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