Synthesis of a Structurally Modified Glycal. (-)-(2R,4S)-2-Methyl-2-vinyl-4-(benzyloxy)-3,4-dihydro-2H-pyran

Article abstract of DOI:10.1021/jo00248a010

A synthetic procedure for the transformation of levoglucosan (4) to (-)-(2R,4S)-2-methyl-2-vinyl-4-(benzyloxy)-3,4-dihydro-2H-pyran (1) in 11 steps is described.The scheme relies on selective deoxygenation of the pair of α-hydroxyl groups, blocking of the β-hydroxyl, and formation of ester 16.The presence of the carboxylate group allows for stereocontrolled methylation of the enolate anion, conversion of ester to vinyl, and ultimate eliminative removal of the methoxyl substituent in methyl glycoside 25.This key transformation takes advantage of regioselective acetal cleavage by trimethylsilyl iodide and in situ dehydroiodination of the product so formed with hexamethyldisilazane.Certain unsuccessful attempts to form the α-lithio anion of 1 are also discussed.