Building blocks for syntheses of guaiane and secoguaiane sesquiterpenoids were prepared by the title reaction. 4-(3-Methylfuran-2-yl)butan-2-one (1) was obtained in five steps from methyl 3-methylfuran-2-carboxylate (4), and treatment of 1 with pentachloroacetone and sodium 2,2,3,3-tetrafluoropropoxide in 2,2,3,3-tetrafluoropropan-1-ol produced a [4 + 3] cycloadduct that was dechlorinated without prior isolation to give the oxabicyclic diketone 2 in a low yield. A better route to diketone 2 was via the oxabicycle 10b, prepared in a high yield from 2-(but-3-en-1-yl)-3-methylfuran (9) and pentachloroacetone, followed by dechlorination. Treatment of 2 with dilute methanolic potassium hydroxide resulted in the cleavage of the oxa bridge, with formation of 3,8-dimethylazulen-4-ol (11). Catalytic hydrogenation of 2 afforded the saturated oxabicyclic diketone 14, intramolecu-

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. 69, 949 (1991).A detailed study directed toward the development of a versatile synthetic route to the interesting dimeric natural product yuehchukene (1) and its epimer, 6a-epi-yuehchukene (2), has been completed. Due to the anti-implantation activity associated with 1 , it was important to provide a synthetic strategy not only to 1 , but to a family of yuehchukene analogues that, hopefully, would reveal superior chemical stability and (or) elevated biological activity. The experiments described herein and which utilize the readily available and inexpensive isophorone (6) satisfy these requirements.

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