Addition reactions of methyl 4,6- O-benzylidene-2,3-dideoxy-α- D- erythro-hex-2-enopyranoside

Abstract

The title alkene ( 1) reacts with bromine in methanol, in the presence of silver acetate, to give a high yield of a crystalline 2,3-dibromide ( 2) having the D- altro configuration. Removal of the O-benzylidene group from 2 followed by acetylation gave the corresponding 4,6-diacetate 5, and treatment of 2 with N-bromosuccinimide gave methyl 4- O-benzoyl-2,3,6-tribromo-2,3,6-trideoxy-α- D-altropyranoside ( 6). Strong base converts the dibromide 2 into methyl 4,6- O-benzylidene-2-bromo-2,3-dideoxy-α- D- threo-hex-3-enopyranoside ( 3); the bromine atom at C-2 is not removed. Acetyl hypobromite reacts with the alkene 1 to give a mixture of two adducts, namely, methyl 2- O-acetyl-4,6- O-benzylidene-3-bromo-3-deoxy-α- D-altropyranoside ( 7) and methyl 3- O-acetyl-4,6- O-benzylidene-2-bromo-2-deoxy-α- D-glucopyranoside ( 8), with the former preponderating; treatment of either 7 or 8 with base gives methyl 2,3-anhydro-4,6- O-benzylidene-α- D-mannopyranoside ( 10). N-Bromosuccinimide converts 7 into the 6-bromo 4-benzoate analog 9. Treatment of alkene 1 with diiodomethane and zinc-copper couple gives the cyclopropyl derivative 4, but 1 is quite inert toward “dichlorocarbene”, “ethoxycarbonylcarbene”, and “ethoxycarbonylnitrene”. The structures assigned to the products 2–10 were confirmed by 100-MHz n.m.r. spectral studies and by spin-decoupling experiments.