Reaction of chiral homobinuclear Fischer chromium carbene complexes with allyl alcohol in the presence of NaH and the following oxidative demetalation gave alpha-allyl esters in up to 97% ee via [3,3]-sigmatropic rearrangement reaction promoted by the metal 1,3-shift. On the other hand, chiral heterobinuclear tungsten carbene complexes with arene chromium complexes afforded alpha-allyl-beta-hydroxy
Asymmetric aldolcondensations using titaniumenolates of (R)-camphorselenoacetone and of methyl (R)-camphorselenoacetate are reported. The reactions with aromatic, α,β-unsaturated or aliphatic aldehydes proceed with good chemical yields giving a mixture of the syn and antialdols. These diastereoisomers were easily separated by column chromatography. The two syn diastereoisomers were obtained in enantiomerically
Investigation of a model for 1,2-asymmetric induction in reactions of .alpha.-carbalkoxy radicals: a stereochemical comparison of reactions of .alpha.-carbalkoxy radicals and ester enolates
作者:David J. Hart、Ramanarayanan Krishnamurthy
DOI:10.1021/jo00042a029
日期:1992.7
The stereochemical course of reductions and allylations of alpha-carbalkoxy radicals with chiral centers at the beta-position are reported. Radicals without polar substituents, with alkoxyl or acetoxyl groups, and with hydroxyl groups at the beta-position were examined. Reactions showed selectivities ranging from low (50:50) to high (99:1). The results are discussed in terms of transition-state models that emphasize the importance of (1) allylic conformational analysis (minimization of A1,3 and A1,2 strain), (2) torisonal strain (minimization of eclipsed interactions), and (3) stereoelectronic effects.