Tandem inter [4+2]/intra [3+2] cycloadditions. 8. Cycloadditions with unactivated dipolarophiles

The tandem intermolecular [4+2]/intramolecular [3+2] cycloaddition of nitroalkenes bearing a pendant olefin is presented. The success of the tandem process is remarkable in view of the potential for intramolecular reaction with the tethered alkene. Cycloadditions of di- and trisubstituted nitroalkenes bearing two- and three-methylene tethers which contain mono-, di- and trisubstituted olefins are described. In all cases the tandem process is high yielding (with n-butyl vinyl ether as the dienophile) and highly stereoselective. Hydrogenolysis of the resulting nitroso acetals leads cleanly to fused pyrrolidines in high yields.