HIGHLY DIASTEREOSELECTIVE ALDOL REACTIONS OF COBALT-COMPLEXED AND UNCOMPLEXED PROPYNALS WITH AN O-SILYL KETENE O,S-ACETAL - HIGHLY STEREOSELECTIVE TOTAL SYNTHESES OF (+/-)-BLASTMYCINONE AND ITS 3 DIASTEREOISOMERS FROM 3-(TRIMETHYLSILYL)PROPYNAL

Total syntheses of (+/-)-blastmycinone (2) and its three diastereoisomers (3-5) are described. The syntheses involve highly stereoselective aldol reactions of cobalt-complexed propynal 6 and uncomplexed propynal 7 with O-silyl ketene O,S-acetal 8, which possesses an n-butyl tether, under Mukaiyama conditions. (+/-)-Blastmycinone (2) and its 2-epimer 3 were stereoselectively synthesized through chelation-controlled reduction, whereas (+/-)-3-epi- and 4-epiblastmycinone (5 and 4) were obtained in a highly stereocontrolled manner with bromolactonization as a key step.