Computational NMR of Carbohydrates. 2. Allopyranoses: Theoretical Conformational Analysis and NMR Calculations

Abstract

Allose, an aldohexose sugar, is a rare natural monosaccharide that occurs in nature in the form of a 6-O-cinnamyl glycoside in the leaves of the African shrub Protea rubropilosa. It differs from glucose in the orientation of the hydroxyl group at the C-3 position, being thus the C-3 substituted epimer of glucopyranose. In this paper, a detailed theoretical modeling of α- and β-D-alloses was performed, including molecular dynamics (MD) simulations, high-level geometry optimizations, and GIAO-DFT NMR studies. Generally, a good correlation between calculated ¹H and ¹³C NMR chemical shifts of the conformationally averaged α- and β-D-alloses against experiment was found. Based on the performed calculations, the optimal combination of functionals with basis sets for the geometry optimization (M06-2X/aug-pc-2) and NMR calculations (BHandHLYP/aug-pcSseg-2) for carbohydrates was suggested.