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PubMed2002

Chemoenzymatic asymmetric total syntheses of antitumor agents (3R,9R,10R)- and (3S,9R,10R)-Panaxytriol and (R)- and (S)-Falcarinol from Panax ginseng using an enantioconvergent enzyme-triggered cascade reaction.

Total asymmetric synthesis of two components of Panax ginseng showing antitumor activity, i.e., (3R,9R,10R)- and (3S,9R,10R)-Panaxytriol and of both enantiomers of Falcarinol was accomplished. Due to the fact that the synthetic strategy was based on enantioconvergent biotransformations, the occurrence of any undesired stereoisomer was entirely avoided. The absolute configuration of naturally occurring Panaxytriol was confirmed to be (3R,9R,10R) on the basis of optical rotation values. It was shown that enzyme-triggered cascade reactions represent a valuable tool for the synthesis of natural products.

Alkynes↗
PubMed2002

Absolute configuration of tert-butyl-1-(2-methylnaphthyl)phosphine oxide.

The enantiomers of tert-butyl-1-(2-methylnaphthyl)phosphine oxide 1 have been separated using a homemade HPLC column and an analytical gradient system. Vibrational absorption and circular dichroism spectra for both enantiomers have been measured in CD2Cl2 and CH2Cl2 solutions in the 2000-900 cm(-1) region. The fully relaxed potential energy surface of (S)-tert-butyl-1-(2-methylnaphthyl)phosphine oxide, obtained using the B3LYP functional with a 6-31G basis set, indicated two stable conformers with their populations in a approximately 2:1 ratio. The vibrational absorption and VCD spectra are predicted for these two conformers using the B3LYP functional with a 6-31G basis set. The comparison of predicted and experimental spectra indicated that (+)-tert-butyl-1-(2-methylnaphthyl)phosphine oxide is in the (S)-configuration. This assignment is supported by the ab initio prediction of positive optical rotation for the most stable conformer with an (S)-configuration and the nonequivalence sense of the tert-butyl group chemical shift observed in the 1H NMR spectrum of this enantiomer measured in the presence of (+)-(S)-mandelic acid as a chiral solvating agent.

Chromatography, High Pressure Liquid↗
PubMed2002

Synthesis of R-(-)-imperanene from the natural lignan hydroxymatairesinol.

A convenient and high yielding method for the synthesis of R-(-)-imperanene, starting from the readily available natural lignan hydroxymatairesinol from Norway spruce, was developed. Hydroxymatairesinol was degraded in strongly basic aqueous conditions to (E)-4-(4-hydroxy-3-methoxyphenyl)-2-(4-hydroxy-3-methoxyphenylmethyl)but-3-enoic acid, which was esterified and then reduced by LiAlH(4) to afford R-(-)-imperanene. The configuration at the crucial stereocenter was preserved in the synthesis, and the obtained product was identified by optical rotation measurements and chiral HPLC analyses as the R-(-)-enantiomer (ee 86-92%).

Indicators and Reagents↗
PubMed2003

Asymmetric synthesis of four diastereomers of 3-hydroxy-2,4,6-trimethylheptanoic acid: proof of configurational assignment.

Four unique diastereomers of 3-hydroxy-2,4,6-trimethylheptanoic acid--(2R,3R,4R), (2S,3R,4R), (2S,3R,4S), and (2R,3R,4S)--the fatty acid component of callipeltin A and D, have been synthesized from commercially available (+)- and (-)-pseudoephedrine propionamide in 6 steps and 59% average overall yield. Comparison of the 1H and 13C NMR and optical rotation data of the resulting isomers with the natural fragment unambiguously verifies the configurational assignment of the natural isomer as (2R,3R,4R).

Depsipeptides↗
PubMed2004

Enantioselective total syntheses of (+)-arborescidine A, (-)-arborescidine B, and (-)-arborescidine C.

