A STEREOSELECTIVE HETERO-DIELS-ALDER APPROACH TO CARBON-CARBON LINKEDDISACCHARIDES

The chiral 1-oxabuta-1,3-diene 1 bearing the thiazol-2-yl ring at C-2 and the D-galacto-pentopyranosid-5-yl ring at C-4 reacts with ethyl vi nyl ether 2 in an endo-specific hetero-Diels-Alder process to give a m ixture of diastereoisomeric 3,4-dihydro-2H-pyran derivatives 3 and 4 i n 4: 1 ratio and 97% overall yield; these compounds are transformed by thiazole-to-formyl unmasking and hydroxylation into stereoisomeric C- 3-C-5 linked dipyranosides 7 and 8.