On the mechanism of Lewis acid promoted ene cyclizations of .omega.-unsaturated aldehydes
作者:James A. Marshall、Marc W. Andersen
DOI:10.1021/jo00048a016
日期:1992.10
The diastereomerically labeled d1 enals 1 and 2 were prepared from (S)-3-bromo-2-methylpropanol 5 by a sequence involving homologation to the allylic alcohol 13 and Sharpless epoxidation to either the alpha- or the beta-epoxide diastereomers 14 or 15. Reduction with LiAlD4 afforded the diastereomerically deuterated diols 16 and 20, respectively. Deoxygenation of the thionocarbonate derivatives 17 and 21 followed by THP ether cleavage and Swern oxidation afforded aldehydes 1 and 2. The undeuterated aldehyde 28 was similarly prepared. Cyclization of 1 and 2 with Me2AlCl afforded the cis-(E)-ethylidenecyclohexanols 3 and 4, respectively, as the major products in accord with a mechanism involving internal proton or deuteron transfer from the vinylic CHD grouping to the aldehyde carbonyl. Product ratios (E:Z, cis:trans) from the two aldehydes were significantly different, indicative of a substantial isotope effect.