The "aqueous" Prins reaction.

[reaction: see text] In this communication we demonstrate that Prins cyclization reactions occur under very mild conditions when cyclic alpha,beta-unsaturated acetals are employed as oxocarbenium ion progenitors and allylsilanes are used as nucleophiles. Cyclizations proceed efficiently inside Lewis acidic micelles in water, demonstrating that colloidal suspensions can protect highly electrophilic intermediates from hydrolysis. Reactions are experimentally facile and useful in the preparation of a variety of vinyl- and aryl-substituted tetrahydropyrans with excellent stereocontrol.