Quinolizidines. XVI. Chiral Syntheses of 9-Demethylcephaeline and 10-Demethylcephaeline

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In order to establish the structure of the Alangium alkaloid demethylcephaeline, chiral syntheses of the two possible alternative structures, (-)-9-demethylcephaeline (1) and (-)-10-demethylcephaeline (2), have been accomplished through a "cincholoipon-incorporating route." The synthesis of (-)-2 started with an initial condensation of the tricyclic acid (-)-12b, prepared from the ester (-)-11b by alkaline hydrolysis, with 3-benzyloxy-4-methoxyphenethylamine and proceeded through the intermediates (-)-13b, (+)-15b, and (-)-14b. The 1'-epimers (-)-18b and (-)-17 were also produced in this reaction sequence. A parallel sequence of conversions starting with (+)-15a afforded (-)-1 via the intermediate (-)-14a, together with the 1'-epimer (-)-16 via (-)-18a. Unfortunately, however, lack of a sufficient amount of natural (-)-demethylcephaeline for a detailed and direct comparison precluded identification of either (-)-1 or (-)-2 with this alkaloid, leaving its chemistry incomplete.