Kinetic resolution of tert-butyl (RS)-3-alkylcyclopentene-1-carboxylates for the synthesis of homochiral 3-alkyl-cispentacin and 3-alkyl-transpentacin derivatives
作者:Mark E. Bunnage、Stephen G. Davies、Richard M. Parkin、Paul M. Roberts、Andrew D. Smith、Jonathan M. Withey
DOI:10.1039/b407559e
日期:——
High levels of stereocontrol are observed in the conjugate addition of lithium dibenzylamide to tert-butyl (RS)-3-alkylcyclopentene-1-carboxylates (alkyl = Et, Bn), with addition occurring exclusively anti- to the 3-alkyl substituent. Treatment of a range of tert-butyl (RS)-3-alkylcyclopentene-1-carboxylates (alkyl = Et, Bn, iPr, tBu) with lithium (RS)-N-benzyl-N-α-methylbenzylamide indicates that good enantiorecognition is observed (E > 80) in their mutual kinetic resolution. In these reactions, conjugate addition of the lithium amide occurs exclusively anti- to the 3-alkyl substituent, with subsequent C(1)-protonation occurring preferably anti- to the 2-amino group in the 3-Et, 3-Bn and 3-iPr cases, giving predominantly the corresponding 1,2-syn-2,3-anti-diastereoisomers. Conjugate addition to (RS)-3-tert-butyl cyclopentene-1-carboxylate results in exclusive 2,3-anti -addition and a reversal in C(1)-protonation selectivity, giving predominantly the 1,2-anti-2,3-anti-diastereoisomer. Furthermore, the kinetic resolution of the tert-butyl (RS)-3-alkylcyclopentene-1-carboxylates (alkyl = Et, Bn, iPr, tBu) with lithium (S)-N-benzyl-N-α-methylbenzylamide proceeds efficiently, giving, at between 47 and 51% conversion, the resolved 3-alkylcyclopentene-1-carboxylates in >85 to >98% ee and the β-amino ester products of conjugate addition in high de, consistent with E > 80 in each case. Subsequent deprotection of the 1,2-syn-2,3-anti-3-alkyl-β-amino esters (alkyl = Et, Bn, iPr) by hydrogenolysis and ester hydrolysis gives the corresponding 1,2-syn-2,3-anti-3-alkylcispentacins in >98% de and 98 ± 1% ee. Selective epimerisation of the 1,2-syn-2,3-anti-3-alkyl-β-amino esters (alkyl = Et, Bn, iPr, tBu) by treatment with KOtBu in tBuOH gives the corresponding 1,2-anti-2,3-anti-3-alkyl-β-amino esters in quantitative yield and in >98% de, with subsequent deprotection by hydrogenolysis and ester hydrolysis giving the corresponding 1,2-anti-2,3-anti-3-alkylcispentacin hydrochlorides in >98% de.
在锂二苯甲酰胺对叔丁基(RS)-3-烷基环戊烯-1-羧酸酯(烷基 = Et, Bn)的共轭加成中,观察到高水平的立体控制,添加反应专门以反位于3-烷基取代基。将一系列叔丁基(RS)-3-烷基环戊烯-1-羧酸酯(烷基 = Et, Bn, iPr, tBu)处理与锂(RS)-N-苄基-N-α-甲基苄基胺显示出良好的对映识别(E > 80),其相互动力学分辨率中。在这些反应中,锂胺的共轭加成专门以反位于3-烷基取代基,随后C(1)-质子化更倾向于反位于2-氨基,在3-Et、3-Bn和3-iPr情况下,主要生成对应的1,2-syn-2,3-anti-立体异构体。对(RS)-3-叔丁基环戊烯-1-羧酸酯的共轭加成则产生专门的2,3-反位加成,并且C(1)-质子化选择性发生逆转,主要生成1,2-反位-2,3-反位的立体异构体。此外,与锂(S)-N-苄基-N-α-甲基苄基胺的叔丁基(RS)-3-烷基环戊烯-1-羧酸酯(烷基 = Et, Bn, iPr, tBu)的动力学分辨率有效进行,在47%至51%的转化率范围内,得到分辨的3-烷基环戊烯-1-羧酸酯,具有>85%至>98%的光学纯度(ee)和在相应共轭加成中得到的β-氨基酯产物具有高的立体选择性,与每一种情况下的E > 80一致。随后,通过氢解和酯水解去保护1,2-syn-2,3-anti-3-烷基-β-氨基酯(烷基 = Et, Bn, iPr)产生相应的1,2-syn-2,3-anti-3-烷基环戊酮,去除立体杂质大于98%且光学纯度为98 ± 1%。通过在叔丁醇中用KOtBu处理1,2-syn-2,3-anti-3-烷基-β-氨基酯(烷基 = Et, Bn, iPr, tBu)选择性表异构化得到相应的1,2-反位-2,3-反位-3-烷基-β-氨基酯,以定量产率和>98%的去保护率,随后通过氢解和酯水解去保护得到相应的1,2-反位-2,3-反位-3-烷基环戊酮盐酸盐,去保护率大于98%。