TANDEM INTER [4 + 2] INTRA [3 + 2] NITROALKENE CYCLOADDITIONS .5. ORIGIN OF THE LEWIS ACID DEPENDENT REVERSAL OF STEREOSELECTIVITY

The stereochemical course of the tandem [4 + 2]/[3 + 2] nitroalkene cycloaddition with chiral enol ethers has been shown to exhibit remarkable sensitivity to the nature of the Lewis acid promoter. Cycloadditions using (+)-camphor-derived vinyl ether 3 or (-)-trans-2-phenylcyclohexanol-derived vinyl ether 6 gave a predominance of the 1S hydroxy lactam 4 when promoted by Ti(O-i-Pr)2Cl2. The same vinyl ethers selectively produced the 1R hydroxy lactam 4 when promoted by MAPh. The (E)- and (Z)-1-propenyl ethers of (-)-trans-2-phenylcyclohexanol have been used to investigate the reversal of selectivity. Cycloadditions promoted by Ti(O-i-Pr)2Cl2 were found to be endo selective (12:1), while MAD and MAPh favored exo approach of the dienophile. High levels of asymmetric induction were observed in the endo mode of the cycloaddition for propenyl ethers (E)-13 and (Z)-13, 97.9% ee and 82.2% ee, respectively, but an erosion of selectivity occurred in the exo mode (72.2% ee) presumably through an s-cis enol ether conformation.