Asymmetric synthesis and Lewis acid mediated type II carbonyl ene cyclisations of (R)-2-isopropyl-5-methylhex-5-enal

被引:12
作者
Braddock, DC [1 ]
Brown, JM
机构
[1] Univ London Imperial Coll Sci Technol & Med, Dept Chem, London SW7 2AY, England
[2] Dyson Perrins Lab, Oxford OX1 3QY, England
关键词
D O I
10.1016/S0957-4166(00)00316-5
中图分类号
O61 [无机化学];
学科分类号
070301 ; 081704 ;
摘要
The asymmetric synthesis of (R)-2-isopropyl-5-methylhex-5-enal in 98% ee is described. It was discovered that the key alkylation step employing an Evans chiral auxiliary and 3-methylbut-3-en-1-yl trifluoromethanesulphonate as the alkylating agent led to significant competing O-alkylation, a phenomenon not previously reported. Type II carbonyl ene cyclisation of the aldehyde with a range of Lewis acids led to either the (R,R)- or (R,S)-5-methylidenecyclohexanols without concurrent racemisation of the alpha stereogenic centre of the aldehyde. Conditions for effecting the easy racemisation of a model enantiomerically pure aldehyde, (S)-2-methylbutanal, were developed. In an effort to secure a dynamic kinetic resolution procedure, these conditions were applied to (R)-2-isopropyl-5-methylhex-5-enal. However, a competing and dominant Prins cyclisation occurred instead leading to a mixture of all possible cycloadducts, all of which were obtained in 98% ee. Any unreacted aldehyde was found to be enantiomerically pure. (C) 2000 Elsevier Science Ltd. All rights reserved.
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页码:3591 / 3607
页数:17
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