Asymmetric Total Synthesis of (+)-Apovincamine and a Formal Synthesis of (+)-Vincamine. Demonstration of a Practical “Asymmetric Linkage” between Aromatic Carboxylic Acids and Chiral Acyclic Substrates
作者:Arthur G. Schultz、William P. Malachowski、You Pan
DOI:10.1021/jo961603p
日期:1997.3.1
Asymmetric syntheses of (+)-apovincamine (1a) and (+)-vincamine (2) are described. Construction of the pentacyclic diene lactam 14, a pivotal intermediate for synthesis of the cis-fused vincane-type alkaloids, began by Birch reduction-alkylation of the chiral benzamide 3 to give the 6-ethyl-1-methoxy-4-methyl-1,4-cyclohexadiene 4. Conversion of 4 to 2,5-cyclohexadienone 5 (92% overall yield from 3) and HPLC analysis of 5 demonstrated the diastereomeric purity resulting from the Birch reduction-alkylation to be >100:1. Dienone 5 was converted to butyrolactone 9 (47% overall yield from 3), and 9 was coupled with tryptamine (10) to give the amide 11a. Amido keto aldehyde 13 was obtained from 11a, and acid-catalyzed tricyclization and subsequent base-induced elimination of MeOH provided the desired cis-fused pentacyclic diene lactam 14. Examination of the two-step process 13 --> 14 revealed a novel base-induced epimerization at C(21) which served to interconvert 14 and 17, possibly by the involvement of a homoenolate. Diene lactam 14 was converted to (+)-apovincaminal 20a, an intermediate in the synthesis of (+)-apovincamine (1a) reported by Winterfeldt and co-workers. A new procedure for conversion of 20a to 1a involves conversion of 20a to the acetal 20b and treatment of 20b with NBS/AIBN in CCl4. The conversion of 1a to vincamine (2) has been reported by Oppolzer and co-workers.