Steroid-based head-to-tail amphiphiles as effective iono- and protonophores

The synthesis of five steroid-oligo(ethyleneglycol) conjugates (1-5) has been accomplished starting from commercially available epi-androsterone (8) and known 3β-[(tert-butyldiphenylsilyl)oxy]-5α-23,24-bisnorchol-16-en-6α,7β,22-triol (27). The synthetic strategy was based on a convergent approach including stereoselective C-17 side chains construction and standard coupling reactions. The activities of the head-to-tail amphiphiles, once incorporated in 95:5 egg PC/PG vesicular membranes, have been assessed by direct determination of transported species by NMR techniques (²³Na⁺) and fluorescence spectroscopy (H⁺). The sodium and proton transmembrane transport was compared to those evaluated for the polyene macrolide antibiotic amphotericin B and those shown by the known related C₂-symmetric sterol-polyether conjugates 6 and 7.     

Synthesis of the aglycone of 26-O-deacetyl pavoninin-5

The aglycone of 26-O-deacetyl pavoninin-5, (25R)-cholest-5-en-3β,15α,26-triol, 5a, was synthesized in 10 steps in 17% overall yield from diosgenin, 3. Removing mercury from the Clemmensen reduction of diosgenin 3, gave a higher yield of (25R)-cholest-5-en-3β,16β,26-triol, 4, by a method, that is also more environmentally friendly. Attempted methods for the transposition of the C-16β hydroxyl to the 15α position are described. A successful method for this transposition via the 15α-hydroxy-16-ketone, 13, using the Barton deoxygenation reaction on the 16-alcohol, 15, is reported.     

Synthesis of 3β,6α-dihydroxy-5α-cholan-23-one

Evidence for the presence of 3β,6α-dihydroxy-5α-chol-9(11)-en-23-one in the aglycone mixture from the starfish Marthasterias glacialis is provided by the synthesis of 3β,6α-dihydroxy-5α-cholan-23-one (19) and its identification in the hydrogenated aglycone mixture. The side-chain is constructed from the 23,24-dinorcholanol (13) by reaction of the 22-tosylate (16) with the acetylide anion, followed by hydration of the resulting 23-yne (17).     

A new route for the preparation of the 22,23-dioxocholestane side chain from diosgenin and its application to the stereocontrolled construction of the 22R,23S-diol function

The new (22R,23S,25R)-3β,16β,26-triacetoxy-cholest-5-ene-22,23-diol (11a) was synthesized from diosgenin (3) through a synthetic route based on chemoselective RuO₄ oxidation of (25R)-3β,16β-diacetoxy-23-ethyl-23¹,26-epoxycholesta-5,23(23¹)-dien-22-one (9) that afforded (20S,25R)-3β,16β,26-triacetoxycholest-5-ene-22,23-dione (10) which was stereoselectively reduced using NaBH₄. Compound 9 was obtained from the known isomeric 22,26-epoxycholest-5-ene steroidal skeleton 8b by treatment with p-TsOH in toluene, amberlyst-15 or directly from diosgenin by treatment with BF₃·OEt₂/Ac₂O. Chemoselective reduction of the 23-keto group of 10, was attained using NaBH₄/ZnCl₂ at −70 °C to give 23S-14. The NMR spectra of all compounds were unambiguously assigned based on one and two dimensional experiments and the C-22 and C-23 stereochemistry in the diacetate derivative 11b, as well as the structure of epoxycholestene 9 were further established by X-ray diffraction analyses. The new route for the functionalization of the side chain of diosgenin can find application in the synthesis of norbrassinosteroid analogues.     

