文摘
Geometry optimizations at the B3LYP level and single point calculations at the MP2 level are reported for the 4H5 and 5H4 conformations of methyl 3,4-di-O-acetyl-1,2-dideoxy-d-arabino-hex-1-enopyranuronate (methyl 3,4-di-O-acetyl-d-glucuronal), and methyl 3,4-di-O-acetyl-1,2-dideoxy-d-lyxo-hex-1-enopyranuronate (methyl 3,4-di-O-acetyl-d-galacturonal). Energy and geometry parameters are presented for the most stable optimized geometries. Conformational analysis of the acetoxy and methoxycarbonyl groups as well as the 1,2-unsaturated pyranoid ring is performed. It is demonstrated that both the acetoxy and methoxycarbonyl groups are planar and prefer cis over trans orientations with respect to the CO-O bond rotations. With regard to the AcO-R bond rotations some of the orientations are forbidden. The 4H5 5H4 conformational equilibrium in both methyl 3,4-di-O-acetyl-d-glucuronal (shifted towards 5H4) and methyl 3,4-di-O-acetyl-d-galacturonal (shifted towards 4H5) is the outcome of the competition between the vinylogous anomeric effect and quasi 1,3-diaxial interactions. It is demonstrated that the orientation of the 4-OAc group influences the strength of the quasi 1,3-diaxial interactions between 3-OAc and 5-COOCH3 groups. Theoretical results are compared with assignments based on 1H NMR studies.