Improved Syntheses and Some Selective Transformations of 2,2,4,4-Tetrachloro-8-oxabicyclo[3.2.1]oct-6-en-3-ones.

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.