30 光学活性の天然Isotetrandrine,PhaeanthineおよびTetrandrineの合成
詳細
Total syntheses of optically active natural phaeanthine(1a), isotetrandrine(1b), and tetrandrine(1c) have been now accomplished through Route B. The intermediate coclaurine type base(14: racemate) was first synthesized, resolved, and N-methylated. The Ullmann condensation of the resulting optically active base(15) with the phenylethyl amine(17) afforded the diphenyl ether(18) which after debenzylation, was submitted to the second Ullmann condensation with the compound(20). The product(21) was then transformed to the cycloamide(24) via the compound(23) by the p-nitrophenyl ester method. The Bischler-Napieralski reactin of the resulting amide(24) gave the dihydroisoquinoline(25). Reduction of the compound(25) with NaBH_4, followed by N-methylation, gave a diastereomeric mixture consisting of isotetrandrine and phaeanthine. Separation of each diastereomer was effected by the aid of different solubilities of the picrates. The synthesized isotetrandrine and phaeanthine were identical with authentic samples of these alkaloids, respectively in all respects including their specific rotations. On the other hand, reduction of the compound(25; racemate) with Zn-dil.-H_2SO_4 gave chiefly dl-tetrandrine. Since dl-tetrandrine has been isolated from the natural source and resolved, this synthesis implies the total synthesis of natural tetrandrine.
