TANDEM [4+2]/[3+2] CYCLOADDITIONS - FACILE AND STEREOSELECTIVE CONSTRUCTION OF POLYCYCLIC FRAMEWORKS

Di- and trisubstituted nitroalkenes tethered to dipolarophiles (unsaturated esters, nitriles) undergo tandem [4 + 2]/[3 + 2] cycloadditions with 2,3-dimethyl-2-butene or butyl vinyl ether in the presence of Lewis acids. For the dimethylene tether 1, the tandem cycloadduct 4 is the direct reaction product. The E configuration of the dipolarophile is preferred, and the products arise selectively from a syn-endo pathway. For a trimethylene-tethered precursors 2 the initial [4 + 2] cycloadducts 9 are isolable and undergo the second [3 + 2]-dipolar cycloaddition upon brief warming via a syn-exo pathway. The resulting nitroso acetals (4bB/B’ and llaB/B’) are cleaved with hydrogen and Raney nickel to afford the tricyclic lactams 12 and 14 stereoselectively in good yield. © 1990, American Chemical Society. All rights reserved.