The microbiological transformation of steroidal saponins by Curvularia lunata

The microbiological transformation of polyphyllin I (compound I), polyphyllin III (compound II), polyphyllin V (compound III) and polyphyllin VI (compound IV) by Curvularia lunata into their corresponding subsaponins, for example, diosgenin-3-O-α-l-arabinofuranosyl (1→4)-β-d-glucopyranoside (compound V), diosgenin-3-O-α-l-rhamnopyranosyl (1→4)-β-d-glucopyranoside (compound VI), diosgenin-3-O-β-d-glucopyranoside (compound VII) and pennogenin-3-O-β-d-glucopyranoside (compound VIII), were studied in this paper. Curvularia lunata is able to hydrolyze terminal rhamnosyls that are linked by 1→2 C- bond to sugar residues of steroidal saponins at C-3 position with high activity and regioselectivity.     

Simple preparation of phenylpropenoid β-d-glucopyranoside congeners by Mizoroki-Heck type reaction using organoboron reagents

Palladium(II)-catalyzed carbon-carbon bond formation between allyl 2,3,4,6-tetra-O-acetyl-β-d-glucopyranoside (3) and arylboronic acid congeners gave the corresponding cinnamyl 2,3,4,6-tetra-O-acetyl- β-d-glucopyranosides (4a-m) in good yield. Among them, coupling products 4a-m were converted to not only the naturally occurring phenylpropenoid β-d-glucopyranoside analogues (1a-e) but also the unnaturally ones (1f-m).     

Organofluorine compounds and fluorinating agents

Starting with 1,2,4,6-tetra-O-acetyl-3-O-dodecyl-β-d-glucose (1), mixed alkyl-perfluoroalkyl substituted sugar derivatives with an anomeric perfluoroalkylthio group and an O-alkyl group in the 3 position were synthesized via 2,4,6-tri-O-acetyl-3-O-dodecyl-1-thio-β-d-glucose (4). The latter was S-perfluorohexylated with 1-iodoperfluorohexane in a dithionite initiated reaction yielding perfluorohexyl 2,4,6-tri-O-acetyl-3-O-dodecyl-1-thio-β-d-glucopyranoside (5). Experiments with the aim compound 5 completely to deacetylate ended in surprising results. Thus, methanolic methanolate solution produced the orthoester 7 as the result of α-fluoride replacement by methoxy groups as well as the methyl glucoside 8 as the result of a transglycosylation reaction. Alumina supported cesium fluoride cleaved regioselectively the two acetyl groups in the 4- and 6-position yielding perfluorohexyl 2-O-acetyl-3-O-dodecyl-1-thio-β-d-glucopyranoside (10). A complete deacetylation of 5 to amphiphile 11 succeeded only with methanolic tert-butanolate. However, the products 8 and 10 were likewise formed.     

Polyhydroxylated pyrrolidines, III. Synthesis of new protected 2,5-dideoxy-2,5-iminohexitols from d-fructose

The readily available 3-O-benzoyl-4-O-benzyl-1,2-O-isopropylidene-β-d-fructopyranose (6) was straightforwardly transformed into 5-azido-3-O-benzoyl-4-O-benzyl-5-deoxy-1,2-O-isopropylidene-β-d-fructopyranose (8), after treatment under modified Garegg's conditions followed by reaction of the resulting 3-O-benzoyl-4-O-benzyl-5-deoxy-5-iodo-1,2-O-isopropylidene-α-l-sorbopyranose (7) with lithium azide in DMF. O-debenzoylation at C(3) in 8, followed by oxidation and reduction caused the inversion of the configuration to afford the corresponding β-d-psicopyranose derivative 11 that was transformed into the related 3,4-di-O-benzyl derivative 12. Cleavage of the acetonide of 12 to give 13 followed by O-tert-butyldiphenylsilylation afforded a resolvable mixture of 14 and 15. Compound 14 was transformed into (2R,3R,4S,5R)- (17) and (2R,3R,4S,5S)-3,4-dibenzyloxy-2′,5′-di-O-tert-butyldiphenylsilyl-2,5-bis(hydroxymethyl)pyrrolidine (18) either by a tandem Staudinger/intramolecular aza-Wittig process and reduction of the resulting intermediate Δ²-pyrroline (16), or only into 18 by a high stereoselective catalytic hydrogenation. When 15 was subjected to the same protocol, (2S,3S,4R,5R)- (21) and (2R,3S,4R,5R)-3,4-dibenzyloxy-2′-O-tert-butyldiphenylsilyl-2,5-bis(hydroxymethyl)pyrrolidine (22) were obtained, respectively.     

