Alkaloids of the Australian Rutaceae: Pentaceras australis Hook. F. III. Identification of 4-Methylthiocanthin-6-one

Abstract

The alkaloid C₁₅H₁₀OSN₂ occurring in the bark of Pentaceras australis Hook. f. is shown to be 4-methylthiocanthin-6-one. The lactam ring is opened by alcoholic alkali giving 2-methylthio-2-(1'-β-carbolyl)acrylic acid, followed by the elimination of methyl mercaptan and recyclization to 4-hydroxycanthin-6-one. Confirmation of the relation- ship of the alkaloid to canthinone is provided by (i) the ultraviolet absorption spectra, (ii) the formation of methyl mercaptan and 4,5-dihydrocanthine by treatment with zinc and hydrochloric acid, and (iii) the formation, by treatment with Raney nickel, of canthinone, 4,5-dihydrocanthinone, or the methyl or ethyl esters of β-carbolylpropionic acid, depending on the conditions employed. The alkaloid has been synthesized from β-carboline-1-carboxylic acid by conversion of the acid chloride with malonic ester, followed by cyclization, to 4-hydroxycanthinone, thence to 4-chlorocanthinone which with potassium methyl mercaptide yields 4-methylthiocanthinone. The low basicity of the alkaloid is discussed, particularly in relation to the basicity of 4-methoxy- canthinone.