(Chemical Equation Presented). We report the synthesis of methyl α-D-glycero-D-idoseptanoside (1) and methyl β-D-glycero-D- guloseptanoside (2) and the characterization of their preferred solution conformations by computational chemistry and 1H NMR 3JH,H coupling constant analysis. Central to the synthetic approach was the epoxidation of glucose-derived oxepine 3 using DMDO. Nucleophilic attack on the resulting 1,2-anhydroseptanose using NaOCH 3 in CH3OH followed by deprotection provided the 1,2-trans diastereomers 1 and 2. The computational approach for determining the preferred low energy septanose conformations began with a pseudo Monte Carlo search for each isomer using minimization with the AMBER force field. Single-point energy calculations (HF/6-31G* and B3LYP/ 6-31+G**) as well as full geometry optimizations in a model for aqueous solvent were then conducted using the conformers within 5 kcal/mol of the AMBER global minimum. Calculated 3JH,H values, based on a Boltzmann distribution of the computed low energy conformers, were compared to experimental 3J H,H values from 1H NMR coupling constant analyses. The correlation between calculated and observed values suggest that septanose carbohydrates are not so flexible and should generally prefer one twist-chair (TC) conformation.
ASJC Scopus subject areas
- Organic Chemistry