Origin of Stereocontrol in the Construction of the 12-Oxatricyclo[6.3.1.0^2,7]dodecane Ring System by Prins−Pinacol Reactions

Polycyclic products containing the 12-oxatricyclo[6.3.1.02,7]dodecane moiety having either the trans (8a−e) or cis (9a−e) relative configuration of the oxacyclic bridge and the cis angular substituents are formed stereospecifically by Prins−pinacol cyclizations of unsaturated α-dithianyl acetals 14a−e or 15a−e. These results show that the topography (boat or chair) of the Prins cyclization of the sulfur-stabilized oxocarbenium ions generated from acetals 14a−e or 15a−e is controlled by the stereoelectronic influence of the allylic substituents, with steric effects playing a minor role. A complex molecular rearrangement that is terminated by a thio-Prins−pinacol reaction is also identified.