The methyl- and alkenyl-substituted furans 1b-h react with pentachloroacetone (2) and sodium 2,2,2-trifluoroethoxide solution to form the title compounds 3b-h in good yield. With the furans 1i-m bearing an oxygen or a sulfur heteroatom in the side chain, moderate yields are obtained. Dechlorination of the [4+3] cycloadducts 3 with the Zn-Cu couple leads to the corresponding oxabicyclic ketones 4. On catalytic hydrogenation of the tetrachlorooxabicyclooctenones 3a-c hydrogenolysis of the exo-carbon-chlorine bonds occurs, leading to the endo-2,endo-4-dichloro-8-oxabicyclooctan-3-ones 8a-c. With lithium aluminum hydride and Grignard reagents, 8a and 3a form the endo-3-alcohols 12a-c and 13, respectively, the latter with uncertain configuration at C-3, in a highly stereoselective manner. The ether bridge in the dechlorinated oxabicyclooctenones 4b, 4f, and 4g can be cleaved by means of trimethylsilyl triflate/triethylamine to produce the tropones 5b, 5f, and 5g. Hydrogenation of 1,5-dimethyl-8-oxabicyclo[3.2.1]oct-6-en-3-one (4c), followed by Wolff-Kishner reduction, affords 1,5-dimethyl-8-oxabicyclo[3.2.1]octane (7). m-Chloroperbenzoic acid epoxidizes the alkenyl side chain of the tetrachlorinated oxabicycles 3d, 3e, 3g, and 3h in a site-selective reaction. In contrast, from the dehalogenated oxabicyclic ketone 4g the bis(epoxide) 14 is obtained.

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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Treatment of the title compounds (I) and (V) with (IV) results in cleavage of the oxygen bridge to afford the monocyclic products (II) and (VI).