Asymmetric synthesis and Lewis acid mediated type II carbonyl ene cyclisations of (R)-2-isopropyl-5-methylhex-5-enal
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.
Different cyclization products are formed with different Lewis acids in the ene cyclization of 5-hexenals (see scheme on the right; a: methylaluminum bisphenoxide, b: Me2 AlCl, c: Sc(OSO2 CF3 )3 ). This is not surprising, but it is gratifying that these result can be rationalized with an internally consistent model.