A novel reductive aminocyclization for the syntheses of chiral pyrrolidines: stereoselective syntheses of (S)-nornicotine and 2-(2 '-pyrrolidyl)-pyridines

(S)-Nornicotine 2 was synthesized in four steps. A key step in the synthesi s involved reductive aminocyclization of a 1,4-keloaldehyde with 2,3,4,6-te tra-O-pivaloyl-beta-D-galactosylamine 1 in the presence of sodium cyanoboro hydride, diastereoselectively affording the corresponding stereoisomer 6 in 45% yield. The aminosugar moiety could be easily removed by acidic hydroly sis to furnish 2. The aminocyclization was further extended to asymmetric s yntheses of novel chiral 2-(2'-pyrrolidyl)-pyridine ligands 12 and 13. (C) 1999 Elsevier Science Ltd. All rights reserved.