Modeling of deoxy- and dideoxyaldohexopyranosyl ring puckering with MM3(92)

Abstract

Extensive variations of the ring structures of three deoxyaldohexopyranoses, l-fucose, d-quinovose, and l-rhamnose, and four dideoxyaldohexopyranoses, d-digitoxose, abequose, paratose, and tyvelose, were studied by energy minimization with the molecular mechanics algorithm MM3(92). Chair conformers, 4 C 1 in d-quinovose and the equivalent 1 C 4 in l-fucose and l-rhamnose, overwhelmingly dominate in the three deoxyhexoses; in the d-dideoxyhexoses, 4 C 1 is again dominant, but with increased amounts of 1 C 4 forms in the α anomers of the three 3,6-dideoxyhexoses, abequose, paratose, and tyvelose and in both α and β anomers of the 2,6-dideoxyhexose d-digitoxose. In general, modeled proton–proton coupling constants agreed well with experimental values. Computed anomeric ratios strongly favor the β configuration except for d-digitoxose, which is almost equally divided between α and β configurations, and l-rhamnose, where the β configuration is somewhat favored. MM3(92) appears to overstate the prevalence of the equatorial β anomer in all three deoxyhexoses, as earlier found with fully oxygenated aldohexopyranoses.