TANDEM INTER[4-TETHER)(2] INTRA[3+2] CYCLOADDITIONS OF NITROALKENES .15. THE BRIDGED MODE (ALPHA)

A new variant of the tandem inter [4 + 2]/intra [3 + 2] cycloaddition of nitroalkenes is described in detail. The scope and limitations of t he bridged mode tandem cycloaddition in which the diene and dienophile are part of the same molecule are documented. Simple 1,4-pentadienes as well as 2-alkoxy-1,4-pentadienes can function effectively as dienop hile and dipolarophile combinations with excellent chemical selectivit y and regio- and diastereoselectivities. Hydrogenation of the bridged nitroso acetals produces aminocyclohexanemethanol derivatives in high diastereo- and enantioselectivities. Further, insights into the mechan istic aspects of the Raney nickel promoted hydrogenation are reported. An intriguing influence of the nitro olefin alpha-substituent on the diastereoselectivity in the [4 + 2] cycloaddition has been documented. The reactivity of the alpha-chlorosubstituted nitroalkene 26 as the h eterodiene in the Diels-Alder reaction is assayed, and the use of the chlorine atom as a hydrogen surrogate is described.