Hydroxymethyl group conformation in saccharides: structural dependencies of (2)J(HH), (3)J(HH), and (1)J(CH) spin-spin coupling constants.

Abstract

Experimental and theoretical methods have been used to correlate (2)J(HH) and (3)J(HH) values within the exocyclic hydroxymethyl groups (CH(2)OH) of saccharides with specific molecular parameters, and new equations are proposed to assist in the structural interpretation of these couplings. (3)J(HH) depends mainly on the C-C torsion angle (omega) as expected, and new Karplus equations derived from J-couplings computed from density functional theory (DFT) in a model aldopyranosyl ring are in excellent agreement with experimental values and with couplings predicted from a previously reported general Karplus equation. These results confirm the reliability of DFT-calculated (1)H-(1)H couplings in saccharides. (2)J(HH) values depend on both the C-C (omega) and C-O (theta) torsions. Knowledge of the former, which may be derived from other parameters (e.g., (3)J(HH)), allows theta to be evaluated indirectly from (2)J(HH). This latter approach complements more direct determinations of theta from (3)J(HCOH) and potentially extends these more conventional analyses to O-substituted systems lacking the hydroxyl proton. (1)J(CH) values within hydroxymethyl fragments were also examined and found to depend on r(CH), which is modulated by specific bond orientation and stereoelectronic factors. These latter factors could be largely, but not completely, accounted for by C-C and C-O torsional variables, leading to only semiquantitative treatments of these couplings (details discussed in the Supporting Information). New equations pertaining to (2)J(HH) and (3)J(HH) have been applied to the analysis of hydroxymethyl group J-couplings in several mono- and oligosaccharides, yielding information on C5-C6 and/or C6-O6 rotamer populations.