Conjugate Addition of Nitroalkanes to Vinyl Sulfone

Significance

The authors describe an asym­metric conjugate addition of nitroalkanes to 1,1-bis(phenylsulfonyl)ethene (2) mediated by a ­cinchona alkaloid derived thiourea catalyst. With the bifunctional catalyst shown in the scheme, various nitroalkanes without α-substituents can be smoothly added to vinyl sulfone 2 with good enantioselectivity. After reduction of the nitro group ­followed by desulfonation, the Michael addition products can be converted into chiral α-branched amines. The synthetic application of this method was exemplified by the authors in the preparation of intermediate A, a key compound for the synthesis of (R)-3-ethyl-1,2,3,4-tetrahydroisoquinoline, which is a potent inhibitor of phenylethanol­amine N-methyltransferase.