Chemoenzymatic approach to optically active phenylglycidates: resolution of bromo- and iodohydrins
摘要:
Enantiomerically enriched phenylglycidates, precursors of the taxol C-13 phenylisoserine side chain and diltiazem. were prepared by kinetic resolution of anti-2-bromo-3-hydroxy- and anti-3-hydroxy-2-iodophenylpropanoates to provide enantioriched (2R,3R)- and (2S,3S)-halohydrins. The bulkiness and inflexibility of bromo and iodo groups in halohydrins have made them inaccessible to the active site of most of the lipases utilized for the hydrolysis of their acyloxy derivatives. In a set of 22 hydrolases screened herein, including native as well as commercial enzymes, only Aspergillus niger (Lipase AS, AMANO) could catalyze the hydrolysis with high enantioselectivity (E = 176). The resolved halohydrins easily underwent transformation to the corresponding (2S,3R)- and (2R,3S)phenylglycidates. (C) 26 E'lsevier Ltd. All rights reserved.
α, β-Unsaturated esters were made to react with N, N-dibromobenzenesulfonamide and formic acid in chloroform to give bromo-formyloxyesters. Methyl acrylate (1a), ethyl crotonate (2a), ethyl trans-cinnamate (3a), methyl tiglate (4a), methyl methacrylate (5a), and methyl phenylacrylate (6a) gave methyl 2-bromo-3-formyloxypropionate (1b), ethyl erythro-2-bromo-3-formyloxybutyrate (2b), ethyl erythro-2-bromo-3-formyloxy-3-phenylpropionate (3b), methyl erythro-2-bromo-3-formyloxy-2-methylbutyrate (4b), a mixture of methyl 3-bromo-2-formyloxy-2-methylpropionate (5b) and methyl 2-bromo-3-formyloxy-2-methylpropionate (5c), and methyl 3-bromo-2-formyloxy-2-phenylpropionate (6b) respectively in good yields.
Et<sub>2</sub>Zn-Mediated Rearrangement of Bromohydrins
作者:Lezhen Li、Peijie Cai、Qingxiang Guo、Song Xue
DOI:10.1021/jo800231s
日期:2008.5.1
and highly efficient method for the rearrangement of bromohydrins mediated by Et2Zn to synthesize carbonylcompounds was described. Various β-bromo alcohols were treated with 0.6 equiv of Et2Zn to form a zinc complex in CH2Cl2 at room temperature for 2 h, followed by 1,2-migration to give the corresponding carbonylcompounds. This remarkable and clean rearrangement is general for acyclic and cyclic bromohydrins
(hydroxy-, acetoxy-, or mesyloxy)-alkanoates with NaN3 or Et3N have been examined. The optimized procedure, in combination with subsequent reduction provides a general synthetic route to α-amino-α-alkenoic acid esters (9). The configuration of 8 and 9 have been shown to be of (Z)-geometry.
very rapid and efficient method has been developed for the synthesis of vicinal bromohydrins and alkoxybromides directly from an olefin without any catalyst. The reaction was performed in CH3CN−water (4:1) or alcohol using N,N-dibromo-p-toluenesulfonamide (TsNBr2) as the brominating agent. Excellent yields and regio- and stereoselectivities have been obtained. Bromohydrins are formed instantaneously
1,2-Asymmetric induction in radical reactions. Deuteration and allylation reactions of β-oxy-α-bromo esters
作者:Dennis P. Curran、P.S. Ramamoorthy
DOI:10.1016/s0040-4020(01)80402-6
日期:1993.5
Chiral radicals were generated by halogen abstraction reactions of β-oxy-α-bromo esters and their asymmetric deuteration and allylationreactions were studied.