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drolysis, affording the enone 18 as a 64:36 mixture of E to Z isomers in 59% yield. The crucial ring closure of 18 via oxyselenation furnished, after reductive workup, the desired tetrahydropyran-Cone 19 in 78% yield along with 6% yield of its epimer. In stark contrast to the result with a model system,88 the stereochemical outcome of this cyclization proved to be independent of the starting olefin geometry (19 and its epimer: 79% and 5% from E-18; 80% and 6% from 2-18), implying that cyclization proceeded through the chair-preferred transition state involving a stable open carbocation allowing rotation about C-12/C-13 bond to direct the methyl group a t an axial position.

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