Conformational analysis of 1-oxaspiro[5.5]undecanes. A powerful probe for the endo and the exo anomeric effects in acetals

A 13C nuclear magnetic resonance spectroscopy study of 1-oxaspiro[5.5]undecanes 1, 4, and 5 as a function of temperature is reported. Compound 1 exists as a rapidly equilibrating mixture of a major and a minor conformer (1a and 1b) at room temperature that can be observed at low temperature. By comparison, 1,7-dioxaspiro[5.5]undecane 2 is conformationally rigid in a single conformation (2a) at room temperature. This completely different behavior demonstrates the importance of the endo and the exo anomeric effects in the acetal function. Keywords: spiro ethers, conformational analysis, 13C NMR.