Biotransformation of two stemodane diterpenes by Mucor plumbeus

The microbiological transformation of 13α,17-dihydroxy-stemodane (2) by the fungus Mucor plumbeus afforded 13α,17,19-trihydroxy-stemodane (3), 3β,13α,17-trihydroxy-stemodane (5), 3-oxo-13α,17-dihydroxy-stemodane (7), 7α,13α,17,19-tetrahydroxy-stemodane (8), 3β,11α,13α,17-tetrahydroxy-stemodane (10), 3β,7α,13α,17-tetrahydroxy-stemodane (12), 3β,8β,13α,17-tetrahydroxy-stemodane (14), 2α,13α,17-trihydroxy-stemodane (16), 2α,13α,17,19-tetrahydroxy-stemodane (17), 2α,3β,13α,17-tetrahydroxy-stemodane (20) and 3β,11β,13α,17-tetrahydroxy-stemodane (22), whilst the incubation of 13α,14-dihydroxy-stemodane (25) gave 3β,13α,14-trihydroxy-stemodane (28), 2α,13α,14-trihydroxy-stemodane (29) and 13α,14,19-trihydroxy-stemodane (30). Preference for hydroxylations of ring A at C-2(α), C-3(β) and C-19 were observed in both incubations. An interesting rearrangement of 13α,14α-dihydroxy-stemodanes to 14-oxo derivatives with an unusual carbon framework has been observed under acetylation conditions. We have named this skeleton prestemodane, which, as a hydrocarbon ion, had been postulated as a biogenetic precursor of stemodane.     

Studies on Wittig rearrangement of furfuryl ethers in steroidal side chain synthesis

Wittig rearrangement of 17(20)-ethylidene-16-furfuryloxy steroids 5-8 was examined. Reaction of 17E(20)-ethylidene-16α-furfuryloxy steroid 5 with t-BuLi in THF afforded (20S,22S)- and (20S,22R)-22-hydroxy steroids 9, 10 and 17Z(20)-ethylidene-16α-(2-furyl)hydroxymethyl steroid 11 in 61, 28, and 9% yields, respectively. Base treatment of 17E(20)-ethylidene-16β-furfuryloxy steroid 7 gave (20R,22R)-22-hydroxy steroid 13 and 17Z(20)-ethylidene-16β-(2-furyl) hydroxymethyl steroid 14 in 60 and 17% yields. In contrast, 17Z(20)-ethylidene-16-furfuryloxy steroids 6, 8 led to the corresponding 2,3-rearranged products in low yields (25% for (20R,22S)-22-hydroxy steroid 12; 31% for (20S,22R)-22-hydroxy steroid 10). Both (20S,22S)- and (20S,22R)-22-hydroxy steroids 9, 10 were converted by catalytic hydrogenation into known compounds 16, 17, key intermediates for the synthesis of biologically active steroids.     

A new taxane composed of two N-formyl rotamers from Taxus canadensis

A taxane with an amino-side chain on C-5 was identified for the first time from rooted cuttings of the Canadian yew, Taxus canadensis. The structure was characterized as 7β,9α,10β,13α-tetraacetoxy-5α-[3′-(N-formyl-N-methylamino)-3′-phenylpropanoyl]oxytaxa-4(20),12-diene (1) on the basis of 1D-, 2D-NMR data, and HR-FABMS analyses. The spectra revealed that in CDCl₃ solution 1 was composed of two rotamers (1a and 1b) in a ratio of approximately 2:1.     

Nucleophilic substitution approach to 4′-substituted thymidines by employing 4′-benzenesulfonyl leaving group

Synthesis of 4′-substituted thymidines was investigated based on nucleophilic substitution using organosilicon and organoaluminum reagents. Two substrates having a benzenesulfonyl leaving group at the 4′-position were prepared for this purpose: 1-[4-benzenesulfonyl-3,5-bis-O-(tert-butyldimethylsilyl)-2-deoxy-α-l-threo-pentofuranosyl]thymine (8α) and the 4′-(benzenesulfonyl)thymidine derivative (8β). The reaction of 8α with organosilicon reagents (Me₃SiCH₂CHCH₂ and Me₃SiN₃) in combination with SnCl₄ gave preferentially the 4′-substituted β-d-isomer: the 4′-allyl (12β) and 4′-azido (15β) derivatives, respectively. The reaction of 8α with AlMe₃, however, gave the 4′-methyl-α-l-isomer (16α) as the major product, presumably through an ion pair mechanism. By employing the substrate 8β in this reaction, the 4′-methylthymidine derivative (16β) was obtained exclusively in high yield. The 4′-ethyl (20β) and 4′-cyano (24β) derivatives were also synthesized by reacting 8β with the respective organoaluminum reagent.     

