The stereochemistry of 1,6,7,12b-Tetrahydro-2H,4H-[1,2] oxazino[3′,4′:1,2]-pyrido[3,4-b]indole and of 1,2,3,6,7,12b-Hexahydro-3-methyl-4H-pyrimido[3′,4′:1,2]pyrido[3,4-6] indole

From an analysis of nmr spectral data, 1,6,7,12b-tetrahydro-2H,4H-1,3 ]oxazino3′, 4′ :1,2]-pyrido 3,4-b ]indole is shown to exist in solution at room temperature almost entirely in the cis-fused ring conformation with the nitrogen lone pair bisecting the C4 methylene group whereas under the same conditions 1,2,3,6,7,12b-hexahydro-3-methyl-4H-pyrimido3′,4′:1,2] pyrido-3,4-b ]indole exists as an approximately 50:50 equilibrium mixture of the cis and trans-fused ring conformations.