bluuebunny/crossref_metadata_2025_split huggingface dataset:
N‐Acylureas in Peptide Synthesis: An X‐Ray Diffraction and IR‐Absorption Study
Authors: Toniolo Claudio , Valle Giovanni , Crisma Marco , Moretto Vittorio , Izdebski Jan , Pelka Jacek , Schneider Conrad H. 
Published: December 2004

AbstractAn X‐ray diffraction analysts of two N‐acyl derivatives of symmetrical dialkylureas, N‐[Nα‐(benzyloxycarbonyl)‐L‐valyl] ‐N, N′‐diisopropylurea (1) and N‐{Nα(tert‐butyloxy)carbonyl ‐L‐valyl}‐N‐N′‐dicyclohexylurea (2), and one N‐acyl derivative of an unsymmetrical N‐N′‐dialkylurea, N‐[Nα‐(benzyloxycarbonyl)‐L‐valyl] ‐N′‐(tert‐butyl)‐N‐ethylurea (3), has been performed. It was established that it is the least hindered O‐acylisourca N‐atom that attacks intramolecularly the carbonyl group of the Nα‐protected amino acid activated by the unsymmetrical carbodiimide to form the major rearrangement product. The occurrence and nature of intra‐ and intermolecularly Hbonded forms of the N‐acylureas in the crystal state were also assessed. It was also shown that soluble N‐acylureas may compete with intermolecular (peptide)NH…OC(peptide) H‐bonds in CH2Cl2.
Pteridine. Teil XCIV. Synthese und Eigenschaften von 5,6‐Dihydro‐6‐(1,2,3‐trihydroxypropyl)pteridinen: Kovalente intramolekulare Addukte
Authors: Soyka Rainer , Pfleiderer Wolfgang , Prewo Roland 
Published: December 2004

Pteridines: Synthesis and Characteristics of 5,6‐Dihydro‐6‐(1,2,3‐trihydroxypropyl)pteridines: Covalent Intramolecular AdductsVarious 5,6‐diaminopyrimidines (1, 15, 24, 33) were condensed with the phenylhydrazones of L‐(2) and D‐arabinose (3) in acidic medium under N2 to give formal 5,6‐dihydro‐6‐(1,2,3‐trihydroxypropyl)pteridines (see, e.g., 4 and 5), the latter turned out to exist preferentially as intramolecular adducts, the hexahydropyrano‐[3,2‐g]pteridines 6, 7, 16, 17, 25, 26, and 34, formed subsequently by addition of the terminal OH group of the side‐chain to the C(7)N(8) bond of the pteridine moiety. Spectroscopically, the isomeric hexahydrofuro‐[3,2‐g]pteridines 10,11,18,19, and 35 were also detected as minor components in the equilibrium mixtures. In the 4‐amino‐2‐(methylthio)pteridine series, crystallization of 6 and 7 led to the stereochemically pure (3S,4R,4aR, 10aS)‐6‐amino‐3,4,4a,5,10,10a‐hexahydro‐8‐(methylthio)‐2H‐pyrano[3,2‐g]pteridine‐3,4‐diol (8) and its corresponding enantiomer 9, respectively Structure 8 was proven by X‐ray analysis. Acylation of the hexahydropyrano[3,2‐g]pteridines yielded the more stable tri‐, tetra‐, and pentaacetyl derivatives 12–14, 20–23, 27–32, and 37–39 which were characterized and of which the absolute and relative configurations were determined (1H‐ and 13C–NMR and UV spectra, chiroptical measurements, elemental analyses).
Stereoselective Coordination to Chiral Matrices. Cobalt(III). Chemistry of Simple Facially Coordinating Chiral Triamines
Authors: Comba Peter , Hörmann Aldo , Martin Lisandra L. , Zipper Luc 
Published: December 2004

