Asymmetric synthesis of (1R,2S,3R)-3-methylcispentacin and (1S,2S,3R)-3-methyltranspentacin by kinetic resolution of tert-butyl (±)-3-methylcyclopentene-1-carboxylate
作者:Mark E. Bunnage、Ann M. Chippindale、Stephen G. Davies、Richard M. Parkin、Andrew D. Smith、Jonathan M. Withey
DOI:10.1039/b306935b
日期:——
Conjugate addition of lithium dibenzylamide to tert-butyl (±)-3-methylcyclopentene-1-carboxylate occurs with high levels of stereocontrol, with preferential addition of lithium dibenzylamide to the face of the cyclic α,β-unsaturated acceptor anti- to the 3-methyl substituent. High levels of enantiorecognition are observed between tert-butyl (±)-3-methylcyclopentene-1-carboxylate and an excess of lithium (±)-N-benzyl-N-α-methylbenzylamide (10 eq.)
(E > 140) in their mutual kinetic resolution, while the kinetic resolution of tert-butyl (±)-3-methylcyclopentene-1-carboxylate with lithium (S)-N-benzyl-N-α-methylbenzylamide proceeds to give, at 51% conversion, tert-butyl (1R,2S,3R,αS)-3-methyl-2-N-benzyl-N-α-methylbenzylaminocyclopentane-1-carboxylate consistent with E > 130, and in 39% yield and 99 ± 0.5% de after purification. Subsequent deprotection by hydrogenolysis and ester hydrolysis gives (1R,2S,3R)-3-methylcispentacin in >98% de and 98 ± 1% ee. Selective epimerisation of tert-butyl (1R,2S,3R,αS)-3-methyl-2-N-benzyl-N-α-methylbenzylaminocyclopentane-1-carboxylate by treatment with KOtBu in tBuOH gives tert-butyl (1S,2S,3R,αS)-3-methyl-2-N-benzyl-N-α-methylbenzylaminocyclopentane-1-carboxylate in quantitative yield and in >98% de, with subsequent deprotection by hydrogenolysis and ester hydrolysis giving (1S,2S,3R)-3-methyltranspentacin hydrochloride in >98% de and 97 ± 1% ee.
(±)-3-甲基环戊烯-1-羧酸叔丁酯与二苄基酰胺锂的共轭加成具有高度的立体控制性,二苄基酰胺锂优先加成于环状 α、β-不饱和受体反 3-甲基取代基的表面。(±)-3-甲基环戊烯-1-甲酸叔丁酯与过量的(±)-N-苄基-N-α-甲基苄基酰胺锂(10 eq.而(±)-3-甲基环戊烯-1-甲酸叔丁酯与(S)-N-苄基-N-α-甲基苄基酰胺锂(10 eq、(1R,2S,3R,αS)-3-甲基-2-N-苄基-N-α-甲基苄基氨基环戊烷-1-羧酸叔丁酯,转化率为 51%,E > 130,收率为 39%,纯化后的脱率为 99 ± 0.5% de。随后通过氢解和酯水解进行脱保护,可得到 (1R,2S,3R)-3-甲基肉毒碱,de > 98%,ee 为 98 ± 1%。在 tBuOH 中用 KOtBu 处理 (1R,2S,3R,αS)-3-甲基-2-N-苄基-N-α-甲基苄基氨基环戊烷-1-羧酸叔丁酯的选择性环化反应,可得到 (1R,2S,3R,αS)-3-甲基-2-N-苄基-N-α-甲基苄基氨基环戊烷-1-羧酸叔丁酯、αS)-3-甲基-2-N-苄基-N-α-甲基苄基氨基环戊烷-1-羧酸叔丁酯,定量收率和de >98%,随后通过氢解和酯水解进行脱保护,可得到(1S,2S,3R)-3-甲基三尖杉酯盐酸盐,de >98%,ee为 97 ± 1%。