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Prins-pinacol cyclizations of unsaturated [alpha]-dithianyl acetals is carried out to stereospecifically generate polycyclic products containing the 12-oxatricyclo[6.3.1.02,7]dodecane moiety bearing either the trans or cis relative configuration of the oxacyclic bridge and the cis angular substituents. The topography (boat or chair) of the Prins cyclization of the sulfur-stabilized oxocarbenium ions generated from acetals is observed to be controlled by the stereoelectronic influence of the allylic substituents, with steric effects playing a minor role.

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. [ABSTRACT FROM AUTHOR]

A cross-metathesis and Sharpless asymmetric dihydroxylation reaction gives an easy access to compound (IV), which can be further converted to the title F-Ring (V) in five steps. [ABSTRACT FROM AUTHOR]