AbstractChiral ligands coordinated to metal ions exert a selectivity towards the additional coordination of racemic substrates. Experimentally determined equilibria distributions of [Co(L3)2]3+ and [Co(L3)(L2)(X)]n+ are compared with calculated data based on strain‐energy minimization (L3: trap = propane‐1,2,3‐triamine; 1,2,4‐trab = butane‐1,2,4‐triamine; 1,2,3‐trab = butane‐1,2,3‐triamine; 1,3,4‐trpe = pentane‐1,3,4‐triamine; 1,3,4‐tmeb = 2‐methylbutane‐1,3,4‐triamine; 1,2,4‐trpe = pentane‐1,2,4‐triamine; L2: en = ethane‐1,2‐diamine; pn = propane‐1,2‐diamine; X: NH3, OH2, OH−). Equilibration of Co(III) complexes was achieved by oxygenation of aqueous solutions of Co(II) salts in presence of the ligands. Quantitative isomer distribution was investigated with HPLC, and quantitative analysis of the enantiomeric excess (ee) of the racemic substrate (present in a two‐fold excess) was studied, after chromatographical recovery, by 1H‐NMR analysis of its Mosher‐acid derivative. There is good agreement between calculated and experimental data. Systems with L = 1,2,4‐trab are, as expected, relatively poorly discriminating (ee([Co(1,2,4‐trab)2]3+) ∼ 5%; ee([Co(1,2,4‐trab)(pn)(X)]n+) ∼ 10%). Calculations indicate that Me substitution of the ligand backbone of 1,2,4‐trab (and trap) leads to an increased enantioselectivity (with practically constant isomer selectivity), and at the optimum site for substitution ∼ 90% ee is predicted.
A Short Synthesis of (±)‐Muscone
Authors: Rautenstrauch Valentin , Snowden Roger L. , Linder Simon M. 
Published: December 2004

Abstract(±)‐Muscone ((±)‐1) has been synthesised in three steps from 2‐(2′‐methylprop‐2′‐enyl)cyclododecan‐1‐one (2). The synthesis involves two key transformations: a Lewis‐acid‐mediated intramolecular ene reaction (2→3) and the β‐cleavage of the bicyclic potassium alkoxide 3a′ to the macrocyclic enone (Z)‐11.
Analogues of Sialic Acids as Potential Sialidase Inhibitors Synthesis of 2‐C‐Hydroxymethyl Derivatives of N‐Acetyl‐6‐amino‐2,6‐dideoxy‐neuraminic Acid
Authors: Bernet Bruno , Murty A. R. C. Bulusu , Vasella Andrea 
Published: December 2004

AbstractThe intramolecular cycloaddition of the previously described azidoalkene 16, the related diacetates 7 and 13, and the monoacetate 8 led diastereoselectivity to the (2R)‐ and (2S)‐configurated hydropyridotriazoles 17, 9 and 11, 14 and 15, and 10 and 12, respectively (Scheme 1). Thermolysis of 16 gave also the aziridine 18, its proportion increasing with reaction time. The diastereoselectivity of the cycloaddition‐ is rationalized on the basis of steric interactions and of Hbonds in the transition state. Photolysis in benzene partially transformed 9 into the aziridine 19. Treatment of 9 with aqueous AcOH gave 19 and the tetrahydrofuran 20, with AcOH in benzene 20 and the triacetate 23, and with aqueous H2SO4 in THF, the primary alcohol 22 (room temperature) or 19 and 22 (0°). Deacetylation of 9 followed by reaction with pyridinium hydrochloride led to the tetrahydrofuran 21 and the chloride 24 (Scheme 2). The diacetate 22 and the triacetate 23 gave the tripl 25 which was deprotected to 26. Reduction of the keto‐aziridine 18 (NaBH4) gave the alcohols 27 and 29 which were acetylated to give 28 and 19, respectively (Scheme 3). Treatment of the aziridine 28 with AcOH in benzene followed by deacetylation gave 30 and hence 31. AcOH in benzene transformed the triazoline 15 first into the aziridine 32 and hence into 33, which was deprotected to give the triol 34 and hence 35. The 2‐(hydroxymethyl)piperidines 26, 31, and 35 inhibited Vibrio cholerae sialidase with K1 = 3.8 · 10−2 M, 3.4 · 10−3 M, and 1.5 · 10−4 M, respectively. The conformation of the glycerol side chain of these compounds and of the unbranched piperidines 2–4 deviates from the one of Neu5Ac (and Neu2en5Ac). This finding is rationalized by an H‐bond between OHC(8) and NHC(6). The conformations and the K1 values of 26, 31, and 35 correlate with each other.
Synthese der (3S,4R,3′S,4′R)‐ und (3S,4S,3′S,4′S)Crustaxanthine sowie weiterer Verbindungen mit 3,4‐Dihydroxy‐β‐Endgruppen
Authors: Buschor Daniel Jacques , Eugster Conrad Hans 
Published: December 2004

