‐configurated spiro epoxides 14 and 16, and the α‐D‐configurated dihydrooxazoles 15 and 17, respectively (Scheme 3), which are either formed by ring‐opening of ß‐D‐epoxides, by competitive interception of the initially formed, hypothetical addition products of the intermediate carbene to the ketones. The glycosylidene carbenes, derived from 1 or 13 are not very reactive towards ketones, yields are good only when sterically unhindered ketones are used in large excess.
7‐Deazaadenosine: Oligoribonucleotide building block synthesis and autocatalytic hydrolysis of base‐modified hammerhead ribozymes
Authors: Seela Frank , Mersmann Karin , Grasby Jane A. , Gait Michael J.
Published: December 2004

A 7‐deazaadenosine ( = tubercidin; c7A;1) building block for solid‐phase oligoribonucleotide synthesis was prepared. The amino group of1was protected with the (dimethylamino)methylidene residue (→3), and the monomethoxytrityl group was introduced at OHC(5′) (→4). Protection of OHC(2′) was carried out by silylation, showing that use of the (i‐Pr)3Si group resulted in high 2′‐O‐selectivity (→5b, 80%). Reaction of5bwith PCl3afforded the phosphonate 7 which was used in solid‐phase oligoribonucleotide synthesis. The autocatalytic hydrolysis of hammerhead ribozymes using pG‐G‐G‐A‐G‐U‐C‐A‐G‐U‐C‐C‐C‐U‐U‐C‐G‐G‐G‐G‐A‐C‐U‐C‐U‐G‐A‐A‐G‐A‐G‐G‐C‐G‐C as substrate strand (S) and modified G‐C‐G‐C‐C‐G‐A‐A‐A‐C‐U‐C‐C‐C as enzyme strand (E) was studied. When c7A replaced A13or A14, a small decrease of catalytic activity was observed, while modification in position A15enhanced the autocatalytic hydrolysis. The results demonstrate, that the atom N(7) of adenosine in any of these positions is not crucial for ribozyme action.
The Effect of Electron‐Donating and Electron‐Withdrawing Substituents on 1H‐and 13C‐NMR chemical shifts of novel 7′‐aryl‐substituted 7′‐apo‐β‐carotenes
Authors: Hand Elli S. , Belmore Kenneth A. , Kispert Lowell D.
Published: December 2004

The synthesis of 7′‐aryl‐7′‐apo‐β‐carotenes, where aryl (Ar) is Ph, 4‐NO2C6H4, 4‐MeOC6H4, 4‐(MeO2C)C6H4, C6F5, and 2,4,6‐Me3C6H2, is described. NMR Chemical shifts of all H‐ and C‐atoms are presented, together with specific examples of the spectra. In contrast to 1H chemical shifts which, except for HC(8′) and HC(7′), did not differ greatly from those of β,β‐carotene, considerable variations in 13C chemical shifts were observed. Signals of the C(α) atoms of the polyene chain [C(β)C(α)] +n Ar were shielded, those of the C(β) atoms were deshielded, with some exceptions when n = 1; the effects decreased with increasing n.
Synthese und Pyrolyse einer Triafulven‐Vorstufe
Authors: Mühlebach Michel , Neuenschwander Markus , Engel Peter
Published: December 2004

Synthesis and Pyrolysis of a Triafulvene Precursor

In view of retro‐Diels‐Alder reactions (RDA reactions), the triafulvene precursor 3 has been prepared in a simple three‐step synthesis by dibromocarbene addition at dibenzo‐barrelene (11→12; 44%), halogen‐Li exchange followed by methylation (12→14, 100%) and HBr elimination (14→3, 62%) (Scheme 3). Reactivity of the so far unknown bridged 1,1‐dibromocyclopropane 12 has been explored, including reductions, allylic rearrangements, and ‘carbene dimerizations’ (Scheme 4). First experiments with respect to the thermal behavior of 3 show that RDA reaction, although occurring in most cases, is not the predominant pathway. When 3 is heated in a sealed tube without solvent, two dimers 26 and 27 are isolated in a total yield of 55% (Scheme 6). On the other hand, gas‐phase pyrolysis of 3 at 400° mainly produces rearranged 28 (56%; Scheme 7). It is assumed that bridged trimethylenemethane 29 is an essential intermediate in thermal rearrangements of 3 (Scheme 8).