An unprecedented gedunin rearrangement reaction converts a methyl group into the methylene group of a cyclopropyl ring

Herein we describe an unprecedented formation of a cyclopropane ring through the conversion of a methyl group that was not functionalized for the purpose. In a one-step reaction, 7-deacetoxy-7α-hydroxygedunin (4) afforded two new gedunin derivatives, namely 7-deacetoxy-13,14,18-cyclopropyl-7α,15β, 17ξ-trihydroxy-gedu-16-oic acid (7) and 7-deacetoxy-9,11-en-7α,15β-dihydroxygedunin (8) along with the known 7-deacetoxy-7,9-diene-15β-hydroxygedunin (5).     

A facile stereoselective synthesis of difunctionalized 1,3-dienes containing tin and halogen via hydrozirconation of (Z)-3-(tributylstannyl)alk-3-en-1-ynes

Sonogashira coupling of (E)-α-iodovinylstannanes 1 with (trimethylsilyl)acetylene gave (Z)-1-(trimethylsilyl)-3-(tributylstannyl)alk-3-en-1-ynes 2, which underwent a desilylation reaction to afford (Z)-3-(tributylstannyl)alk-3-en-1-ynes 3 in high yields. (1E,3Z)-1-Halo-3-(tributylstannyl)-substituted 1,3-dienes 5 could be synthesized stereoselectively via hydrozirconation of (Z)-3-(tributylstannyl)alk-3-en-1-ynes 3, followed by trapping with iodine or NBS.     

Bis-spiro-azaphilones and azaphilones from the fungi Chaetomium cochliodes VTh01 and C. cochliodes CTh05

Four new dimeric spiro-azaplilones, cochliodones A-D (1-4), two new azaphliones, chaetoviridines E and F (5 and 6), a new epi-chaetoviridin A (7), together with five known compounds, chaetoviridin A (8), ergosterol (9), chaetochalasin A (10), 24(R)-5α,8α-epidioxyergosta-6-22-diene-3β-ol (11), and ergosterol-β-d-glucoside (12) were isolated from the fungi Chaetomium cochliodes VTh01 and C. cochliodes CTh05. Structures and stereochemistry of the atropisomers 1-3 were determined by single-crystal X-ray diffraction analysis. Compounds 5, 10, and 11 exhibited antimalarial activity against Plasmodium falciparum, while 3, 5, 6, 10, and 11 showed antimycobacterial activity against Mycobacterium tuberculosis. In addition, 5 and 6 also showed cytotoxicity against the KB, BC1, and NCI-H187 cell lines.     

Synthesis of bulky tris[(dimethyl(alkyl/aryl)silyl)methyl]stannanes as radical based reducing agents

