Quinolizidines. VII. Structure of O-methylpsychotrine: The endocyclic versus the exocyclic double bond structure in the dihydroisoquinoline moiety.

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Abstract

By comparison of its ultraviolet spectra in H2O at various pH's 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 13C 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 H2O. On heating in H2O at 90°C 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 H2O at 20°C were spectrometrically determined to be 9.16±0.02 and 8.80±0.02, respectively.

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