A short asymmetric synthesis of a C19–C27 segment of rifamycin S. Kinetic resolution in the aldol reactions of ethylketones using chiral boron reagents

Abstract The chiral reagents (+)- and (−)-(Ipc) 2 BOTf are used to control the aldol addition reactions of ethylketone 3 with methacrolein. Under suitable conditions, racemic 3 can be converted into the SS aldol adduct (+)- 4 in ≥95% ee with >95% diastereoselectivity. Resolved starting ketone (+)- 3 can be recovered in ≥95% ee. Adduct (+)- 4 was converted into the rifamycin S segment (−)- 2 via stereoselective ketone reduction and hydroboration.