A formal synthesis of optically active clavicipitic acids, unusual azepinoindole-type ergot alkaloids
作者:Hirofumi Shinohara、Tsutomu Fukuda、Masatomo Iwao
DOI:10.1016/s0040-4020(99)00626-2
日期:1999.9
An enantioselective synthesis of (−)-cis- and (−)-trans-clavicipitic acid methyl esters (15a,b) has been achieved. The key steps of the synthesis were 1) C-4 selective functionalization of the indole ring via directed lithiation of 1-(triisopropylsilyl) gramine (2), and 2) stereoselective alkylation of the lithiated (2R)-(−)-2,5-dihydro-2-isopropyl-3,6-dimethoxypyrazine (Schöllkopfs bislactim ether)
(-)-顺式-和(-)-反式-克拉维酸甲酯(15a,b)的对映选择性合成已经实现。合成的关键步骤是1)通过1-(三异丙基甲硅烷基)草胺(2)的直接锂化对吲哚环进行C-4选择性官能化,以及2)锂化(2 R)-(-)-2的立体选择性烷基化带有1-叔丁氧羰基-3-氯甲基-4-[(E)-3-甲氧基-3-甲基-1-的1,5-二氢-2-异丙基-3,6-二甲氧基吡嗪(Schöllkopfs双内酯醚)(4)丁烯基]吲哚(10)。发现后者反应的非对映选择性高度依赖于溶剂或添加剂的配位能力。通过烷基化产物11a的轻度酸水解制备的旋光氨基醇13通过PPTS催化的脱水环化成功地转化为to庚啶吲哚14a,b。在N- Boc基团的最终脱保护过程中,观察到在15a和15b之间的有趣的顺式-反式异构化,其通过酸催化的开环和在C-6处的环化进行。