Properties of the canthin-6-ones

A second alkaloid S-methoxycanthin-6-one was demethylated with hydrobromic acid in acetic add to a phenol which formed water insoluble alkali salts. Upon treatment of the phenol with diazomethane in methylene chloride or its O-acetate in moist ether die alkaloid was regenerated. Like canthin 6-one, potassium permanganate oxidation gave )S-carboIine-l-carboxylic acid and the phenol with o-phenylenediamine furnished a hydroxyquinoxa-line. These facts established its structure. Reduction of the alkaloid with zinc in acetic acid resulted in a mixture of canthin-6-one and its 4,5-dihydro-derivative. The latter derivative could be easily dehydrogenated to the former with sulphur or selenium. 

A synthesis of 5-hydroxycanthin-6-one was accomplished by condensing the dilithium derivative of l-methyl-jS-carboline with diethyloxalate. A formal reversal of this synthesis was brought about by heating 5-methoxy-canthin-6-one methiodide in base whereupon anhydro-l,2-dimethyl-j8-carboline crystallized out. 

The third alkaloid from the Australian plant turned out to be 4-methylthiocanthin-6-one. Its chemistry paralleled that of the above bases. Oxidation yielded jS-carboUne-l-carboxylic acid, reduction gave canthin-6-one. It was synthesized as follows j8-carboline-l-carboxylic acid chloride was condensed with the magnesium ethoxy derivative of malonic ester to yield after hydrolysis 4-hydroxycanthin-6-one. This compound first treated with phosphorous oxychloride then reacted with potassium methyl mercaptide gave the alkaloid. 

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