Assignment of 13C and 1H resonances of methyl groups in the tri-O-methyl derivatives of methyl pentopyranosides; some observations on the methoxy 13C chemical shifts
By studies of specifically labelled trimethyl ethers of methyl β-L-arabinopyranoside, methyl α-D-lyxopyranoside, methyl β-D-ribopyranoside, and methyl α-D-xylopyranoside, in which some methyl groups are replaced by trideuteriomethyl groups, the 13C and 1H resonances of each of the methyl groups were identified. A rationalization is presented, in terms of syn-axial δ-effects and rotamer populations, for the deshielding experienced in six-membered rings by carbon-13 nuclei in equatorially disposed methoxy-groups, which are flanked by two equatorially attached methoxy(hydroxy)-groups. Benzoylation of methyl β-L-arabinopyranoside with benzoyl cyanide was shown, by 13C n.m.r. spectroscopy, to afford the 2,4-dibenzoate as the major product.