Stereochemistry of cyclic compounds—I : Synthesis and configurational assignment of diastereomeric 2,4-dioxaspiro[5.5]undec-8-enes

Several 2,4 - dioxaspiro[5.5]undec - 8 - enes have been prepared from the Diels-Alder adducts of butadiene and 2,3-dimethylbutadiene with acrolein. Contrary to literature reports on some 3-substituted derivatives, which were assumed to be pure compounds, it is shown that these spiro compounds are generally obtained as 1:1 mixtures of two diastereomers. The isomers have been separated in the case of the 3-methyl and 3-ethyl derivatives by fractional distillation. On the basis of 1 H NMR spectroscopy, the nature of the isomerism is shown to be of cis , trans type and an unambiguous cofigurational assignment of the isomers has been achieved. Although the diastereomers are of similar topography, they have distinctly different olfactory properties.