Synthesis of (3S,4R,3′S,4′R)‐ and (3S,4S,3′S,4′S)‐Crustaxanthins and Further Compounds with 3,4‐Dihydroxy β‐End‐groupsStarting from 3, the enantiomerically pure title compounds were synthesized in eight steps. Spectra and HPLC systems are presented that allow a distinction between these isomers.
Photodissociation of Halonaphthalenes in Solution: Comparative Photochemistry of 1‐Iodo‐, 1‐Bromo‐, and 1‐Chloronaphthalenes
Authors: Haselbach Edwin , Rohner Yves , Suppan Paul 
Published: December 2004

AbstractFluorescence measurements have been used to follow the build‐up of photoproducts during the direct and benzophenone‐sensitized irradiation of the title compounds 1‐IN, 1‐BrN, and 1‐ClN (HN = naphthalene). Compounds 1‐IN and 1‐BrN react by homolytic dissociation through their lowest triplet and singlet excited states, respectively. Compound 1‐ClN does not undergo C–Cl bond fission, except through electron transfer in the presence of an amine A,

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In the absence of this electron transfer, 1‐ClN reacts only through substitution and oxidation processes.
The Reaction of Singlet Oxygen with Adamantylideneadamantane Mediated by Rose Bengal
Authors: Jefford Charles W. , Estrada Manuel Jimenez , Barchietto Giacomo , Berclaz Théo , Geoffroy Michel 
Published: December 2004

AbstractThe photo‐oxygenation of adamantylideneadamantane (1) on siliceous supports using admixed granules of ion‐exchange resin fixed to methylene blue (MB) and rose bengal (RB) gave exclusively the corresponding dioxetane derivative 2 for the former sensitizer, while the latter gave 2 and traces of the epoxide 3. RB and the charge‐transfer complex produced from N‐ethylcarbazole and 2,4,5,6‐tetranitrofluoren‐9‐one both reacted with chemically generated singlet oxygen to give superoxide radical anion. Trapping of the latter with 5,5‐dimethyl‐1‐pyrroline 1‐oxide gave an adduct exhibiting a characteristic ESR spectrum. The treatment of 1 in MeOH with 30% aqueous H2O2 for 22 h at 60° gave 3 in 100% yield. Repetition of this experiment in the presence of 2,6‐di(tert‐butyl)‐p‐cresol caused no significant change. These results indicate that singlet oxygen reacts with 1, in the presence of RB, by two different processes. The first leads to dioxetane formation. The second process involves conversion of singlet oxygen by RB to superoxide radical anion which subsequently gives H2O2 so producing epoxide 3 from 1.
Synthese neuer Heptafulvene, Röntgenstrukturanalyse von ‘8,8‐(1′,4′‐Dioxotetramethylen)heptafulven’(2‐(Cyclohepta‐2,4,6‐trien‐1‐yliden)cyclopentan‐1,3‐dion)
Authors: Bönzli Peter , Neuenschwander Markus , Engel Peter 
Published: December 2004

