TOTAL SYNTHESIS OF SIALYLGALACTOSYLGLOBOSIDE - STAGE-SPECIFIC EMBRYONIC ANTIGEN-4

A versatile total synthesis of sialylgalactosvlgloboside (SGG, 1), car rying the stage-specific embryonic antigen 4 (SSEA-4) is reported, ill ustrating a more general strategy for the synthesis of complex globe-s eries glycosphingolipids. Starting from readily available building blo cks 7, 8, and 10, two different approaches to the synthesis of the key tetrasaccharide 6 have been developed in a highly convergent manner. Further glycosylations with galactosyl trichloroacetimidate (5) and si alyl phosphite.(a) donors successively afforded the penta- and hexasac charides 3 and 11. The latter was finally converted into the target mo lecule (SGG, 1) with the help of a azidosphingosine glycosylation proc edure, favored in this case by the stereocontrolling properties of the 2a-O-pivaloyl protecting group. Valuable intermediates 6 and 3, havin g the oligosaccharidic skeletons of Gb(4) and Gb(5) (SSEA-3), respecti vely, were obtained in the course of the synthesis.