Tandem Inter [4 + 2]/Intra [3 + 2] Cycloadditions of Nitroalkenes. 15. The Bridged Mode (α-Tether)

A new variant of the tandem inter [4 + 2]/intra [3 + 2] cycloaddition of nitroalkenes is described in detail. The scope and limitations of the 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 dienophile and dipolarophile combinations with excellent chemical selectivity and regio- and diastereoselectivities. Hydrogenation of the bridged nitroso acetals produces aminocyclohexanemethanol derivatives in high diastereo- and enantio-selectivities. Further, insights into the mechanistic aspects of the Raney nickel promoted hydrogenation are reported. An intriguing influence of the nitro olefin α-substituent on the diastereoselectivity in the [4 + 2] cycloaddition has been documented. The reactivity of the α-chloro-substituted nitroalkene 26 as the heterodiene in the Diels-Alder reaction is assayed, and the use of the chlorine atom as a hydrogen surrogate is described.