An enantioselective synthesis of benzylidene-protected syn-3,5-dihydroxy carboxylate esters via osmium, palladium, and base catalysis.

[reaction: see text]. The enantioselective syntheses of several protected syn-3,5-dihydroxy carboxylic esters have been achieved from the corresponding achiral 1,3-dieneoates. The route relies upon an enantio- and regioselective Sharpless dihydroxylation and a palladium-catalyzed reduction to form delta-hydroxy-1-enoates. The resulting delta-hydroxy-1-enoates are subsequently converted into benzylidene-protected 3,5-dihydroxy carboxylic esters in one step. The benzylidene-protected 3,5-dihydroxy carboxylic esters are produced in good overall yields (25% to 51%) and high enantiomeric excesses (80% to >95%).