Compounds with bridgehead nitrogen 66 —NMR spectra and stereochemistry of derivatives of 1,2,3,4,6,7,12,12b‐ octahydropyrimido [1′, 6′:1,2] pyrido [3,4‐ b ]indol‐2‐one and of 1,3,4,6,7,11b‐hexahydropyrimido [6,1‐ a ] isoquinolin‐2‐one

Comparison of the NMR spectra of 3‐methyl‐1,2,3,4,6,7,12,12b‐octahydropyrimido [1′,6′ : 1,2]pyrido[3,4‐ b ] indol‐2‐one and of cis ‐(H‐6, H‐12b)‐3,6‐dimethyl‐ 1,2,3,4,6,7,12,12b‐octahydropyrimido [1′,6′:1,2]pyrido[3,4‐ b ] indol‐2‐one shows the former to exist in solution at 25°C as an equilibrium mixture containing ca. 20% N ‐inside‐ cis ‐ and ca. 80% trans ‐fused conformers, and the latter to adopt exclusively the trans ‐fused conformer. The related 3‐methyl‐ 1,3,4,6,7,11b‐ hexahydropyrimido [6,1‐ a ] isoquinoline‐2‐one adopts a ca. 53% trans ‐fused 47% cis ‐fused equilibrium.