Tungsten-promoted [3+2]- and [3+3]-cycloaddition of epoxides with alkynes. A facile enantiospecific synthesis of bicyclic lactones

[[abstract]]The cycloaddition of alkynes and alkenes with organic substrates is widely used for the synthesis of carbocyclic and heterocyclic compounds. 1,2 Epoxides and aziridines are important substances, and the cycloaddition of these molecules with alkenes and alkynes is an interesting topic in organic synthesis. Scheme 1 shows two types for [3+2]-cycloaddition of alkenes and alkynes with epoxides or aziridines via cleavage of their C-C and C-X bonds, respectively. 3,4 The former (eq 1) involves 1,3-dipolar cycloaddition of electron-deficient alkynes and alkenes with a zwitterionic intermediate A produced by the thermal and photolytic activation of epoxides and aziridines. The presence of an electron-withdrawing group R2 (R2 ) CN, CO2R) is crucial for formation of this intermediate. 3,4 This system has been thoroughly studied and is useful in organic synthesis. An alternative route 5 involves the use of a Lewis acid to cleave of the C-X bond, to give the intermediate B (eq 2). Despite its synthetic significance, there have been very few successful examples of this system despite its synthetic significance. It is only applicable to special types of functionalized olefins. 5 Yamamoto 6 recently reported the synthesis of tetrahydrofuran derivatives from vinyloxirane with activated alkenes using palladium catalyst. Normally, addition products are formed exclusively when epoxides and aziridienes are treated with activated alkenes or alkynes in the presence of a Lewis acid. 7,8 [3+2]-Cycloaddition of epoxides and aziridines with alkynes remains unknown to our best knowledge. In this study, we describe two new cycloadditions for common epoxides and functionalized alkynes. These methods are applicable to the enantiocontrolled synthesis of complex bicyclic lactones.[[fileno]]2010327010102[[department]]化學