Selective transformations of threo-2,3-dihydroxy esters

Abstract

Two highly regio- and stereoselective transformations of threo-2,3-dihydroxy esters have been developed. In the first reaction, the α-hydroxy group is converted into a sulfonate group (tosylate or nosylate); the α-tosylates and α-nosylates are then subjected to basic conditions (K 2 CO 3 /ROH) to give erythro glycidic esters in high yield. The α-nosylates are also suitable electrophiles for azides, giving access to erythro-α-azido-β-hydroxy esters. The second reaction involves conversion of the diol esters to acetoxy bromo esters. The β-substituent plays a key role in determining the regiochemistry since cases with β-alkyl substituents afford β-acetoxy-α-bromo esters exclusively, whereas a β-phenyl substituent directs formation of the α-acetoxy-β-bromo ester. The acetoxy bromo esters can subsequently be converted to the threo glycidic esters (via the bromohydrin esters); selective hydrogenolysis of the bromine substituent can also be achieved