Synthesis of New Heptafulvenes; X‐Ray Analysis of ‘8,8‐(1′,4′‐Dioxotetramethylene)heptafulvene’ (2‐(Cyclohepta‐2,4,6‐trien‐1‐ylidene)cyclopentane‐1,3‐dione)Experimental procedures for the synthesis of heptafulvene (3a), 8,8‐tetramethylene heptafulvene (3c) and ‘8,8‐(1′,4′‐dioxotetramethylene) heptafulvene’ (2‐(cyclohepta‐2,4,6‐trien‐1‐ylidene)‐cyclopentane‐1,3‐dion; 3d) are described. The most important sequences include a low‐temperature reaction of tropylium salts with lithium or Grignard carbenoids (Scheme 1) to give 3a and 3b as well as hydride abstraction from substituted cycloheptatrienes followed by deprotonation to give 3c and 3d. Limitations of these sequences are discussed. Two other heptafulvenes 3h and 3i are available by silylation of heptafulvenolates according to well‐known procedures. NMR‐Spectroscopic evidence as well as an X‐ray analysis of 3d are presented. Compound 3d is a relatively polar heptafulvene with a planarised seven‐membered ring as well as a partly delocalized π system.
Glycosylidene Carbenes. Part 2. Synthesis of O‐Aryl Glycosides
Authors: Briner Karin , Vasella Andrea 
Published: December 2004

AbstractPhenol, 4‐methoxyphenol, 4‐nitrophenol, methyl orsellinate (1), and 2,6‐di(tert‐butyl)‐4‐methylphenol (BHT; 2) have been glycosylated by thermal reaction (20–60°) with various glycosylidene‐derived diazirines.4‐Methoxyphenol reacted with the D‐glucosylidene‐derived diazirine 3 to give O‐glucosides (4 and 5, 69%, 3:1) and C‐glucosides (6 and 7, 16%, 1:1). Similarly, phenol yielded O‐glucosides (10 and 11, 70%, 4:1) and C‐glucosides (12 and 13, 13%, 1:1). 4‐Nitrophenol gave only O‐glycosides, 3 leading to 14 and 15 (75%, 3:2; Scheme 1), and the D‐galactosylidene‐derived diazirine 17 to 22 and 23 (52% (from 16), 65:35; Scheme 2). The reaction of phenol with 17 yielded 58% (from 16) of the O‐galactosides 18 and 19 (4:1) and 14% of the C‐galactosides 20 and 21 (1:1). From the D‐mannosylidene‐derived diazirine 25, we predominantly obtained the α‐D‐configurated 26 (38 % from 24). These results are interpreted by assuming that an intermediate (presumably a glycosylidene carbene) first deprotonates the phenol to generate an ion pair which combines to give O‐ and ‐ with electron‐rich phenolates ‐ also C‐glycosides. A competition experiment of 3 with 4‐nitro‐ and 4‐methoxyphenol gave the products from the former (14 and 15) and the latter phenol (4‐7) in almost equal amounts. Differences in the kinetic acidity of OH groups, however, may form the basis of a regioselective glycosidation, as evidenced by the reaction of 3 with methyl orsellinate (1) yielding exclusively the 4‐O‐monoglycosylated products 27 and 28 (78%, 85:15), although diglycosidation is possible (27→ 31 and 32; 67%, 4:3; Scheme 3). Steric hindrance does not affect this type of glycosidation; 3 reacted with the hindered BHT (2) to afford 33 and 34 (81 %, 4:1). The predominant formation of 1,2‐trans ‐configurated O‐aryl glycosides is rationalized by a neighbouring‐group participation of the 2‐benzyloxy group.
Conversion of Light into Electricity with Trinuclear Ruthenium Complexes Adsorbed on Textured TiO2 Films
Authors: Nazeeruddin Mohammad K. , Liska Paul , Moser Jacques , Vlachopoulos Nick , Grätzel Michael 
Published: December 2004