Simple synthesis of phenylpropenoid β-d-glucopyranoside congeners based on Mizoroki-Heck type reaction of organoboron reagents

Palladium(II)-catalyzed carbon-carbon bond formation between allyl 2,3,4,6-tetra-O-acetyl-β-d-glucopyranoside (3) and phenylboronic acid congeners gave the phenylpropenoid 2,3,4,6-tetra-O-acetyl-β-d-glucopyranoside (4a-f) in good yield. Among them, compounds 4a-c were converted to the naturally occurring phenylpropenoid β-d-glucopyranoside analogues (1a-c).     

Total synthesis of apigenin 7,4′-di-O-β-glucopyranoside, a component of blue flower pigment of Salvia patens, and seven chiral analogues

We have succeeded in the first total synthesis of apigenin 7,4′-di-O-β-d-glucopyranoside (1a), a component of blue pigment, protodelphin, from naringenin (2). Glycosylation of 2 according to Koenigs-Knorr reaction provided a monoglucoside 4a in 80% yield, and this was followed by DDQ oxidation to give apigenin 7-O-glucoside (12a). Further glycosylation of 4′-OH of 12a with 2,3,4,6-tetra-O-acetyl-α-d-glucopyranosyl fluoride (5a) was achieved using a Lewis acid-and-base promotion system (BF₃·Et₂O, 2,6-di-tert-butyl-4-methylpyridine, and 1,1,3,3-tetramethylguanidine) in 70% yield, and subsequent deprotection produced 1a. Synthesis of three other chiral isomers of 1a, with replacement of d-glucose at 7 and/or 4′-OH by l-glucose (1b-d), and four chiral isomers of apigenin 7-O-β-glucosides (6a,b) and 4′-O-β-glucosides (7a,b) also proved possible.     

Cyclooxygenase-2 enzyme inhibitory withanolides from Withania somnifera leaves

Four novel withanolide glycosides and a withanolide have been isolated from the leaves of Withania somnifera. The structures of the novel compounds were elucidated as physagulin D (1→6)-β-d-glucopyranosyl-(1→4)-β-d-glucopyranoside (1), 27-O-β-d-glucopyranosyl physagulin D (2), 27-O-β-d-glucopyranosyl viscosalactone B (3), 4,16-dihydroxy-5β, 6β-epoxyphysagulin D (4), and 4-(1-hydroxy-2,2-dimethylcyclo-propanone)-2,3-dihydrowithaferin A (5) on the basis of 1D-, 2D NMR and MS spectral data. In addition, seven known withanolides withaferin A (6), 2,3-dihydrowithaferin A (7), viscosalactone B (8), 27-desoxy-24,25-dihydrowithaferin A (9), sitoindoside IX (10), physagulin D (11), and withanoside IV (12) were isolated. These withanolides were assayed to determine their ability to inhibit cycloxygenase-1 (COX-1) and cyclooxygenase-2 (COX-2) enzymes and lipid peroxidation. The withanolides tested, except compound 9, showed selective COX-2 enzyme inhibition ranging from 9 to 40% at 100 μg/ml. Compounds 4, 10 and 11 also inhibited lipid peroxidation by 40, 44 and 55%, respectively. The inhibition of COX-2 enzyme by withanolides is reported here for the first time.     

δ-Lactone formation from δ-hydroxy-trans-α,β-unsaturated carboxylic acids accompanied by trans-cis isomerization: synthesis of (−)-tetra-O-acetylosmundalin

(±)-(4,5-anti)-4-Benzyloxy-5-hydroxy-(2E)-hexenoic acid 6 was subjected to δ-lactonization in the presence of 2,4,6-trichlorobenzoyl chloride and pyridine to give the α,β-unsaturated-δ-lactone congener (±)-7 (87% yield) accompanied by trans-cis isomerization. This δ-lactonization procedure was applied to the chiral synthesis of (+)-(4S,5R)-7 or (−)-(4R,5S)-7 from the chiral starting material (+)-(4S,5R)-6 or (−)-(4R,5S)-6. Deprotection of the benzyl group in (+)-(4S,5R)-7 or (−)-(4R,5S)-7 by the AlCl₃/m-xylene system gave the natural osmundalactone (+)-(4S,5R)-5 or (−)-(4R,5S)-5 in good yield, respectively. Condensation of (−)-(4R,5S)-5 and tetraacetyl-β-d-glucosyltrichloroimidate 22 in the presence of BF₃·Et₂O afforded the condensation product (−)-8 (97% yield), which was identical to tetra-O-acetylosmundalin (−)-8 derived from natural osmundalin 9.     

