HIGHER-CARBON SUGARS .14. THE EFFECT OF DOUBLE-BOND GEOMETRY ON THE CRYSTALLOGRAPHIC CONFORMATIONS OF 2 ENOPYRANURONATES

In the crystal structure of methyl (Z)-8,9-dideoxy-1,2:3,4:6,7-tri-O-isopropylidene-a-d-threo-d-galacto-dec-8- enopyranuronate (10), the allylic system adopts an eclipsed conformation with the smallest group, H-7, at the adjacent stereocentre eclipsing the olefinic double bond. The allylic system of methyl (E)-6-O-benzyl-7,8-dideoxy-1,2:3,4-di-O-isopropylidene-a-d-glycero-d-galacto-non-7-enopyranuronate (11) also adopts an eclipsed conformation in the crystal, but with the benzyloxy group at the adjacent stereocentre eclipsing the olefinic double bond. These ground-state conformations in the solid state provide some insight into the vastly different facial stereoselectivities of 10 and 11 towards catalytic osmylation. © 1990.