The synthesis of novel bulky tris[dimethyl(ethyl/benzyl/p-tolyl/α-naphthyl)silylmethyl]stannanes (1-4) is described. Alkylation of SnCl₄ with Me₂(ethyl/p-tolyl)SiCH₂MgBr (10-11) gave mainly the triorganotin chlorides [(Me₂(ethyl/p-tolyl)SiCH₂)]₃SnCl 14 and 15, which were isolated by silica gel chromatography. Reduction of 14 and 15 with LiAlH₄ in THF gave the corresponding triorganotin hydrides 1 and 2, respectively. [Me₂(benzyl/α-naphthyl)SiCH₂]₃SnCl 16 and 17, generated by the alkylation of SnCl₄ with Me₂(benzyl/α-naphthyl)SiCH₂MgBr 12 and 13, were inseparable from the minor product [Me₂(benzyl/α-naphthyl)SiCH₂]₂SnCl₂18 and 19, respectively. Treatment of the mixtures of 16/18 and 17/19 with NaOH furnished the corresponding mixtures of stannoxanes, from which the hexakisdistannoxanes [Me₂(benzyl/α-naphthyl)SiCH₂]₆Sn₂O 20 and 22 were isolated from the minor dialkyltin oxide derivatives [Me₂(benzyl/α-naphthyl)SiCH₂]₂SnO in good yields. Reduction of 20 and 22 with BH₃ in THF gave [Me₂(benzyl/α-naphthyl)SiCH₂]₃SnH (3 and 4), respectively in good yields. ¹H, ¹³C, ¹¹⁹Sn, ²⁹Si NMR characteristics of the newly synthesized compounds are included.     

First cross-coupling reactions on halogenated 1H-1,2,4-triazole nucleosides

The halogenated 1H-1,2,4-triazole glycosides 6-10 were synthesized by BF₃-activated glycosylation of 3(5)-chloro-1,2,4-triazole (2), 3,5-dichloro-1,2,4-triazole (3), 3,5-dibromo-1,2,4-triazole (4), and 3(5)-bromo-5(3)-chloro-1,2,4-triazole (5) with 1,2,3,4-tetra-O-pivaloyl-β-d-xylopyranose (1). The β-anomeric major products 3-chloro-1-(2,3,4-tri-O-pivaloyl-β-d-xylopyranosyl)-1,2,4-triazole (6β), 3,5-dichloro-1-(2,3,4-tri-O-pivaloyl-β-d-xylopyranosyl)-1,2,4-triazole (7β), and 3,5-dibromo-1-(2,3,4-tri-O-pivaloyl-β-d-xylopyranosyl)-1,2,4-triazole (8β) were used as starting materials for transition metal catalyzed C-C-coupling reactions. Arylations of the triazole ring of 7β, and 8β were successful in 5-position with phenylboronic acid, 4-vinylphenylboronic acid, and 4-methoxyphenylboronic acid, respectively, under Suzuki cross-coupling conditions (products 11-17). Moreover, a Cu-catalyzed perfluoroalkylation of 8β is reported with 1-iodo-perfluorohexane yielding 3-perfluorohexyl-1-(2,3,4-tri-O-pivaloyl-β-d-xylopyranosyl)-1,2,4-triazole (18). Compound 18 was depivaloylated to the trihydroxy derivative 19. The copper-mediated reaction of 8β with Rupert's reagent gave the bis(3-bromo-1-(2,3,4-tri-O-pivaloyl-β-d-xylopyranosyl)-1,2,4-triazol-5-yl) (20).     

Reversible pyranyl complex formation and the mechanism of rearrangement to (η-6-oxocycloheptadienyl)Mn(CO)3 complexes in the reaction of β-cyclomanganated 1,5-diarylpenta-1,4-dien-3-ones and alkynes; the crystal structure of [2,4-diphenyl-6-(2-phenylethenyl)pyranyl-η]tricarbonylmanganese