AbstractA series of CN‐bridged trinuclear Ru complexes of the general structure [RuL2(μ‐(CN)Ru(CN)L2′)2] where L is 2,2′‐bipyridine‐4,4′‐dicarboxylic acid and L′ is 2,2′‐bipyridine (1)2,2′‐bipyridine‐4,4′‐dicarboxylic acid (2), 4,4′‐dimethyl‐2,2′‐bipyridine (3), 4,4′‐diphenyl‐2,2′‐bipyridine (4), 1,10‐phenanthroline (5), and bathophenanthrolinedisulfonic acid (6) have been synthesized, and their spectral and electrochemical properties investigated. The two carboxylic functions on the 2,2′‐bipyridine ligand L serve as interlocking groups through which the dye is attached at the surface of TiO2 films having a specific surface texture. The role of these interlocking groups is to provide strong electronic coupling between the π* orbital of the 2,2′‐bipyridine and the 3d‐wave‐function manifold of the conduction band of the TiO2, allowing the charge injection to proceed at quantum yields close to 100 %. The charge injection and recombination dynamics have been studied with colloidal TiO2, using laser photolysis technique in conjunction with time‐resolved optical spectroscopy. Photocurrent action spectra obtained from photo‐electrochemical experiments with these trinuclear complexes cover a very broad range in the visible, making them attractive candidates for solar light harvesting. Monochromatic incident photon‐to‐current conversion efficiencies are strikingly high exceeding 80% in some cases. Performance characteristics of regenerative cells operating with these trinuclear complexes and ethanolic triiodide/iodide redox electrolyte have been investigated. Optimal results were obtained with complex 1 which gave a fill factor of 75 % and a power conversion efficiency of 11.3% at 520 nm.
Kinetic Isotope Effects and exo/endo‐Face Selectivity in the Unimolecular Dehydrogenation of Norborneol Alkoxides in the Gas Phase
Authors: Dreyer Uwe , Sülzle Detlev , Schröder Detlel , Schwarz Helmut 
Published: December 2004

AbstractThe study of specifically labeled alkoxides generated in the gas phase by deprotonation of epimeric norbernols demonstrates, beyond any doubt, that ‘solvated’ hydride ions are formed as intermediates in the course of enolate formation. Kinetic isotope effects and exo/endo‐face selectivities for the dehydrogenation are discussed.
Coenzyme F430 from Methanogenic Bacteria: Oxidation of F430 Pentamethyl Ester to the Ni(III) Form
Authors: Jaun Bernhard 
Published: December 2004

AbstractF430M, the pentamethyl ester of coenzyme F430, can be oxidized reversibly by one electron. The oxidation potential has been determined, and the electrolytically prepared oxidation product was characterized by its UV/VIS and ESR spectrum. The strongly anisotropic and nearly axial ESR spectrum is consistent with a S = ½ species with the unpaired‐electron spin density predominantly in a d‐type orbital of the central nickel ion. The properties of Ni(III)F430M are discussed in the context of two hypothetical mechanisms for the catalytic role of coenzyme F430 in methyl coenzyme M reductase, which catalyses the last step of methane formation in methanogenic bacteria.
New Synthetic Route to Polyhedral Organylsilsesquioxanes
Authors: Herren Daniel , Bürgy Heribert , Calzaferri Gion 
Published: December 2004

AbstractHydrosilation of methylenecyclohexane and hex‐1‐ene by octa(hydridosilsesquioxane) catalysed by hexachloroplatinic acid is a new route to polyhedral organylsilsesquioxanes. Quantitative yield of octa(cyclohexylmethylsilsesquioxane) is reached. This reaction opens a vast field of yet unknown polyhedral silsesquioxanes.
Rogiolenyne A, B, and C: The First Branched Marine C15 Acetogenins. Isolation from the Red Seaweed Laurencia microcladia or the Sponge Spongia zimocca of II Rogiolo
Authors: Guella Graziano , Pietra Francesco 
Published: December 2004

