Nitroalkene [4 + 2] Cycloaddition as a General and Stereoselective Route to the Synthesis of 3,3- and 3,4-Disubstituted Pyrrolidines

2,2-Disubstituted 1-nitroalkenes 12 undergo Lewis acid-promoted [4 + 2] cycloadditions with n-butyl vinyl ether to afford cyclic nitronates 14 as anomeric mixtures in good yields. The resulting nitronates were reduced with hydrogen in the presence of Adam's catalyst to afford 3,3-disubstituted pyrrolidines which were isolated in good yield as their N-p-toluenesulfonamides 17. Cycloadditions of (E)-2-nitrostyrene with ethyl (E)- and (Z)-1-propenyl ethers provided for the stereoselective synthesis of cis- and trans-3-phenyl-4-methyl-N-(p-tolylsulfonyl)pyrrolidine. Similarly, trans-3,4-diphenyl-N-(p-tolylsulfonyl)pyrrolidine was prepared from (E)-2-nitrostyrene and ethyl (E)-2-styryl ether.