A Flexible Common Approach to α-Substituted Serines and Alanines: Diastereoconvergent Syntheses of Sphingofungins E and F

A flexible common approach for the asymmetric syntheses of sphingofungins E and F is reported, with efficient use of both diastereomers of the Baylis-Hillman adduct in a stereoconvergent manner. Pronounced steric effects of the 2-substituents in diastereoselective dihydroxylations of (E)-2-hydroxymethyl-2,3-alkenoates were observed. This strategy also allowed full control over the absolute configurations of the quaternary stereocenters in alpha-substituted amino acids through tunable site-specific adjustment of oxidation states. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)