Substrate modification to increase the enantioselectivity of hydrolases. A route to optically-active cyclic allylic alcohols.
摘要:
The esterase-catalyzed resolution of the cyclic allylic acetates - 1-acetyloxy-2-cyclopentene, 1-acetyloxy-2-cyclohexene, and 1-acetyloxy-2-cycloheptene - was not enantioselective. We hypothesized that this inefficiency stems from the similarity in size of the substituents at the stereocenter (CH2-CH2 vs. CH=CH). To increase the enantioselectivity, we resolved precursors to these cyclic allylic alcohols: esters of trans-2-bromocycloalkanols (C5, C6, C7). These esters had a larger difference in the size of the substituents (CH2 vs. CHBr) at the stereocenter and were efficiently resolved by both cholesterol esterase and lipase from Pseudomonas cepacia (Amano P, PCL). A synthetic-scale resolution with PCL yielded the (1S,2S)-1-butanoyloxy-2-bromocycloalkanes in >98% ee. Heating with DBU to eliminate HBr, followed by reduction with LiAlH4 to cleave the ester, yielded the allylic alcohols: (S)-(-)-2-cyclopenten-1-ol (65% ee), (S)-(-)-2-cyclohexen-1-ol (>99% ee), and (S)-(-)-2-cyclohepten-1-ol (>98% ee).
Eu(dpm)3 [dpm: dipivaloylmethanate] catalyzes the ring-opening reaction of epoxides with acylhalides affording the corresponding 2-haloalkyl esters. The stereochemical course was confirmed as trans-addition in the case of the reaction of cyclohexene oxide.
Enantioselective Synthesis of Vicinal Halohydrins via Dynamic Kinetic Resolution
作者:Abel Ros、Antonio Magriz、Hansjörg Dietrich、Rosario Fernández、Eleuterio Alvarez、José M. Lassaletta
DOI:10.1021/ol052821k
日期:2006.1.1
[reaction: see text] Expanding the scope of enantioselective catalysis via DKR, transfer hydrogenation of a variety of cyclic alpha-halo ketones was accomplished using the Noyori/Ikariya (R,R)- or (S,S)-I catalysts and either HCO(2)H/Et(3)N or HCO(2)Na/n-Bu(4)NBr in H(2)O/CH(2)Cl(2) as the hydrogen sources. Good yields of vicinal bromo-, chloro-, and fluorohydrins with excellent de and ee levels were
Trifluoromethylation of Alkyl Radicals in Aqueous Solution
作者:Haigen Shen、Zhonglin Liu、Pei Zhang、Xinqiang Tan、Zhenzhen Zhang、Chaozhong Li
DOI:10.1021/jacs.7b06044
日期:2017.7.26
The copper-mediated trifluoromethylation of alkyl radicals is described. The combination of Et3SiH and K2S2O8 initiates the radical reactions of alkyl bromides or iodides with BPyCu(CF3)3 (BPy = 2,2'-bipyridine) in aqueous acetone at room temperature to afford the corresponding trifluoromethylation products in good yield. The protocol is applicable to various primary and secondary alkyl halides and
includes a six-memberedring bromolactonization of alkenyl carboxylic acids catalyzed by AZADO as the nitroxyl radical catalyst, and an intermolecular bromoesterification of alkenes with carboxylic acids using NMO as the N-oxide catalyst. We also accomplished a remote diastereoselective bromohydroxylation via an AZADO-catalyzed six-memberedring bromolactonization and a subsequent ring cleavage reaction
Solvent and catalyst-free bromofunctionalization of olefins using a mechanochemical approach
作者:Jonathan Wong、Ying-Yeung Yeung
DOI:10.1039/d1ra01816g
日期:——
Bromofunctionalizations of olefins are an important class of chemical transformations. N-Bromoimide reagents are commonly used in these reactions but catalysts and chlorinatedsolvents are often employed to achieve a reasonable reaction rate. In this report, we present a solvent and catalyst-free bromofunctionalization of olefins using mechanical force.