Stereoselective (3+3)-Carbocyclization of Enamines with Nitroallylating Reagents

Abstract

The enamines from open-chain (3-pentanone) and cyclic (cyclopentanone, cyclohexanones, β-tetralone) ketones and the amines pyrrolidine, morpholine or (S)-2-methoxymethyl-pyrrolidine combine with E-3-phenyl-2-nitro-2-propen-1-yl or E-2-nitro-2-hepten-1-yl pivalate (NPP derivatives 6) to form six-membered rings. Monocyclic (7, 8) and bicyclic (9-14) products containing four new asymmetric carbon atoms are obtained stereoselectively. The diastereoselectivity observed with the chiral, proline-derived enamines of cyclohexanones is generally higher, and they furnish enantiomerically pure products (cf. the (+)-2-butyl-7-tert-butyl-3-nitro-bicyclo[3.3.1]nonan-9-one 14, formed in 37% yield as one of sixteen stereoisomers!).