Quinolizidines. VII. Structure of O-Methylpsychotrine : The Endocyclic versus the Exocyclic Double Bond Structure in the Dihydroisoquinoline Moiety

By comparison of its ultraviolet spectra in H_2O at various pH\u27s with those of model compounds, 11,14,16,17,and 18,the Ipecac alkaloid O-methylpsychotrine has been shown to have the genuine 3,4-dihydroisoquinoline structure (1), not the exocyclic double bond structure (4), in the free base form as well as in the protonated form. The ^1H nuclear magnetic resonance (NMR) and ^C NMR spectra of the alkaloid have also confirmed this endocyclic double bond structure in the dihydroisoquinoline moiety. These results indicate that the position of the double bond for simple 1-alkyl-3,4-dihydroisoquinolines is endocyclic, and factors that stabilize the exocyclic double bond structure are discussed. 1-tert-Butyl-3,4-dihydro-6,7-dimethoxy-2-methylisoquinolinium iodide (30) has been found to be unstable in H_2O. On heating in H_2O at 90℃ for 10 min, it underwent ring opening to give 27 in good yield. The acid dissociation constants for 1-methyl-(16) and 1-tert-butyl-3,4-dihydro-6,7-dimethoxyisoquinoline (18) in H_2O at 20℃ were spectrometrically determined to be 9.16±0.02 and 8.80±0.02,respectively