Synthesis of NK109, an Anticancer Benzo[c]phenanthridine Alkaloid

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Abstract

A total synthesis of NK109 (7-hydroxy-8-methoxy-5-methyl-2,3-methylenedioxybenzo[c]phenanthridinium hydrogensulfate dihydrate), an anticancer benzo[c]phenanthridine alkaloid, is reported. The primary structure of this compound was erroneously communicated in 1973 as fagaridine (from Fagara xanthoxyloides) which is the 8-hydroxy regioisomer. NK109 has not yet been isolated from a natural source and therefore can only be obtained by synthesis. To study a wide variety of analogues, we decided to use a synthetic route via substituted benzylamine 5, which was obtained from the appropriate benzaldehyde and naphthylamine units. The benzo[c]phenanthridine ring was constructed by radical cyclization with tri-n-octyltin hydride and 2,2‘-azobis(2-methylbutyronitrile), followed by oxidative aromatization with MnO2. The resulting benzo[c]phenanthridine 6 was successfully methylated with methyl 2-nitrobenzenesulfonate. After deprotection of the benzyl group and subsequent hydration, NK109 was obtained. All reactions were performed under normal conditions. Purification was achieved only by recrystallization to give an overall yield of 40%.

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