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.
Convergent Evolution of Diastereomeric Mixtures of 5-Methoxy-pentylzirconocenes toward Trans-1,2-substituted Cyclopentanes
Original Synthesis of 1,2-Oxathiolan-2-oxides and Vinyl Cyclopropane Carboxylic Esters
作者:Alain Krief、Laurent Provins
DOI:10.1055/s-1997-6140
日期:1997.6
1,2-Oxathiolan-2-oxides or vinyl cyclopropane carboxylic esters are prepared chemoselectively and in rather good yields from methyl 2,4-pentadienoate, tributylstannyllithium and carbonyl compounds. The later are formally deoxygenated in the process and play the role of carbenes.