AbstractIt is shown here that the red seaweed Laurencia microcladia, collected off the torrent II Rogiolo, south of Livorno, contains rogiolenyne A (= (−)‐(1R*,2S*,3R*,5S*,7S*)‐2‐(bromomethyl)‐5‐[(Z)‐1‐chlorohex‐3‐en‐5‐ynyl]‐3‐ethyl‐4,8‐dioxabicyclo[5.1.0]octane; (−)‐1) while the sponge Spongia zimocca, which grows in the same small area, contains rogiolenyne B (= (−)‐(2R*3R*,4R*,5R*,7S*)‐3‐(bromomethyl)‐5‐chloro‐7‐[(Z)‐1‐chlorohex‐3‐en‐5‐ynyl]‐2‐ethyloxepan‐4‐ol; (−)‐4a) and its acetate, rogiolenyne C((−)‐4b). These structures, which are based on extensive NMR and MS data and on chemical transformation, are the first examples of branched marine C15 acetogenins. Biogenesis of (−)‐1 in L. microcladia is thought to involve C(12) extrusion form a C15 linear tetraen‐1‐yne precursor via H+‐induced cyclopropane ring closure, followed by Br+‐induced cyclopropane ring opening, aided by C–O− attack (Scheme 2). It is also proposed that transfer of (−)‐1 to S. zimocca is followed by epoxide ring opening by Cl− to give (−)‐4a and acetylation to give (−)‐4b.
Generation and Trapping of Cyclopropenes from 2‐Alkoxy‐1,1‐dichlorocyclopropanes
Authors: Müller Paul , Pautex Nicole 
Published: December 2004

AbstractIn presence of crown ether, 2‐alkoxy‐1,1‐dichlorocyclopropanes react with t‐BuOK/THF preferentially via ring opening to 2‐chloroalk‐2‐en‐1‐ones and alkynones or to chlorocyclopropenes. The latter may be intercepted with 1,3‐diphenylisobenzofuran, but in the absence of trapping agent, the rearrangement to vinylcarbenes does not occur.
Stereoselective Synthesis of Indazole 2′‐Deoxy‐β‐D‐ribonucleosides: Glycosylation of the Nucleobase Anion
Authors: Seela Frank , Bourgeois Werner 
Published: December 2004

AbstractThe glycosylation of indazolyl anions derived from 4a, b with 2‐deoxy‐3,5‐bis‐O‐(4‐methylbenzoyl)‐α‐D‐erythro‐pentofuranosyl chloride (5) is described. The reaction was Stereoselective – exclusive β‐D‐anomer formation – but regioisomeric N1‐ and N2‐(2′‐deoxy‐β‐D‐ribofuranosides) (i.e. 6a and 7a, resp., and 6b and 7b, resp.) were formed in about equal amounts. They were deprotected to yield 8a, b and 9a, b. Compound 1, related to 2′‐deoxyadenosine (3), and its regioisomer 2 were obtained from 8b and 9b, respectively, by catalytic hydrogenation. The anomeric configuration as well as the position of glycosylation were determined by 1D NOE‐difference spectroscopy. The first protonation site of 1 and 2 was found to be the NH2 group. The N‐glycosylic bond of 1H‐indazole N1‐(2′‐deoxyribofuranosides) is more stable than that of the parent purine nucleosides. Compound 1 is no substrate for adenosine deaminase.
On the Regioselectivity Control in the Palladium‐Catalyzed Hydro‐alkoxycarbonylation of α,β‐Unsaturated Esters
Authors: Consiglio Giambattista , Nefkens Sylvia C. A. , Pisano Carmelina , Wenzinger Fritz 
Published: December 2004

AbstractThe regioselectivity of the hydro‐alkoxycarbonylation of methyl acrylate, methacrylate, and crotonate catalyzed by [PdCl2L2] complexes (L = phosphine ligands) can be largely controlled by variation of the ligands. PPh3, promotes preferential carbonylation at the α‐position, whereas with [(2,2‐dimethyl‐1,3‐dioxolane‐4,5‐diyl)bis‐(methylene)]bis[diphenylphosphine] as ligand, the β‐position is overwhelmingly carbonylated.
Synthesis of 4‐{{[(Isopropyloxy)methylphosphoryloxy]imino}methyl}‐1‐methylpyridinium Iodide and Its Characterisation
Authors: Portmann Rudolf , Niederhaiiser Andreas , Hofmann Werner , Frey Alfred , Stoeckli‐Evans Helen 
Published: December 2004

