NITROALKENE [4+2]-CYCLOADDITIONS WITH 2-(ACYLOXY)VINYL ETHERS - STEREOSELECTIVE SYNTHESIS OF 3-HYDROXY-4-SUBSTITUTED-PYRROLIDINES

2-(Acyloxy)vinyl ethers undergo regioselective [4 + 2] cycloadditions with nitroalkenes (promoted by SnCl4) to afford subsutituted 5-acetoxy nitronates in good yields (68-91%). Endo/exo selectivity in the cycloadditions has been found to be dependent on the nitroalkene substitution; 2-aryl-1-nitroalkenes provided exclusively exo cycloadducts while 2-cyclohexyl-1-nitroalkene 28 afforded predominately endo cycloadducts in a ratio of 12:1. The resulting nitronates can be elaborated to N-tosyl-4-substituted-3-hydroxypyrrolidines by hydrogenolysis (160 psi of H-2/PtO2) or to bicyclic alpha-hydroxy lactams by [3 + 2] cycloaddition followed by hydrogenation (14.7 psi of H-2/Raney nickel). A chiral 2-acetoxyvinyl ether derived from (R)-2,2-diphenylcyclopentanol has been employed in the cycloaddition-hydrogenation sequence to prepare an optically active N-tosyl-3-hydroxypyrrolidine in 96% ee.