[1-Phenyl-2-[(E)-3-phenylprop-2-en-1-oyl-κO]ethenyl-κC¹]tetracarbonylmanganese (1a) reacts with PhCCH in CCl₄ at room temperature to form [2,4-diphenyl-6-(2-phenylethenyl)pyranyl-η⁵]tricarbonylmanganese (2a), whose X-ray crystal structure is reported to complement that of its isomer [6-oxo-2,4,7-triphenylcyclohepta-1,4-dienyl-1,2,3,4,5-η]tricarbonylmanganese (3a), previously obtained from the reaction under reflux; but for 1a and PhCCPh the pyranyl complex cannot be isolated before rearrangement to the 3a analogue occurs. More forcing reaction conditions for 1a with Me₃SiCCH and for [1-(2-trifluoromethylphenyl)-2-[(E)-3-(2-trifluoromethylphenyl)prop-2-en-1-oyl-κO]ethenyl-κC¹]tetracarbonylmanganese (1b) with Me₃SiCCH and PhCCH give new analogues of 3a where previously only 2a analogues had been isolated.The reaction in CCl₄ under reflux of PhCCH and the β-deuterio analogue of 1a, [1-phenyl-2-[(E)-3-phenylprop-2-en-1-oyl-3d-κO]ethenyl-κC¹]tetracarbonylmanganese, gave deuteriated 3a with exo-D at the α-carbon, C7. This is inconsistent with the Mn-mediated Ph migration mechanism originally proposed to accommodate the endo position of Ph in 3a, and instead it implicates a cyclopropyl carbonyl-addition intermediate or a cyclopropyl acyl-substitution transition state in the key rearrangement step for 2a → 3a.     

Synthesis of novel ferrocene labelled steroidal derivatives via palladium-catalysed carbonylation. X-ray structure of 17-(N-(4′-((2-ferrocenyl-ethenyl)-carbonyl)-phenyl)carbamoyl)-5α-androst-16-ene

Homogeneous catalytic carbonylation of some representative steroidal substrates (alkenyl iodides/enol triflates 1-5) has been carried out in the presence of (E)-1-(4′-aminophenyl)-3-ferrocenyl-prop-2-en-1-one (6) as the nucleophile. The products 1a-4a were obtained in moderate to good yield (43-75%) and were characterised with various spectroscopic methods (¹H-, ¹³C NMR, IR, MS).The solid state structure of 17-(N-(4′-((2-ferrocenyl-ethenyl)-carbonyl)-phenyl)-carbamoyl)-5α-androst-16-ene (1a) has also been determined by X-ray crystallography.     

Syntheses of rac/meso-{PhP(3-t-Bu-C5H3)2}-Zr{RN(CH2)3NR}, structural analyses of rac-{PhP(3-t-Bu-C5H3)2}Zr{RN(CH2)3NR} (where R is SiMe3 or Ph), and meso to rac isomerization

Syntheses of rac/meso-{PhP(3-t-Bu-C₅H₃)₂}Zr{Me₃SiN(CH₂)₃NSiMe₃} (rac-3/meso-3) and rac/meso-{PhP(3-t-Bu-C₅H₃)₂}Zr{PhN(CH₂)₃NPh} (rac-4/meso-4) were achieved by metallation of K₂[PhP(3-t-Bu-C₅H₃)₂] · 1.3 THF (2) with Zr{RN(CH₂)₃NR}Cl₂(THF)₂ (where R = SiMe₃ or Ph, respectively) using ethereal solvent. These isomeric pairs were characterized by ¹H, ¹³C{¹H}, and ³¹P{¹H} NMR spectroscopy; rac-3 and rac-4 were also examined via single crystal X-ray crystallography. The structures of rac-3 and rac-4 are notable in the tendency of the cyclopentadienyl rings towards η³ coordination. While isolated samples of rac-3/meso-3 and rac-4/meso-4 slowly isomerize in tetrahydrofuran-d₈ to equilibrium ratios, the isomerization rate for 3 is more than 15-fold greater than that for 4. In addition, equilibrium ratios are rapidly reached when isolated samples of rac-3/meso-3 and rac-4/meso-4 are exposed to tetrabutylammonium chloride in tetrahydrofuran-d₈ solvent. We propose that a nucleophile (either chloride or the phosphine interannular linker) brings about dissociation of one cyclopentadienyl ring, thus promoting the rac/meso isomerization mechanism.     

