Acid-promoted cycloisomerizations of phenylallenes bearing acetalic functions at the ortho position: a stereocontrolled entry to indeno-fused dioxepanes, dioxocanes and thioanalogues.

The cycloisomerization reactions of allenes bearing cyclic acetal, thioacetal and dithioacetal subunits, when triggered either by the catalytic action of AgSbF6 or by one equiv. of CF3COOH, gave rise to four different classes of indeno-fused 1,4-dioxa, oxathia and dithia heterocycles, in most cases as a single diastereomer. Acyclic acetals and dithioacetals are also suitable starting materials in similar transformations yielding 1,2-disubstituted indenes and 1,3-disubstituted 2-alkylideneindanes.