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

A rotational population Study of the hydroxymethyl group of alkyl beta-D-mannopyranosides was performed by means of CD and NMR spectroscopy. Three different benzyl, acetyl, and p-bromobenzoyl series of alkyl beta-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 pK(a) of the bonded alcohol (aglycon). The population of the gt rotamer gradually increased as the pK(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 or the hydroxymethyl group on the anomeric configuration and a greater dependence on the aglycon structure in the beta anomers. (C) 2004 Elsevier Ltd. All rights reserved.