Lewis base catalyzed aldol additions of chiral silyl enol ethers and total synthesis of RK-397

The double diastereodifferentiation in chiral Lewis base catalyzed aldol additions was examined using chiral silyl enol ethers derived from hydroxyisobutyrate and hydroxybutyrate. Trichlorosilyl enolates derived from the chiral methyl and ethyl ketones were subjected to the Lewis base catalyzed conditions, and the intrinsic selectivity of these enolates and the external stereoinduction from chiral catalyst were studied. In all cases, the catalyst-controlled diastereoselectivities were observed, and the resident stereogenic centers exerted marginal stereoinduction. Corresponding TMS enol ethers were employed in SiCl4/bisphosphoramide catalyzed aldol additions, and the effect of double diastereodifferentiation was investigated. The internal stereoinduction was again controlled by the strong external induction from the catalyst. The methodologies developed in these studies were applied in the total synthesis of polyene macrolide antibiotic RK-397. The synthesis features the highly site- and enantioselective vinylogous aldol addition using a chiral bisphosphoramide which allowed efficient construction of the key building block. The polyene fragment was constructed utilizing the sequential palldium-catalyzed cross-coupling reactions of organosilanes.