Described are the first enantioselective total syntheses of (+)-arborescidine A ((+)-1), (-)-arborescidine B ((-)-2), and (-)-arborescidine C ((-)-3), via routes that proceeded in five steps and 50% overall yield, eight steps and 61% overall yield, and nine steps and 51% overall yield, respectively, from 6-bromotryptamine (7). The syntheses feature the use of the Noyori catalytic asymmetric hydrogen-transfer reaction to introduce chirality in dihydro-beta-carbolines 6 and 8. On the basis of an ample precedent from Noyori's work, the reduction produces dihydro-beta-carbolines, and ultimately the natural products, possessing the R absolute configuration. The synthetic arborescidines displayed optical rotations that were opposite in sign those of the natural products, thereby supporting the S configuration for natural arborescidines A (1) and B (2) and the (3S,17S) configuration for natural arborescidine C (3). Our results are in agreement with the initial stereochemical assignment by Païs and co-workers, and are counter to their recently revised assignment.

Alkaloids↗
PubMed2004

Eremophilane sesquiterpenes from capsidiol.

A series of eremophilane sesquiterpene alcohols and hydrocarbons was prepared from the phytoalexin capsidiol (1) for mechanistic studies with epiaristolochene synthase and epiaristolochene dihydroxylase. Among them, 3-deoxycapsidiol (10) was obtained through selective derivatization and reductive cleavage of the equatorial 3 alpha hydroxyl group. Two novel isomers of aristolochene and eremophilene were accessed from the 1- and 3-deoxycapsidiol isomers. 4-Epieremophilene (17) was obtained by conjugate reduction of epiaristolochen-1-one tosylhydrazone with catecholborane followed by sulfinate elimination and diimide rearrangement. Epimerization of epiaristolochen-3-one (27a) at the C4 methyl followed by reductions led to the previously unknown aristolochene isomer, eremophila-9(10),11(12)-diene (30). Optical rotations and characteristic (1)H NMR data for the related eremophilenols and dienes are collected in Tables 1 and 2. Finally, bioassays were used to assess the antifungal potencies of capsidiol and its synthetic derivatives. The minimum inhibitory concentration for capsidiol (3-10 ng) was at least 1 order of magnitude lower than that of any of the derivatives and considerably lower than those previously reported for ketoconazole, nystatin, and propiconazole.

Antifungal Agents↗
PubMed1997

Enantiospecific Formation of Trans 1,3-Disubstituted Tetrahydro-beta-carbolines by the Pictet-Spengler Reaction and Conversion of Cis Diastereomers into Their Trans Counterparts by Scission of the C-1/N-2 Bond.

The factors which effect the stereoselective formation of trans-1-alkyl-2-benzyl-3-(alkoxycarbonyl)-1,2,3,4-tetrahydro-beta-carbolines and trans-3-(alkoxycarbonyl)-1-alkyl-2-(diphenylmethyl)-1,2,3,4-tetrahydro-beta-carbolines by the Pictet-Spengler cyclization were examined by heating tryptophan derivatives with aldehydes of varied steric bulk under aprotic and acidic conditions, followed by determination of the ratio of cis to trans diastereomers so formed. The presence of a benzyl group at the N(b)-nitrogen atom alters the diastereochemical outcome of this condensation to provide 100% trans stereoselectivity when the cyclization is carried out with cyclohexanecarboxaldehyde. Furthermore, when N(b)-(diphenylmethyl)tryptophan isopropyl ester was condensed with aldehydes of any size, trans diastereomers are formed with 100% stereoselectively. The trans N(b)-substituted diastereomers are thermodynamically more stable than their cis congeners as shown by equilibration experiments in TFA. Conversion of the cis diastereomers into the more stable trans diastereomers is believed to occur under acidic conditions by cleavage of the carbon (C-1)-nitrogen (N-2) bond with complete retention of configuration at the C-3 stereocenter. Evidence from deuterium exchange experiments as well as optical rotations support this model for epimerization. In addition, when cis diastereomer 66a was allowed to stir in CF(3)COOD, the trans isomer 66b was isolated in 90% yield, while treatment of cis 66a with CF(3)COOH/NaBH(4) provided a mixture of the ring cleaved [scission across C(1)-N(2) bond] product 67 and the trans isomer 66b. Treatment of 66b (control experiment) with NaBH(4)/CF(3)COOH under the same conditions returned only starting trans 66b in excellent yield. The Pictet-Spengler reaction of substrates with sufficiently large substituents, followed by treatment with acid, permits the 100% enantiospecific formation of trans-1,3-disubstituted-1,2,3,4-tetrahydro-beta-carbolines for alkaloid total synthesis.

