Aluminum- and boron-mediated c-glycoside synthesis from 1,2-anhydroglycosides

(C6F5)3B·(HF)n-catalyzed glycosylation of disarmed glycosyl fluorides and reverse glycosyl fluorides

(C₆F₅)₃B·(HF)_n-catalyzed glycosylation of disarmed glycosyl fluorides and reverse glycosyl fluorides with structurally diverse nucleophiles has been achieved.

The Reactivity of Thymine and Thymidine 5,6-Epoxides with Organometallic Reagents - A Route to Thymidine (6-4) Photoproduct Analogues

This report describes an efficient procedure for the generation and isolation of various thymine and thymidine 5,6-epoxides from the corresponding trans-5,6-bromohydrins by reaction with triethylamine. The quantitative isolation of the epoxides, accomplished by solvent precipitation of triethylamine hydrobromide, enabled their regiospecific ring-opening at C6 position by organometallic nucleophiles. The reaction was amenable to a broad range of alkyl, aryl, alkenyl, and alkynyl organomagnesium, -zinc, -aluminum, or -boron reagents, although the reactivity was strongly affected by the electronic effects of N3 protecting group. Additionally, the reaction featured excellent cis-diastereoselectivity providing access to C6-carbon-functionalized dihydrothymidine cis-alcohols, which are synthetic derivatives of UV-induced DNA lesions, namely, thymidine (6-4) photoproducts.

Synthesis and reactivity of 4'-deoxypentenosyl disaccharides

4-Deoxypentenosides (4-DPs) are versatile synthons for rare or higher-order pyranosides, and they provide an entry for structural diversification at the C5 position. Previous studies have shown that 4-DPs undergo stereocontrolled DMDO oxidation; subsequent epoxide ring-openings with various nucleophiles can proceed with both anti or syn selectivity. Here, we report the synthesis of α- and β-linked 4'-deoxypentenosyl (4'-DP) disaccharides, and we investigate their post-glycosylational C5' additions using the DMDO oxidation/ring-opening sequence. The α-linked 4'-DP disaccharides were synthesized by coupling thiophenyl 4-DP donors with glycosyl acceptors using BSP/Tf2O activation, whereas β-linked 4'-DP disaccharides were generated by the decarboxylative elimination of glucuronyl disaccharides under microwave conditions. Both α- and β-linked 4'-DP disaccharides could be epoxidized with high stereoselectivity using DMDO. In some cases, the α-epoxypentenosides could be successfully converted into terminal l-iduronic acids via the syn addition of 2-furylzinc bromide. These studies support a novel approach to oligosaccharide synthesis, in which the stereochemical configuration of the terminal 4'-DP unit is established at a post-glycosylative stage.

Studies on the synthesis of a sugar triazole based ligand for protein and DNA binding

Bis-triazole derivatives accomplished by [3 + 2] cycloaddition methodology were well characterized. Interaction of bis-triazoles with BSA and CT-DNA had good correlation with experimental and docking studies. Compounds showed moderate to excellent antibacterial activity.

5-(5-Aryl-1,3,4-oxadiazole-2-carbonyl)furan-3-carboxylate and new cyclic C-glycoside analogues from carbohydrate precursors with MAO-B, antimicrobial and antifungal activities

Cyclization of acyclic C-glycoside derivatives 1a,b to 2a,b as the major isomers, and 4a,b as the minor isomers were carried out. The isopropylidene derivatives 3a,b were prepared, as well as the hydrazide derivative 6, which was condensed with a variety of aldehydes to give hydrazones 7a–e which were also prepared from the compounds 12a–e. Acetylation of 7a,d gave the corresponding acetyl derivatives 8a,d, respectively. In addition, the dicarbonyl compound 9 was prepared in the hydrate form, which reacted with a number of aroylhydrazines to give the corresponding bisaroyl-hydrazones 10a–d, which were cyclized into 1,3,4-oxadiazoles 11a–d. Furthermore, two of the prepared compounds were examined to show the ability to activate MAO-B. In addition a number of prepared compounds showed antibacterial and antiviral activities.

Stereoelectronic factors in the stereoselective epoxidation of glycals and 4-deoxypentenosides

Glycals and 4-deoxypentenosides (4-DPs), unsaturated pyranosides with similar structures and reactivity profiles, can exhibit a high degree of stereoselectivity upon epoxidation with dimethyldioxirane (DMDO). In most cases, the glycals and their corresponding 4-DP isosteres share the same facioselectivity, implying that the pyran substituents are largely responsible for the stereodirecting effect. Fully substituted dihydropyrans are subject to a "majority rule", in which the epoxidation is directed toward the face opposite to two of the three groups. Removing one of the substituents has a variable effect on the epoxidation outcome, depending on its position and also on the relative stereochemistry of the remaining two groups. Overall, we observe that the greatest loss in facioselectivity for glycals and 4-DPs is caused by removal of the C3 oxygen, followed by the C5/anomeric substituent, and least of all by the C4/C2 oxygen. DFT calculations based on polarized-π frontier molecular orbital (PPFMO) theory support a stereoelectronic role for the oxygen substituents in 4-DP facioselectivity, but less clearly so in the case of glycals. We conclude that the anomeric oxygen in 4-DPs contributes toward a stereoelectronic bias in facioselectivity whereas the C5 alkoxymethyl in glycals imparts a steric bias, which at times can compete with the stereodirecting effects from the other oxygen substituents.

