Studies on the chemical constituents of rutaceous plants. LV. The development of a versatile method for the synthesis of antitumor-active benzo(c)phenanthridine alkaloids. 5. A new method for quaternization of the benzo(c)phenanthridine nucleus.

Abstract

A versatile method of synthesis of the quaternary benzo [c] phenanthridine alkaloid (1), having a tertiary benzo [c] phenanthridine skeleton, from the norbase (3) is described. Treatment of the norbase (3) with sodium borohydride in formic or in acetic acid gave the N-methyl-(5) or the N-ethyl-(7) dihydrobase, respectively, in good yield. The N-methyldihydrobase (5) could also be prepared by treatment of the norbase (3) with sodium borohydride and dimethyl sulfate in hexamethylphosphoric triamide. The dihydrobases (5 and 7) were readily convertible to the corresponding quaternary benzo [c] phenanthridine alkaloids by oxidation with Jones reagent or with 2, 3-dichloro-5, 6-dicyano-1, 4-benzoquinone (DDQ) in good yields. In addition, we found that the air-oxidation of the carbanions derived from the ψ-cyanides (11) of the quaternary bases (1) gave the corresponding oxybases (10) in excellent yields.