Journal Article↗
PubMed1996

Stereoselective Synthesis of 8,12-Furanoeudesmanes from Santonin. Absolute Stereochemistry of Natural Furanoeudesma-1,3-diene and Tubipofurane.

Ketobutenolide 3, easily obtained from santonin (1), has been transformed into two natural furanoeudesmanes 4 and 5, isolated from Commiphora molmol and Tubipora musica, respectively. trans- And cis-decalin systems were obtained by stereoselective reduction of the C(4)-C(5) double bond in 3 in the following way: hydrogenation of 3 over Pd/C followed by acidic treatment gave the cis isomer 10 as the major product; selective hydrogenation of the C(1)-C(2) double bond with the Wilkinson's catalyst followed by reduction with NaTeH yielded mainly the trans isomer 9. Compounds 9 and 10 were transformed into 4 and 5 in parallel sequences. Optical rotation and CD measurements of the synthetic products revealed that the stereochemistry of both natural products should be revised to their enantiomeric form.

Journal Article↗
PubMed1996

Total Synthesis of (-)-Muscoride A.

The recently isolated cyanobacterium metabolite muscoride A was synthesized in 15 steps and in 4.3% overall yield. Novel structural features of this peptide antibiotic include the presence of a threonine-derived bioxazole core and an N-(1,1-dimethyl)allyl ("reverse prenyl") valine residue. In the context of our synthesis, efficient new strategies for the preparation of these segments were developed. The synthesis of two epimers of muscoride A allowed the unambiguous assignment of the relative and absolute configuration of the natural product by NMR and optical rotation analyses.

Journal Article↗
PubMed1997

Novel Bioactive Cubé Insecticide Constituents: Isolation and Preparation of 13-homo-13-Oxa-6a,12a-dehydrorotenoids.

Two novel rotenoid-like compounds isolated from cubé resin, used as an insecticide and piscicide, have an unprecedented skeleton consisting of a 1,5-benzodioxepin (A and B rings) and a chromone (C and D rings). Their structures are assigned as (-)-13-homo-13-oxa-6a,12a-dehydrorotenone (5) and 13-homo-13-oxa-6a,12a-dehydrodeguelin (6) based on (1)H and (13)C NMR, MS, UV, and optical rotation. X-ray structure determination confirmed the assignment of 5 and revealed that the 1-methylethenyl group is disordered into two sets of atoms. Compound 5 from cubé resin was identical with a product previously reported as being formed by reaction of rotenone (1) with acetyl chloride and DMF and misassigned as "6a,12a-epoxyrotenone" (7). A modified procedure was developed for preparation of the new oxarotenoid component of the commercial insecticide. The cubé resin from which 5 and 6 were isolated in equal amounts contains 1 and deguelin (2) in a >10:1 ratio; this large difference in ratio indicates that 5 and 6 are not artifacts formed from 1and 2, respectively. Rotenoid 5 acts as a respiratory inhibitor with 50% inhibition of NADH:ubiquinone oxidoreductase activity at 0.11 &mgr;M, of goldfish survival at 1 ppm, and of the viability of three cell lines at 4-8 &mgr;M.

Journal Article↗
PubMed1997

Total Synthesis of the Enantiomer of the Furanocembrane Rubifolide.

The total synthesis of 57, the enantiomer of the marine furanocembrane rubifolide (3), is described starting from (S)-(-)-perillyl alcohol (5). The successful route proceeded by oxidative cleavage of 5 to ester aldehyde 30 which was protected, reduced, and homologated to the acetylene 34, the left-hand segment of the synthetic target. Addition to the right-hand aldehyde 39 afforded alcohol 40. The carbonate derivative 41 was converted to the allenylstannane aldehyde 44, which cyclized upon treatment with BF(3).OEt(2). Oxidation with the Dess-Martin periodinane reagent followed by treatment with Et(3)N yielded allenone 45. Allenone 45 cyclized to furan 46 in the presence of catalytic AgNO(3) on silica gel. Brief exposure to p-TsOH effected elimination of the OMOM ether, affording the diastereomeric (Z)-vinylfuran carbonates 47 and 49. Saponification of the former led to alcohol 48, which was converted to the final product by sequential treatment with (CF(3)CO)(2)O, then Pd(PPh(3))(4) and CO in THF-H(2)O, and then AgNO(3) on silica gel. The resulting product, 57, was identical to natural rubifolide on the basis of spectral comparison. The optical rotation was equal and opposite in sign to that of the natural material. A second, but unsuccessful approach is also described.