Synthesis of (3′R,5′S)-3′-hydroxycotinine using 1,3-dipolar cycloaddition of a nitrone

To synthesize (3′R,5′S)-3′-hydroxycotinine [(+)-1], the main metabolite of nicotine (2), cycloaddition of C-(3-pyridyl)nitrones 3a, 3c, and 15 with (2R)- and (2S)-N-(acryloyl)bornane-10,2-sultam [(2R)- and (2S)-8] was examined. Among them, l-gulose-derived nitrone 15 underwent stereoselective cycloaddition with (2S)-8 to afford cycloadduct 16, which was elaborated to (+)-1.     

Pheromone synthesis. Part 245: Synthesis and chromatographic analysis of the four stereoisomers of 4,8-dimethyldecanal, the male aggregation pheromone of the red flour beetle, Tribolium castaneum

All four stereoisomers of 4,8-dimethyldecanal (1) were synthesized from the enantiomers of 2-methyl-1-butanol and citronellal. Enantioselective GC analysis enabled separation of (4R,8R)-1 and (4R,8S)-1 from a mixture of (4S,8R)-1 and (4S,8S)-1, when octakis-(2,3-di-O-methoxymethyl-6-O-tert-butyldimethylsilyl)-γ-cyclodextrin was employed as a chiral stationary phase. Complete separation of the four stereoisomers of 1 on reversed-phase HPLC at −54 °C was achieved after oxidation of 1 to the corresponding carboxylic acid 12 followed by its derivatization with (1R,2R)-2-(2,3-anthracenedicarboximido)cyclohexanol, and the natural 1 was found to be a mixture of all the four stereoisomers.     

Absolute configuration of gomadalactones A, B and C, the components of the contact sex pheromone of Anoplophora malasiaca

Absolute configuration of gomadalactones A (1), B (2) and C (3), the pheromone components of the white-spotted longicorn beetle (Anoplophora malasiaca) was assigned as (1S,4R,5S)-1, (1R,4R,5R)-2 and (1S,4R,5S,8S)-3 by comparing their published CD spectra with those of (1R,5R)-(+)-4,4,8-trimethyl-3-oxabicyclo[3.3.0]oct-7-ene-2,6-dione (4) and (1S,5R,8S)-(+)-4,4,8-trimethyl-3-oxabicyclo[3.3.0]octane-2,6-dione (5) prepared from (R)-(−)-carvone (6).     

Separation and identification of three epimeric pairs of new C-glucosyl anthrones from Rumex dentatus by on-line high performance liquid chromatography–circular dichroism analysis

Six C-glucosyl anthrones were characterized as three pairs of epimers by on-line high performance liquid chromatography–circular dichroism (HPLC–CD) analysis and isolated from the roots of Rumex dentatus by column chromatography. Their structures were elucidated by mass spectrometry, nuclear magnetic spectroscopy and HPLC–CD analysis. They are 10R-C-β-d-glucosyl-10-hydroxyemodin-9-anthrone (rumejaposide E, 1) and 10S-C-β-d-glucosyl-10-hydroxyemodin-9-anthrone (rumejaposide F, 2), 10R-C-β-d-glucosylemodin-9-anthrone (rumejaposide G, 3) and 10S-C-β-d-glucosylemodin-9-anthrone (rumejaposide H, 4), 10S-C-β-d-glucosyl-10-hydroxychrysophanol-9-anthrone (cassialoin, 5) and 10R-C-β-d-glucosyl-10-hydroxychrysophanol-9-anthrone (rumejaposide I, 6). Rumejaposides F–I (2–4 and 6) were new C-glucosyl anthrones. Rumejaposide E (1) and cassialoin (5) were isolated for the first time in Rumex plants. On-line HPLC–UV–CD analysis was a useful tool for structure elucidating epimeric C-glycosides anthrones 3–6 because of the poor stability of the pure isomers (3 and 4) and the minute quantity of 5 and 6 in the mixture.     

