Experimental evidence on the hydroxymethyl group conformation in alkyl β- d-mannopyranosides

Abstract

A rotational population study of the hydroxymethyl group of alkyl β- d-mannopyranosides was performed by means of CD and NMR spectroscopy. Three different benzyl, acetyl, and p-bromobenzoyl series of alkyl β- d-mannopyranosides with different chiral and nonchiral aglycons were synthesized and analyzed. Different rotational populations were observed for each series by changing the structure of the aglycon. The results showed a clear correlation between the rotational population of the hydroxymethyl group around the C5–C6 bond and the p K a of the bonded alcohol (aglycon). The population of the gt rotamer gradually increased as the p K a increased while that of the gg rotamer decreased and the population of the tg rotamer remained almost constant. This is explained by the exo-anomeric effect. For chiral alkyl derivatives, the results also showed a close dependence on the absolute configuration of the aglycon. Comparison of rotational population anomers revealed the dependence of the hydroxymethyl group on the anomeric configuration and a greater dependence on the aglycon structure in the β anomers.