Journal Article↗
PubMed1999

Total Synthesis of the Alkoxydioxines (+)- and (-)-Chondrillin and (+)- and (-)-Plakorin via Singlet Oxygenation/Radical Rearrangement.

The sequential application of singlet oxygenation and peroxyl radical rearrangement provides an asymmetric entry to 4-peroxy-2-enols and 4-peroxy-2-enones. Enantiomerically enriched 2-hydroperoxy-3-alkenols, obtained via hydroxyl-directed addition of (1)O(2) to Z-allylic alcohols, undergo stereospecific radical rearrangement to form 4-hydroperoxy-2-alkenols. The yields of the rearrangement are improved in the presence of excess tert-butyl hydroperoxide, which limits dimerization of the substrate peroxyl radicals. However, the rearrangement equilibrium is unaffected by the presence of polar co-solvents or by the incorporation of a group able to selectively hydrogen bond to the product hydroperoxide. Photoisomerization of the (E)-4-hydroperoxy-2-enone rearrangement products results in irreversible ring closure to furnish diastereomeric mixtures of enantiomerically enriched dioxinols. The strategy is applied to the total synthesis of the alkoxydioxine natural products chondrillin and plakorin. Comparison of the optical rotation of the synthetic material against literature reports indicates that the natural products are either enantiomerically pure or highly enriched in one enantiomer. In addition, our results conclusively demonstrate that the reported configuration of chondrillin is in error.

Journal Article↗
PubMed1999

Chemistry of Unique Chiral Olefins. 4. Theoretical Studies of the Racemization Mechanism of trans- and cis-1,1',2,2',3,3',4,4'-Octahydro-4,4'-biphenanthrylidenes.

The minimum energy conformations and racemization barriers for the chiral sterically overcrowded helical alkenes, trans- and cis-1,1',2,2',3,3',4,4'-octahydro-4,4'-biphenanthrylidenes (1 and 2), are reported. The trans-1 and cis-2 isomers can each adapt three different conformations, (P,P) and (M,M) (an enantiomeric pair) and an achiral (P,M) meso form, of which only the chiral isomers were obtained by synthesis. The conformations and heats of formation of (M,M)-(E)-1, (P,M)-(E)-1, (M,M)-(Z)-2, and (P,M)-(Z)-2 isomers were determined by MOPAC AM1 calculations. The racemization process for both the trans- and cis- isomers is postulated to occur via the (P,M) isomers by two successive inversions of the cyclohexenyl ring; (M,M) <--> (P,M) <--> (P,P). The (M,M) --> (P,M) and reverse (P,M) --> (M,M) isomerizations were simulated by reaction path calculations, providing the molecular structure and the activation energy of the transition state for each isomerization. For each racemization process, the activation enthalpy (DeltaH()) was calculated as 23.9 and 19.9 kcal mol(-)(1) for trans-olefin 1 and cis-olefin 2, respectively. These values reasonably agree with the experimental values obtained by temperature-dependent circular dichroism, optical rotation, and (1)H NMR magnetization transfer measurements: DeltaH() = 24.6 and 20.8 kcal mol(-)(1) for trans-olefin 1 and cis-olefin 2, respectively. While the racemization of cis-isomer 2 is controlled by the steric interaction of H5 with C4'a and C4'b, the surprisingly high barrier for trans-olefin 1 is due to the severe steric interaction between H5 and H3'alpha and/or H3'beta protons.

Journal Article↗
PubMed2000

Asymmetric Samarium-Reformatsky Reaction of Chiral alpha-Bromoacetyl-2-oxazolidinones with Aldehydes.

