Stereospecific chain-branching by c-alkylation at the ketonic and enolic positions of 1,6-anhydro-2,3- o-isopropylidene-β- D- lyxo-hexopyranos-4-ulose

Abstract

Attempted base-catalyzed C-formylation at the enolic (C-3) position of 1,6-anhydro-2,3- O-isopropylidene-β- D- lyxo-hexopyranos-4-ulose ( 1) gives instead a dimer ( 8) of 1. The ketone 1 is, however, C-alkylated stereospecifically at C-3 by the action of paraformaldehyde in methanolic potassium carbonate; concomitant reduction at C-4 occurs, and 1,6-anhydro-3- C-(hydroxymethyl)-2,3- O-isopropylidene-β- D-talopyranose ( 3) is formed. The ketone 1 undergoes stereospecific attack at C-4 by Grignard reagents, and the 4- C-methyl ( 13), 4- C-vinyl ( 10), and 4-C-ethynyl ( 11) derivatives of 1,6-anhydro-2,3- O-isopropylidene-β- D-talopyranose were so prepared. Ozonolysis of 10 and 11 gives the corresponding 4- C-formyl ( 14) and 4- C-carboxy ( 16) derivatives, respectively. The spiro epoxide 12 formed by action of diazomethane on the ketone 1 has been characterized as the D- talo isomer by its conversion on reduction into the 4- C-methyl derivative 13.