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

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 min imization with the molecular mechanics algorithm MM3(92). Chair conformers, C-4(1) in D-quinovose and the equivalent C-1(4) in L-fucose and L-rhamnose , overwhelmingly dominate in the three deoxyhexoses; in the D-dideoxyhexose s, C-4(1) is again dominant, but with increased amounts of C-1(4) forms in the alpha anomers of the three 3,6-dideoxyhexoses, abequose, paratose, and tyvelose and in both alpha and beta anomers of the 2,6-dideoxyhexose D-digi toxose. In general, modeled proton-proton coupling constants agreed well wi th experimental values. Computed anomeric ratios strongly favor the P confi guration except for D-digitoxose, which is almost equally divided between a and P configurations, and L-rhamnose, where the P configuration is somewha t favored. MM3(92) appears to overstate the prevalence of the equatorial P anomer in all three deoxyhexoses, as earlier found with fully oxygenated al dohexopyranoses. (C) 2001 Published by Elsevier Science Ltd.