Proton magnetic resonance studies of compounds with bridgehead nitrogen atoms. Part XXII. The stereochemistry and 1H nuclear magnetic resonance spectra of some perhydro-2-methylimidazo[1,5-a]-pyridin-1-ones and perhydro-2-methylpyrido[1,2-c]pyrimidin-3-ones

Abstract

cis-Fused and trans-fused ring conformations have been assigned to a range of perhydro-2-methylimidazo[1,5-a]-pyridin-1-ones and perhydro-2-methylpyrido[1,2-c]pyrimidin-3-ones from a study of their i.r. and n.m.r. spectra. Unexpectedly the conformational preferences of these compounds and the values of the geminal coupling constants for the methylene group protons situated between the nitrogen atoms are similar to those observed for the corresponding compounds not substituted with a carbonyl function. An unusual long range coupling between 3ax′-H and 8a-H of ca. 2·5 Hz is shown by the perhydro-2-methylimidazo[1,5-a]pyridin-1-ones. cis(2-H,3-H)-3,N-Dimethyl-2-piperidylcarboxamide reacts with formaldehyde to give, instead of the expected cis(8-H,8a-H)-perhydro-8-methylimidazo[1,5-a]pyridin-1-one, cis(10-H,10a-H)-perhydro-2,10-dimethylpyrido[1,2-c][1,3,6]-oxadioxadiazepin-1-one.