Tandem [4+2]/[3+2]-cycloadditions. 2. Asymmetric induction with a chiral vinyl ether

A tandem [4+2]/[3+2]-cycloaddition of nitroalkenes tethered to α,β-unsaturated ester dipolarophiles can be effectively triggered with a vinyl ether. Using the chiral vinyl ether 6 as the dienophile, nitroalkenes 1 and 2 undergo highly selective tandem cycloaddition. Hydrogenolytic cleavage of the resulting nitroso acetals produce α-hydroxy lactams (-)-14 and (-)-21 in >98% e.e. with excellent recovery of the chiral auxiliary. The high diastereoselectivity is seen to arise from a strong endo preference for the vinyl ether together with an intrinsic facial bias in the auxiliary.