Diterpenoids. XXXVII. Rearrangement of methyl 13-isopropyl-7-oxo-podocarpa-5,8,11,13-tetraen-15-oate by means of aluminum chloride.

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The phenacylidene ester 2, methyl 13-isopropyl-7-oxo-podocarpa-5, 8, 11, 13-tetraen-15-oate, underwent novel rearrangements on refluxing with an excess of aluminum chloride to afford 13-isopropyl-1α-methyl-1β-methoxycarbonyl-7-oxo-15, 16-bisnor-10α-podocarpa-5, 8, 11, 13-tetraene (7) and 10α-hydroxy-13-isopropyl-7-oxo-1, 10-seco-spiro(1.5)-podocarpa-8, 11, 13-trien-15-oic acid 15→10 lactone (8) in 57 and 5% yields, respectively. Their structures were unequivocally established by chemical conversions and spectral examinations. The reaction courses (a) and (b) through 6 were proposed for the mechanisms of these rearrangements which somewhat differ from the rearrangement of 2 by means of protonic acid. It was further found that boron trifluoride etherate promoted also these rearrangement, but was less effective as catalyst, and treatment of 7 with aluminum chloride under the same condition led to the formation of 2 and 8 though in respective small yields.

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