16 吐根塩基O-Methylpsychotrineの構造 : Dihydroisoquinoline部の二重結合の位置について

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By comparison of its UV spectra in H_2O at various pH's with those of model compounds 10, 13, 15, 16, and 17, the Ipecac alkaloid O-methylpsychotrine has been shown to have the genuine 3,4-dihydroisoquino-line structure (1), not the exocyclic double bond structure (4), in the free base form as well as the protonated form. The ^1H NMR and ^<13>C NMR spectra of the alkaloid have also confirmed this endocyclic double bond structure in the dihydroisoquinoline part. These results indicate that the position of the double bond for simple 1-alkyl-3,4-di-hydroisoquinolines is endocyclic, and factors to stabilize the exocyclic double bond structure are discussed. 1-tert-Butyl-3,4-dihydro-6,7-dimethoxy-2-methylisoquinolinium iodide (27) 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 24 in good yield. The acid dissociation constants for 1-methyl- (15) and 1-tert-butyl-3,4-dihydro-6,7-dimethoxy-isoquinoline (17) in H_2O at 20℃ were UV spectrometrically determined to be 9.16±0.02 and 8.80±0.02, respectively.