Concise synthesis of di- and trisaccharides related to the O-antigens from Shigella flexneri serotypes 6 and 6a,based on late stage mono-O-acetylation and/or site-selective oxidation

Shigella flexneri serotypes 6 and 6a are closely related bacteria causing shigellosis in humans. Their O-antigens are {→4)-β-d-GalpA-(1→3)-β-d-GalpNAc-(1→2)-[3Ac/4Ac]-α-l-Rhap-(1→2)-α-l-Rhap-(1→}_n acidic polysaccharides ({AB_AcCD}_n), which only differ in the degree of O-acetylation. A concise synthesis of two disaccharides (BC, B_AcC) and four trisaccharides, representing portions and/or analogs of the O-antigens, is described. A protected intermediate compatible with late stage 3_C-O-acetylation, and/or galactosyl (A°) to galacturonic acid (A) conversion, was designed and assembled from trichloroacetimidate and thioglycoside donors tuned for high yielding glycosylation and excellent stereocontrol. The galacturonic moiety was efficiently introduced from galactose using a TEMPO/NaOCl/NaClO₂-based oxidation protocol optimized for full compatibility with sensitive moieties, such as allyl ethers and acetates. Final Pd/C-mediated deprotection provided the targets, including the propyl glycoside AB_AcC, its non O-acetylated counterpart ABC, and the non acidic analogs A°B_AcC and A°BC. The BC and ABC oligosaccharides are also portions of the O-antigen from Escherichia coli O147, which causes diarrhea in pigs.     

Biotransformation of triptolide by Cunninghamella blakesleana

Biotransformation of triptolide 1 by Cunninghamella blakesleana (AS 3.970) was carried out. Seven biotransformation products were obtained and four of them were characterized as new compounds. On the basis of their NMR and mass spectral data, their structures were characterized as 5α-hydroxytriptolide 2, 1β-hydroxytriptolide 3, triptodiolide 4, 16-hydroxytriptolide 5, triptolidenol 6, 19α-hydroxytriptolide 7 and 19β-hydroxytriptolide 8. All the new transformed products (2, 3, 7 and 8) were found to exhibit potent in vitro cytotoxicity against some human tumor cell lines.     

Synthesis of branched tri- to pentasaccharides representative of fragments of Shigella flexneri serotypes 3a and/or X O-antigens

Fragments of the {2)-[α-d-Glcp-(1→3)]-α-l-Rhap-(1→2)-α-l-Rhap-(1→3)-[Ac→2]-α-l-Rhap-(1→3)-β-d-GlcpNAc-(1→}_n ((E)AB_AcCD)_n polymer were synthesized. D(E)A, CD(E)A, _AcCD(E)A were obtained according to a linear strategy, whereas BCD(E)A and B_AcCD(E)A were derived from the condensation of appropriate BC and D(E)A building blocks. Oligosaccharides were synthesized as their propyl glycoside, relying on (i) the efficient trichloroacetimidate chemistry, (ii) a common EA allyl glycoside, and (iii) a 2-trichloroacetamido-d-glucopyranose precursor to residue D. Final Pd/C-mediated deprotection, run under a high pressure of hydrogen, ensured O-acetyl stability. All targets are parts of the O-antigen of Shigella flexneri 3a, a prevalent serotype. Non-O-acetylated oligosaccharides are shared by the S. flexneri serotype X O-antigen.     

N-Nosyl as a stereoselectivity-improving and easily removable group in the O-glycosylation of d-allal and d-galactal-derived allyl aziridines. Stereospecific synthesis of 4-amino-2,3-unsaturated-O-glycosides

The glycosylation of alcohols, phenol, and partially protected monosaccharides with the diastereoisomeric d-allal and d-galactal-derived N-nosyl aziridines 2α and 2β leads to the corresponding 4-N-(nosylamino)-2,3-unsaturated-α-O- (6α) and β-O-glycosides and disaccharides (6β), respectively, in a stereospecific substrate-dependent O-glycosylation process. The N-(nosylamino) group of 6α and 6β can easily be deprotected to give the corresponding 4-amino-2,3-unsaturated-O-glycosides 7α and 7β, with an increased value to our glycosylation protocol.