Title: Synthesis of trilobine, isotrilobine, and obaberine.
Abstract: Bisbenzylisoquinoline alkaloids, trilobine (2 : R-H) possessing a dibenzo-p-dioxin nucleus and obaberine (3) were synthesized. dl-O-Benzyl-8-bromo-N-norarmepavine (10 : R=H) synthesized through an established route was resolved and N-benzoylation of the resolved free base, followed by condensation with the amine (12) gave the diphenyl ether (13a). Debenzylation of 13a, followed by condensation with the compound (14) gave the ester (15a) which was cyclized to the cycloamide (17) via the phenethylammonium trifluoroacetate (16). Bischler-Napieralski reaction of the amide (17) and reduction of the imine (18), followed by N-methylation afforded the compound (19b) and (5) in a 2 : 5 ratio. Successive treatments of the compound (19b) with LiAlH4-AlCl3, hydrogen on Pd-C catalyst and HCHO-NaBH4 afforded obaberine (3). On the other hand, the N-benzyl base (19c) from the compound (5) was demethylated to give the demethyl base (20) which was heated in a saturated aqueous HBr solution to provide the compound (21). Finally, treatment of the compound (21) with CH2N2, followed by hydrogenolysis on Pd-C catalyst gave trilobine (2 : R=H).