Total synthesis of brevenal

This Article describes the total synthesis of the marine ladder toxin brevenal utilizing a convergent synthetic strategy. Critical to the success of this work was the use of olefinic-ester cyclization reactions and the utilization of glycal epoxides as precursors to C-C and C-H bonds.

Stereoselective synthesis of beta-phenylselenoglycosides from glycals and rationalization of the selenoglycosylation processes

Beta-phenylselenoglycosides have been efficiently and stereoselectively synthesized by direct oxidative glycosylation of benzenselenolate (PhSe(-)) with glycals. A rationalization of the presently described beta-selectivity and the opposite alpha-selectivity reported by Danishefsky in the ring-opening of epoxy glycals with benzeneselenol (PhSeH) is proposed.

A method for the preparation of differentiated trans-1,2-diol derivatives with enantio- and diastereocontrol

We describe a synthetic sequence that allows for the preparation of optically active trans-1,2-diol monosilyl ether derivatives from ketones, providing a new means for retrosynthetic simplification of differentiated diol and polyol targets. The sequence involves silyl enol ether formation, Shi asymmetric epoxidation, and then regio- and stereospecific addition of hydride, methide, or higher alkylide. The tactical combination presented has not been integrated in synthetic problem solving, so far as we are aware, but has promise for broad application.

Highly enantioselective vinyl additions of vinylaluminum to ketones catalyzed by a titanium(IV) catalyst of (S)-BINOL

We report a novel asymmetric addition of vinyl group to ketones using vinylaluminum reagents catalyzed by in situ prepared Ti(O(i)Pr)(4) complexes of (S)-BINOL to afford diversified tertiary allylic alochols. Varieties of aromatic ketones bearing either an electron-donating or an electron-withdrawing substituent on the aromatic ring were examined to afford products in excellent enantioselectivities of up to 98% ee with high yields. A 10-fold scale-up reaction afforded the product in a similar yield with a comparable enantioselectivity. More importantly, additions of a variety of vinyl reagents including functionalized vinyls were demonstrated, affording tertiary allylic alcohols with good to excellent enantioselectivities of up to 96% ee.

1,3-Bis[N-sulfonyl-(1R,2S)-1,3-diphenyl-2-aminopropanol]benzene: an excellent ligand for titanium-catalyzed asymmetric AlPh3(THF) additions to aldehydes

Asymmetric AlPh(3) (THF) additions to a wide variety of aldehydes catalyzed by a titanium catalyst of 20 mol % 1,3-bis[N-sulfonyl-(1R,2S)-1,3-diphenyl-2-aminopropanol]benzene (1) are reported. The catalytic system works excellently for aromatic aldehydes bearing either an electron-donating or an electron-withdrawing substituent on the aromatic ring to afford secondary diaryl alcohols in excellent isolated yields of >or=95% and excellent enantioselectivities of >or=94% ee. The phenyl addition to cinnamaldehyde or 2-furylaldehyde gave corresponding secondary alcohols in 85% and 95% ee, respectively. For aliphatic aldehydes, increasing enantioselectivities of the addition products in terms of increasing steric sizes of aldehydes are observed, and this trend goes from the linear 1-pentanal (87% ee), the secondary cyclohexylaldehyde (95% ee) or the 2-methylpropanal (97% ee), to the tertiary 2,2-dimethylpropanal (99% ee).

Synthetic use of epoxy-sugar nucleosides

Preparation of 1',2 '-, 3 ',4 '-, and 4 ',5 '-epoxy derivatives of nucleosides and their use for the stereoselective synthesis of 1'- and 4 '-branched analogues are described.

Total synthesis of theopederin D

The total synthesis of the potent cytotoxin theopederin D has been achieved through the use of an oxidative carbon–carbon bond cleavage reaction to form an acyliminium ion in the presence of acid labile acetal groups Other key transformations include an acid mediated functionalization of a tetrahydrofuranyl alcohol in the presence of a tetrahydropyranyl alcohol, a syn -selective glycal epoxide opening, and a catalytic asymmetric aldehyde-acid chloride condensation.

syn additions to 4alpha-epoxypyranosides: synthesis of L-idopyranosides

The syn-selective addition of organozinc compounds to 4alpha-epoxypyranosides (4alpha-EPs), generated from methyl beta-D-glucoside or aminoglucoside, provides an efficient route to pyranosides with alpha-L-ido configurations, including L-iduronic acid, neosamine B, and higher monosaccharide derivatives.

Remarkably efficient enantioselective titanium(IV)-(R)-H8-BINOLate catalyst for arylations to aldehydes by triaryl(tetrahydrofuran)aluminum reagents

Novel asymmetric triarylaluminum AlAr3(THF) additions to aldehydes catalyzed by 10 mol % of the titanium(IV) complex of (R)-H8-BINOL ligand are reported. The catalytic system is extremely efficient with reactions completing within 10 min. The system applies to the most diversified aldehydes to date, and more than 20 aldehydes were examined to afford diarylmethanols having an electron-donating or an electron-withdrawing group at the 2-, 3-, or 4-position on the aryl moiety, linear or branched 1-aryl aliphatic alcohols, aryl furyl methanols, 1-aryl allylic alcohols, and, especially, 1-aryl propargylic alcohols in excellent enantioselectivities of >/=90% ee, except for the case of 1-naphthyl addition to benzaldehyde. Noteworthily, diarylmethanols in both R- and S-configurations can be obtained.