Sukhendu B. Mandal

A simple route to optically active, functionalized five-, six- and seven-membered carbocyclic derivatives

Abstract The isoxazolidinocarbocyclic derivatives 6, 7 and 13, useful as precursors for unnatural bioactive chiral carbocyclic nucleosides and for glycosidase inhibitors have been synthesized from D-glucose through intramolecular 1,3-dipolar cycloaddition.

An entry to unusual classes of nucleoside analogues

Abstract Nucleosides 6a , 6b and 16–18 having additional carbocycle or oxygen heterocycle fused to the ribose moiety have been synthesized by two routes. While 6a and 6b could be obtained from thymidine using 1 , 3-dipolar cycloaddition approach, Vorbruggen reaction on appropriate preformed carbocyclic derivatives furnished 16–18 .

Stereospecific C=β-Glycosidation and Synthesis of 4,7-Anhydro-5,6-isopropylidene-4 (S), 5(S), 6(R), 7(R)-Tetrahydroxyoxocan-2-one

Abstract Wittig reaction of 2,3-O-isopropylidene-5-O-panisyldiphenylmethyl-β-D-ribofuranose (2) with methoxycarbonyl-methylene triphenylphosphorane was accompanied by spontaneous cyclization to generate 3 stereospecifically. Deprotection of 5-OH followed by mild base hydrolysis and cyclization with Py-Ac2O-DBU then yielded 4,7-anhydro-5,6-isopropylidene-4(S), 5(S), 6(R), 7(R)-tetrahydroxyoxocan-2-one (5) in good overall yield.

Carbohydrate nitrone and nitrile oxide cycloaddition approach to chiral sulfur heterocycles and nucleosides

S-Alkenyl and -alkynyl carbohydrate derivatives were prepared from 1,2:5,6-O-diisopropylidene-α-D-allofuranose. Nitrones and nitrile oxides generated from these derivatives led to the formation of 6, 7, and 11-membered chiral sulfur heterocycles fused to isoxazolidine, isoxazoline and isoxazole rings. Some of these sulfur heterocycles were converted to nucleosides. The 11-memebered sulfur compound was found to gelate hydrocarbon solvents.

Allyloxy and propargyloxy group migration: role of remote group participation in the synthesis of 5-C-nucleosides and other sugar derivatives.

In 1-deoxy-xylofuranose derivatives possessing a good leaving group at 2-C, participation of allyloxy and propargyloxy substituents at 5-C results in loss of the 2-C substituent and attack of various nucleophiles at 5-C of the oxonium intermediate. Such participation of a benzyloxy or crotyloxy group leads to dioxabicyclo[2.2.1]heptane rings.

A Convenient Reduction of Substituted Amino-acid Esters

Esters having N-alkyl-N-acyl (or thioacyl) functionality at the α-position are smoothly reduced with NaBH4-MeOH at 0–5°C in very good yields.

An unusual carbo-carbon bond cleavage by KMnO4-CuSO4 Reagent

Abstract Sugars of furanose skeleton having a carbonyl or secondary hydroxyl at C-5 undergo smooth cleavage of C-4-C-5 bond with the reagent.

Nucleoside synthesis from 3-alkylated sugars: role of 3β-oxy substituents in directing nucleoside formation

Using Vorbrüggens protocol, reaction of persilylated uracil with xylofuranose derivatives having 3beta-oxy-3alpha-alkyl substitution produced both alpha- and beta-nucleosides. Only the beta-nucleosides were formed from substrates having the reverse stereochemistry at C-3 or lacking the 3-alkyl substituent. Participation of the 3beta-oxy substituent in stabilizing the incipient C-1 carbonium ion (or oxonium ion) intermediate has been suggested from analysis of energy-minimized conformations.

Synthesis of monoglycosides, bisglycosides and open-chain ketoacetals from sugar derived exocyclic enol ethers

Under acidic conditions, the reaction of pent-4-enofuranosides with various nucleophiles of mostly alcohol origin furnished bisglycosylated products along with open-chain ketoacetals and monoglycosylated products depending upon the reagents and reaction conditions. The reactions appear to proceed via nucleophilic attack at C-1 and/or C-4 of the reaction intermediates.