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

摘要

Extensive variations of the ring sructure of three deoxyladohexopyranoses,L-fucose,D-quinovose,andL-rhamnose,and four dideoxyaldohexopyranoses,D-digitoxose,abequose,paratose,and tyvelose,were studied by energy minimization with the molecular mechanics algorithm MM3(92).Chair conformers,~4C_1 in D-quinovose and the equivalent ~4C_1 L-fucose and L-rhamnose,over whelmingly dominate int he rhree deoxyhexoses;in the D-dideoxyhexoses,~4C_1 is again dominant,but with increasedamounts of ~4C_1 forms in the #alpha# anomers of the three 3,6-dideoxyhexoses,abeuose,paratose,and tyvelose and in both #alpha# and #beta# 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 #beta# configuration except for D-digitoxose,which is almost equaly divided between #alpha# and #beta# configurations,and L-rhamnose,where the #beta# configurationis somewhat favored.MM3(92) appears to overstate the prevalence of the equatorial #beta# anomer in all three deoxyhexoses,as earlier found with fully oxygenated aldohexopyranoses.