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  • Title: Acetylated methyl 1,2-dideoxyhex-1-enopyranuronates in density functional theory conformational studies.
    Author: Nowacki A, Liberek B.
    Journal: Carbohydr Res; 2013 Apr 19; 371():1-7. PubMed ID: 23454200.
    Abstract:
    Geometry optimizations at the B3LYP level and single point calculations at the MP2 level are reported for the (4)H(5) and (5)H(4) 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 (4)H(5)⇌(5)H(4) conformational equilibrium in both methyl 3,4-di-O-acetyl-D-glucuronal (shifted towards (5)H4) and methyl 3,4-di-O-acetyl-D-galacturonal (shifted towards (4)H(5)) 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-COOCH(3) groups. Theoretical results are compared with assignments based on (1)H NMR studies.
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