Asymmetric Synthesis of Nitrogen Heterocycles
The asymmetric synthesis of nitrogen heterocycles is of continuing interest to medicinal and bio-organic chemistry. Gareth Rowlands of the University of Sussex reports (Tetrahedron Lett. 2004, 45, 5347. ) diastereoselective routes to nitrogen heterocycles from the aziridine starting material 1. Cu-mediated cyclization of 2 gives the bicyclic amine 3, and Cu-mediated addition of 4 to 1 followed by in situ cyclization gives the lactam 5.
The reaction chemistry of aziridines is underdeveloped. This is particularly surprising because aziridines are easily prepared in high enantiomeric purity. Arlette Solladié-Cavallo of the Université L. Pasteur, Strasbourg, reports (J. Org. Chem. 2004, 69, 1409. ) that addition of the pulegone-derived sulfonium ylide 6 to an aldehyde tosylimine such as 7 proceeds to give the N-tosyl aziridine 8 in high ee and with good diastereocontrol. The sulfide precursor to 6 is recovered in almost quantitiative yield. N-Tosyl aziridines such as 8 are readily opened both by carbon and by heteroatom nucleophiles.
Amino acids continue to be useful starting materials for the preparation of enantiomerically-pure heterocycles. Henk Hiemstra of the University of Amsterdam and Floris Rutjes of the University of Nijmegen report (J. Am. Chem. Soc. 2004, 126, 4100. ) that cyclization of the allyl silane 9 followed by ring-closing metathesis leads to the highly-functionalized quinolizidine 11.