Investigations aimed at the enantioselective total synthesis of indolizidine 223A, a recently described 5,6,8-trisubstituted indolizidine alkaloid from a dendrobatid frog, are described. tert-Butyl (2R,3R)-3-amino-2-ethylhexanoate and its (2S,3R)-diastereomer, prepared in several steps from lithium N-benzyl-N-[(1R)-1-phenylethyl]amide and tert-butyl (2E)-hex-2-enoate by the Davies protocol, served as chiral building blocks from which two complementary suites of diastereomeric intermediates were made en route to pivotal tert-butyl 3-[2-(alkoxycarbonylmethylene)pyrrolidin-1-yl]-2-ethylhexanoate intermediates 20 and 21. Cyclisation of these enaminones, achieved by acid hydrolysis of the tert-butyl esters and activation of the liberated carboxylic acids as mixed anhydrides, afforded 6-ethyl-7-oxo-5-propyl-1,2,3,5,6,7-hexahydroindolizine-8-carboxylate esters 28 and 29. Several further transformations of these potential scaffolds for the synthesis of the target alkaloidal systems are also reported.
本文介绍了旨在对
吲哚利嗪 223A 进行对映选择性全合成的研究,
吲哚利嗪 223A 是一种最近从一种石斛蛙中发现的 5,6,8-三取代
吲哚利嗪
生物碱。(2R,3R)-3-
氨基-
2-乙基己酸叔丁酯及其(2S,3R)-非对映异构体由 N-苄基-N-[(1R)-1-苯基乙基]酰胺
锂和(2E)-己-2-烯酸叔丁酯按戴维斯方案分几步制备而成、作为手性结构单元,从中制备出两套互补的非对映中间体,进而制备出关键的 3-[2-(烷氧羰基亚甲基)
吡咯烷-1-基]-
2-乙基己酸叔丁酯中间体 20 和 21。通过酸
水解叔丁酯并将释放出的
羧酸活化为混合酸酐,使这些烯酰胺酮环化,得到 6-乙基-7-氧代-5-丙基-1,2,3,5,6,7-六氢
吲哚嗪-8-
羧酸酯 28 和 29。此外,还报告了这些潜在支架在合成目标
生物碱系统过程中的几种进一步转化。