Comparison of the conformational properties of carbasugars and glycosides: the role of the endocyclic oxygen

A series of carbasugars were prepared and their conformational properties studied by means of NMR spectroscopy. The results were compared to those previously found for O-, S-, and C-β-glycoside analogs. While the rotational populations of the hydroxymethyl group in O-, S-, and C-glycosides are known to depend on the structural nature of their aglycon, in carbasugars it proved to be independent of the pseudo-aglycon. This result confirms that endocyclic oxygen is necessary for the observed relationship between the structure of the aglycon and the rotational populations of the hydroxymethyl group, and indicates that the stereoelectronic exo-anomeric effect is mainly responsible for such conformational dependence.

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► The rotational properties of carbasugars differ from those of glycoside analogs.
► The endocyclic oxygen is crucial for the rotational populations in glycosides.
► The rotational populations in carbasugars are independent of the pseudo-aglycon.