SELECTIVE TRANSFORMATIONS OF THREO-2,3-DIHYDROXY ESTERS

Two highly regio- and stereoselective transformation of threo-2,3-dihydroxy esters have been developed. In the first reaction, the alpha-hydroxy group is converted into a sulfonate group (tosylate or nosylate); the alpha-tosylates and alpha-nosylates are then subjected to basic conditions (K2CO3/ROH) to give erythro glycidic esters in high yield. The alpha-nosylates are also suitable electrophiles for azides, giving access to erythro-alpha-azido-beta-hydroxy esters. The second reaction involves conversion of the diol esters to acetoxy bromo esters. The beta-substituent plays a key role in determining the regiochemistry since cases with beta-alkyl substituents afford beta-acetoxy-alpha-bromo esters exclusively, whereas a beta-phenyl substituent directs formation of the alpha-acetoxy-beta-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.