Unusual α-glycosylation with galactosyl donors with a C2 ester capable of neighboring group participation

Glycosylation of 4-methoxyphenyl 2,3,6-tri-O-benzoyl-β-d-glucopyranoside (2) with isopropyl 3-O-allyl-2,4,6-tri-O-benzoyl- (9) or 6-O-allyl-2,3,4-tri-O-benzoyl-1-thio-β-d-galactopyranoside (7) as the donor, afforded an α- and β-linked mixture, whereas with isopropyl 3-O-chloroacetyl-2-O-benzoyl-4,6-O-benzylidene- (13) and isopropyl 3-O-allyl-2-O-benzoyl-4,6-O-benzylidene-1-thio-β-d-galactopyranoside (15) as the donor, glycosylation of 2 gave α-linked products only, indicating that 4,6-O-benzylidenation led to α-stereoselectivity in spite of the C2 ester capable of neighboring group participation. Using 15 as the donor, glycosylation of mannose derivatives with 2- or 3-OH's, glucose with 2- or 3-OH's, galactose with 2-, or 3-, or 4-OH's, glucosamine and glucuronic acid with a 4-OH, and a lactose derivative with a 4-OH, also furnished α-linked products. However, when using 15 as the donor, glycosylation of aglycon alcohol or sugars with 6-OH's yielded normal β-linked products.     

Preparation of both enantiomers of β-(3,4-dihydroxybenzyl)-β-alanine, higher homologues of Dopa

β²-(3,4-Dihydroxybenzyl)-β-alanine [β²-Homo-Dopa, 1] is a novel β-amino acid homologue of Dopa, the most successful therapeutic agent in the treatment of Parkinson's disease. Enantioenriched (R)-1 and (S)-1 were obtained via the diastereoselective alkylation of enantiopure pyrimidinone (R)- and (S)-3, chiral derivatives of β-alanine, with veratryl iodide. The major diastereomeric products (2S,5R)-4 and (2R,5S)-4 were hydrolyzed with 57% HBr, and the desired β-amino acids were purified by silica gel chromatography. Alternatively, enantioenriched (R)- and (S)-1 were prepared by means of the highly diastereoselective alkylation (3,4-dimethoxybenzyl iodide) of open-chain β-aminopropionic acid derivatives (R,R,S)-8 and (S,S,R)-8 containing the chiral auxiliary α-phenylethylamine. Finally, nearly enantiopure (R)- and (S)-1 were obtained by resolution of racemic N-benzyloxycarbonyl-2-(3,4-dibenzyloxybenzyl)-3-aminopropionic acid, rac-12, with (R)- or (S)-α-phenylethylamine, followed by catalytic hydrogenolysis.     

Organofluorine compounds and fluorinating agents : Part 28. New perfluoroalkyl substituted chiral mesogens

The perfluorohexyl-aryl-thioethers 3 and 4, building blocks for the synthesis of the chiral target mesogens 12-15, were prepared by dithionite-mediated S-perfluoroalkylation of the p-substituted thiophenols 1 and 2. The phenolic HO group of 3 was O-glucosylated with pentaacetyl-d-glucopyranose to 5 followed by deacetylation forming the tetrol 6 and by acetalizing with 4-(4-perfluorohexylsulfanyl-benzoyloxy)-benzaldehyde-dimethylacetal (8) generating the dihydroxy-intermediate 9. The latter contains two perfluorohexylthio chains. Alternatively, the dimethylacetal 8 was linked to p-octylphenyl-β-d-glucopyranoside (10) giving the mixed octyl/perfluorohexyl substituted p-octylphenyl-4,6-O-[4′-(4″-perfluorohexylsulfanyl)-benzoyloxy]-benzylidene-β-d-glucopyranoside (11). Compound 8 was obtained via esterification of 4 with p-hydroxy-benzaldehyde to 4-(4-perfluorohexylsulfanyl-benzoyloxy)-benzaldehyde (7). Finally, the diols 9 and 11 were dehydroxylated to 12 and 13 followed by hydrogenation yielding 14 and 15, respectively. Tetrol 6, diols 9, 11 and the non-amphiphilic compounds 7, 12-15 are thermotropic liquid crystals.     

Effect of α-fluorination on asymmetric epoxidation of trans-olefins using α-fluorinated cyclohexanone dioxiranes

Epoxidations of trans-β-methylstyrene, trans-stilbene and trans-methyl p-methoxycinnamate using chiral dioxiranes derived from both enantiopure diastereomers of α-fluoro cyclohexanones, (2S, 5R)-3a-6a and (2R, 5R)-3e-6e are studied and compared. From ab initio calculations at the HF/6-31G^∗ level of conformational inter-conversion for (2S, 5R)-D5a and (2R, 5R)-D5e dioxiranes it was found that, due to the α-fluorine atom, conformer K1 is more stable in the case of (2S, 5R)-D5a while conformer K2 is more stable in the case of (2R, 5R)-D5e. However, in both cases, the more stable conformers, K1 and K2, undergo rapid inter-conversion. Therefore, based on slow epoxidation reactions and rapid ring inversion of six-membered ring dioxiranes the Curtin-Hammett principle holds. Conformation K2 with axial fluorine having been found to be more reactive, the inversion of configuration observed for the epoxides obtained with ketones 3e-6e (compared with ketones 3a-6a) could be rationalized from competitive reactions of K2 and K1 conformations leading to simultaneous production of both (−) and (+) epoxides in the case of ketones 3e-6e.     

