Higher-carbon sugars. Part 6. The synthesis of some octose sugars via the epoxidation of unsaturated precursors

Abstract

Titanium-catalysed asymmetric epoxidation of (E)-6,7-dideoxy-1,2:3,4-di-O-isopropylidene-α-D-galacto-oct-6-enopyranose (1) with di-isopropyl L-(+)-tartrate afforded 6,7-anhydro-1,2:3,4-di-O-isopropylidene-β-L-threo-D-galacto-octopyranose (4), which was identified by its conversion on basic hydrolysis into 1,2:3,4-di-O-isopropylidene-α-D-erythro-D-galacto-octopyranose (6)via preferential ring-opening of the Payne-rearrangement product (5) at the terminal position. Oxidation of (1) with m-chloroperbenzoic acid gave principally the isomeric epoxy alcohol (8), which basic hydrolysis similarly transformed into 1,2:3,4-di-O-isopropylidene-β-L-erythro-D-galacto-octopyranose (10). The latter sequence of reactions also yielded 1,2:3,4-di-O-isopropylidene-α-D-threo-D-galacto-octopyranose (3) from (Z)-6,7-dideoxy-1,2:3,4-di-O-isopropylidene-α-D-galacto-oct-6-enopyranose (12). D-erythro-D-galacto-Octitol (7), L-erythro-D-galacto-octitol (11), and D-threo-D-galacto-octitol (16) are accessible from the protected octoses (6), (10), and (3), respectively.