Canthin-4,5-diones

Synthesis of 3alkyl-3H-5,6-dihydro-5,6-dioxoindolo[3,2,l-de][l, 5]naphthyridines (Canthin-5,6-diones)... 

UV spectra of canthin-5,6-dione alkaloids 3 and 33, which were isolated by Ohmoto and Koike (3,48) from Picrasma quassioides, showed a characteristic absorption between 400 and 500 nm. This absorption was hypochromically shifted under acidic conditions. The UV spectrum of 3 in acidic solvents resembles that of 5-hydroxycanthin-6-one (16) (Fig. 3). This fact, that is, that 3 undergoes chemical shift in acidic solvents, indicates that the carbonyl at position 5 of the canthin-5,6-dione skeleton in 16 is protonated, producing 42a. On the basis of the above observations, Ohmoto and Koike refluxed 16 and 33 with dimethyl sulfate in acetone and synthesized 3 and 28 (Scheme 3). 

Similarly, Matus and Fischer (100) heated l-alkyl-/3-carboline with an excess of dialkyl oxalate and synthesized canthin-5,6-dione derivatives in 20-65% yield (Scheme 4). Catalytic hydrogenation of TV -benzylcanthin-... 

To the best of my knowledge, canthin-4,5-diones are the only known representatives belonging to the class of bridgehead nitrogen containing quinones represented by 192. A literature search did not reveal any compounds containing fragment 193. 

Methoxycanthin-2,6-dione (2), Canthin-2,6-dione (31), and IO-Hydroxy-3-methoxycanthin-2,6-dione (32)... 

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