Polyhydroxylated pyrrolidines: synthesis from d-fructose of new tri-orthogonally protected 2,5-dideoxy-2,5-iminohexitols

The readily available 3-O-benzyl-1,2-O-isopropylidene-β-d-fructopyranose (2) was transformed into its 5-O- (3) and 4-O-benzoyl (4) derivative. Compound 4 was straightforwardly transformed into 5-azido-4-O-benzoyl-3-O-benzyl-5-deoxy-1,2-O-isopropylidene-β-d-fructopyranose (7) via the corresponding 5-deoxy-5-iodo-α-l-sorbopyranose derivative 6. Cleavage of the acetonide in 7 to give 8, followed by regioselective 1-O-silylation to 9 and subsequent catalytic hydrogenation gave a mixture of (2S,3R,4R,5R)- (10) and (2R,3R,4R,5R)-4-benzoyloxy-3-benzyloxy-2′-O-tert-butyldiphenylsilyl-2,5-bis(hydroxymethyl)pyrrolidine (12) that was resolved after chemoselective N-protection as their Cbz derivatives 11 and 1a, respectively. Stereochemistry of 11 and 1a could be determined after total deprotection of 11 to the well known DGDP (13). Compound 2 was similarly transformed into the tri-orthogonally protected DGDP derivative 18.     

Polyhydroxylated pyrrolizidines. Part 8. Enantiospecific synthesis of looking-glass analogues of hyacinthacine A5 from DADP

(1R,2S,3S,5R,7aR)-1,2-Dihydroxy-3-hydroxymethyl-5-methylpyrrolizidine[(−)-3-epihyacinthacine A₅, 1a] and (1S,2R,3R,5S 7aS)-1,2-dihydroxy-3-hydroxymethylpyrrolizidine[(+)-3-epihyacinthacine A₅, 1b] have been synthesized either by Wittig's or Horner-Wadsworth-Emmond's (HWE's) methodology using aldehydes 4 and 9, both prepared from (2S,3S,4R,5R)-3,4-dibenzyloxy-2′-O-tert-butyldiphenylsilyl-2,5-bis(hydroxymethyl)pyrrolidine (2, partially protected DADP), and the appropriate ylides, followed by cyclization through an internal reductive amination process of the resulting α,β-unsaturated ketones 5 and 10, respectively, and total deprotection.     

Regioselective Baeyer-Villiger lactonization of 2-substituted pyrrolidin-4-one. Synthesis of statine

Nine 2-substituted pyrrolidin-4-ones 4a-i were obtained via a series of functional group transformation of known prolinol 5 by facile six kinds of methodologies. The target structure of 1,3-amino alcohols 2a-i was constructed in the regioselective Baeyer-Villiger lactonization of ketones 4a-i and reduction of the resulting 4-substituted tetrahydro-1,3-oxazin-6-ones 3a-i. A new and straightforward synthesis of (3S,4S)-statine (6) has been established starting from trans-(2S,4R)-4-hydroxyproline (1).     

The first asymmetric synthesis of all four isomers of cis- and trans-3,4-dihydroxy-3,4-dihydromollugin

Asymmetric synthesis of all the four stereoisomers of cis-3,4-dihydroxy-3,4-dihydromollugins 4 and 6 and trans-3,4-dihydroxy-3,4-dihydromollugins 5 and 7 was achieved. The O-methoxymethyl mollugin derivatives were dihydroxylated to (−)- and (+)-cis-3,4-dihydroxy-3,4-dihydromollugin derivatives using both AD-mix-α and AD-mix-β. Deprotection of the MOM-ethers of cis-dihydroxy compounds resulted in the targeted stereoisomers (−)-(3R,4R)-cis-3,4-dihydroxy-3,4-dihydromollugin 4, (−)-(3R,4S)-trans-3,4-dihydroxy-3,4-dihydromollugin 5, (+)-(3S,4S)-cis-3,4-dihydroxy-3,4-dihydromollugin 6 and (+)-(3S,4R)-trans-3,4-dihydroxy-3,4-dihydromollugin 7. These routes were paved with difficulties, for example, incompatibility of the substrates with AD-mixes, the unexpected formation of trans-dihydroxy compounds and failures in deprotection protocols.