CONFORMATIONS OF 11-MEMBERED AND 14-MEMBERED RING MONOLACTAMS

The conformations of the 11- and 14-membered ring lactams (1-azacyclou ndccanZone and 1-azacyclotetradecan-2-one) were studied by low-tempera ture NMR, infrared spectroscopy and molecular mechanics. Low-temperatu re NMR spectroscopy revealed that both compounds are present in severa l conformations in solution. By dynamic NMR between 25 and -120 degree s C of both lactams two different conformational processes were observ ed: a higher barrier process, identified as geminal H-1 site exchange of the protons in the methylene groups, caused by inversion of the chi ral conformation to its mirror image, and a lower barrier process, cau sed by interconversion between the different conformers in the solutio n. At least three different conformers were found for the 11-membered ring and at least two for the 14-membered ring. Infrared spectroscopy showed that one of the conformers of the 14-membered ring in solution was identical with the crystal conformation. Possible structures are p roposed for the two lactams in solution and their energies are calcula ted by molecular mechanics.