Proton magnetic resonance studies of compounds with bridgehead nitrogen. Part 36. Stereochemistry of 7-methyl-6,7,8,9,10,11,11a,11b,-12,13-decahydro-7aH-quino[1,2-c]quinazolines and 7-methyl-6,7,7a,8,9-,10,10a,10b,11,12-decahydrocyclopenta[4,5]pyrimido[1,6-a]quinolines

Abstract

Three diastereoisomeric 7-methyl-6,7,8,9,10,11,11a,11b,12,13-decahydro-7aH-quino[1,2-c] quinazolines and two 7-methyl-6,7,7a,8,9,10,10a,10b,11,12-decahydrocyclopenta[4,5]pyrimido[1,6-a]quinolines have been synthesised and their configurations assigned by a combination of ¹H n.m.r. and i.r. spectroscopy and the application of conformational analysis. For the quino[1,2-c]quinazolines the r-7a,c-11a,c-11b-compound adopts (in solution) the trans-BC conformation with an equatorial N-methyl group. The r-7a,c-11a,t-11b-compound exists in solution as an equilibrium mixture of two conformations, both of which possess the trans-BC ring junction with that conformation with the axial N-methyl group predominating over the equatorial N-methyl conformation. The predominant conformation for the r-7a,t-11a,t-11b compound is that with the cis-BC ring junction and an equatorial N-methyl group. Similar conformational preferences are shown by the r-7a,c-10a,c-10b- and r-7a,t-10a,t-10b-cyclopenta[4,5]pyrimido [1,6-a] quinolines.