The samarium(II) iodide mediated asymmetric Reformatsky-type reaction of chiral 3-bromoacetyl-2-oxazolidinones with various aldehydes was studied. A series of chiral 4-substituted 2-oxazolidinones 1-3 and 5,5-disubstituted "SuperQuat" oxazolidinones 4-5 were employed as chiral auxiliaries of the alpha-bromoacetic acid. The reaction of 1 with various aldehydes gave the alpha-unbranched beta-hydroxy carboximides in good yields with high diastereomeric excess values (up to >99% de). The majority of the reaction product derived from 5,5-diphenyl SuperQuat 5 were highly crystallinity; a single recrystallization yielding a diastereomerically pure product with the other diastereomer not detectable by spectroscopic methods. The absolute configurations of the beta-hydroxy carboximides were determined by signs of optical rotations of the corresponding known ethyl esters referring to the literature values. Hydrolytic cleavage of the appended of beta-hydroxy moieties from the auxiliary SuperQuats was readily achieved under mild conditions using lithium hydroxide; the corresponding carboxylic acids and the returned SuperQuats were obtained in good yields without any evidence of racemization. The first step of the reaction is the reduction of the alpha-bromo group to produce the samarium enolate, which adds to an aldehyde. The absolute configuration of the adduct (7i) derive from benzaldehyde was found to be R, with the samarium enolate favoring the transition state predicted from chelation control of the reagent; this is in analogy to the discussion that has been used for the corresponding titanium enolate. The stereochemistry of the reaction may be explained by incorporating the Nerz-Stormes-Thornton chair transition structure model.

Journal Article↗
PubMed2004

Ultralow interfacial tensions in an aqueous phase-separated gelatin/dextran and gelatin/gum Arabic system: a comparison.

Many protein/polysaccharide mixtures phase separate when the concentrations ofthese biopolymers are sufficiently high. One of the properties involved in this phenomenon is the interfacial tension. Here we present measurements of the interfacial tension of two different protein/polysaccharide mixtures. The protein gelatin was mixed with either dextran or gum arabic, all used in a variety of food products. The phase diagrams were constructed using optical rotation. Although both polysaccharides have the same molecular weight, the phase diagrams differed. The interfacial tension of samples, varying in the distance from the critical point, was determined using the spinning drop method. The interfacial tension was found to be in the range of 1-15 microN/m. For both systems, the scaling behavior of the interfacial tension was investigated. The investigated gelatin/dextran system gave critical exponents of 2.5+/-0.1 and 1.4+/-0.1, in reasonable agreement with the mean-field values 3 and 1.5, respectively. The gelatin/gum arabic system did not show critical behavior. For this system, the interfacial tension shows a logarithmic dependence on the distribution of the gelatin and the gum arabic molecules in the separated phases.

Dextrans↗
PubMed2006

Gelatin hydrogels cross-linked with bisvinyl sulfonemethyl. 2. The physical and chemical networks.

This paper deals with the physical and the chemical gelation of gelatin in the presence of a reactant, bisvinyl sulfonemethyl (BVSM). The strategy of this investigation is to separate the contributions of the two types of cross-links in order to deduce the resultant elasticity of the network. In addition, the question raised by several authors concerning an increase of the thermal stability of the triple helices in the presence of cross-links was examined by using several techniques. In this study, the concentration of gelatin and BVSM were kept constant, while the influence of the thermal protocols was put in evidence. The gel formation was followed by rheological, thermodynamic (microcalorimetry), and optical spectroscopy (optical rotation) measurements. The results demonstrate the large differences which arise on the storage moduli by changing the thermal protocols. Cross-linking of the networks in the presence of the triple helices induce a heterogeneous repartition of the bonds, which can form along the triple helices and at the end of the sequences. Consequently, the rubber like network obtained by denaturation of the triple helices is still reminiscent of the initial twist of the chains, and a large modulus is observed, as if rigid segments were still present (storage modulus 10 times larger than for random cross-linking). The hydrogels have an elastic modulus which is larger that the addition of the physical and chemical contributions. The interpretation of the network elasticity is based on the predominant role of the rigid rods of triple helices, where the BVSM cross-links can either modify the ratio between the apparent length and distance between rods, l/d, and/or increase the rigidity of the interchain connections, which are loose coils for the physical gels. The hydrogels investigated have a network which is still close to the percolation threshold of the physical gel, and therefore, the statistical models known for well developed networks cannot be directly validated in these experimental conditions.

Journal Article↗