An 8π Electron Electrocyclization Leading to a 9,19-Methano-Bridged Analogue of 1α,25-Dihydroxyvitamin D₃

[reaction: see text] Lindlar semihydrogenation of a vitamin D type trienyne leads spontaneously to 9 alpha,19-methano-1 alpha,25-dihydroxyvitamin D3. The intermediate tetraene resulting from the reduction undergoes a rapid, stereoselective 8pi electron electrocyclization affording a novel steroid containing a linearly fused ABC (six-eight-six) 1,3,5-cyclooctatriene carbon framework.