AbstractOximes are used in the therapy of organophosphate poisoning. They form an interaction product with the organophosphate which is claimed to be more toxic than the organophosphate itself. We have synthesized one of these compounds which would be formed by the reaction of pyridine‐4‐carbaldehyde oxime methiodide with sarin (GB). Thus, reaction of oxime 1 with sarin chloridate (2), yielded 4‐{{[(isopropyloxy)methylphosphoryloxy]‐imino}methyl}‐1‐methylpyridinium iodide (4) which was characterized by NMR spectroscopy and X‐ray crystallography.
Synthesis of a Phosphonic Acid Analogue of N‐Acetyl‐2,3‐didehydro‐2‐deoxyneuraminic Acid, an Inhibitor of Vibrio cholerae Sialidase
Authors: Vasella Andrea , Wyler René 
Published: December 2004

AbstractThe synthesis of the phospha analogue 10 of DANA (2) is described. Bromo‐hydroxylation of the known 11 (→ 12 and 13) followed by treatment of the major bromohydrin 13 with 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) gave the oxirane 14 (Scheme 1). Depending on the solvent, TiBr4 transformed 14 into 16 or into a 15/16 mixture. Reductive debromination of 16 (→ 17), followed by benzylation provided 18. Oxidattve decarboxylation (Pb(OAc)4) of the acid, obtained by saponification of 18, yielded the anomeric acetates 19 and 20. While 19 was inert under the conditions of phosphonoylation, the more reactive imidate 22, obtained together with 23 from 19/20 via 21 (Scheme 2), gave a mixture of the phosphonates 24/25 and the bicyclic acetal 26. Debenzylation of 24/25 and acetylation led to the acetoxyphosphonates 27/28. Since β‐elimination of AcOH from 27/28 proved difficult, the bromide 34 was prepared from 27/28 by photobromination and subjected to reductive elimination with Zn/Cu (→ 35; Scheme 3). This two‐step sequence was first investigated using the model compounds 30 and 31. Transesterification of 35, followed by deacetylation gave 10, which is a strong inhibitor of the Vibrio Cholerae sialidase.
On the First Marine Stigmastane Sterols and Sterones Having a 24,25‐Double Bond. Isolation from the Sponge Stelletta sp. of Deep Coral Sea
Authors: Guerriero Antonio , Debitus Cécile , Pietra Francesco 
Published: December 2004

AbstractThe sponge Stelletta sp. (Astrophorida, Stellettidae), collected at −700 m in the Coral Sea, is shown to contain sterones and sterols of the stigmastane type with a C(24)C(25) bond for which there is no precedent in the sea. Structure elucidation of the second abundant of these steroids, stigmasta‐4,24(25)‐dien‐3‐one ((+)‐1), is based on 1D and 2D NMR spectra and chemical transformation to acetate (−)‐5. Stigmasta‐4,24(25)‐diene‐3,6‐dione ((−)‐3), present in trace amounts in the sponge, was obtained in sufficient quantity for NMR study by oxidation of the also present, inseparable, and abundant 4:1 mixture of stigmasta‐5,24(25)‐dien‐3β‐ol (6) and its 5,6‐dihydro derivative 7 (Scheme 1). This oxidation also afforded the ketone analogues (+)‐8 and (+)‐9, which could be separated, thus making structure elucidation possible. The 6β‐hydroxystigmasta‐4,24(25)‐dien‐3‐one ((+)‐4), also present in trace amounts in the sponge, was obtained in sufficient amount for NMR study, together with its C(6) epimer (+)‐11, by hydroperoxidation of (+)‐8 followed by deoxygenation (Scheme 2). The last trace steroid of the sponge, stigmasta‐4,6,24(25)‐trien‐3‐one ((−)‐2), was structurally elucidated using limited NMR data and comparison with the other stigmastanes. These stigmastanes, as the only steroids of this sponge, are likely to function as stabilizers of its cell walls; their phytosteroid structure, for a sponge which lives in the dark of deep waters, suggest origin through a complex food chain, possibly followed by bioelaboration in the sponge.