Daphniphyllum alkaloids. 11. Biomimetic total synthesis of methyl homosecodaphniphyllate. Development of the tetracyclization reaction
作者:Clayton H. Heathcock、Marvin M. Hansen、Roger B. Ruggeri、John C. Kath
DOI:10.1021/jo00035a008
日期:1992.4
A biomimetic total synthesis of (+/-)-methyl homosecodaphniphyllate has been developed. The synthesis starts with a triply convergent, tandem Michael addition-enolate alkylation, wherein amide 9, enoate 7, and alkyl iodide 5 are assembled in essentially quantitative yield to obtain compounds 13, 14, and 15. The major isomer 13 is converted in three steps into a 1:1 mixture of diols 18a and 18b. These diols are subjected to a two-step process involving Swern oxidation and treatment of the resulting dialdehyde sequentially with ammonia and acetic acid; pentacyclic unsaturated amine 23 is obtained in 82% yield. Three additional functional group steps are used to convert 23 into racemic methyl homosecodaphniphyllate ((+/-)-4). The synthesis requires nine steps and proceeds in 48% overall yield from 5, 7, and 9. The tetracyclization process was shown to proceed via dialdehyde 26, tricyclic aza diene 27, and tetracyclic imine 28. An interesting and potentially useful variant of the tetracyclization procedure employs methylamine or benzylamine instead of ammonia. In this modification, the final reaction product is pentacyclic amine 29, in which the isopropenyl double bond has also been reduced. It is suggested that this reduction occurs by intramolecular hydride transfer at the stage of cationic intermediate 33.