Level 2 – Case 19
130
This mechanism would explain the formation of the final product. However,
this is only a mechanistic proposal and additional experimental evidence would
need to be compiled to confirm its validity.
In Summary
The stereoselectivity of the addition of pinacolone enolsilane 1 to E-alkoxy aldehydes bearing two stereocenters depends on the ability of the metal to form intermediate chelates. Those metals that monocoordinate the carbonyl group form Felkin products and the stereochemistry of these aldols is predicted by the FelkinAnh’s model. For metals chelating both the carbonyl and alkoxy groups, antiFelkin products are obtained. In these cases the cyclic-Cram’s model has to be
used to predict the stereochemical outcome of the reaction. Therefore, nonchelated (Felkin-Ahn) and chelated models (cyclic-Cram) have been successively
applied to understand the stereochemistry of the final reaction products.
A Ad dd di it ti io on na al l C Co om mm me en nt ts s
This problem is based on the work by Evans DA, Allison BD, Yang MG (1999)
Tetrahedron Letters 40:4457-4460 and by the work by Evans DA, Allison BD,
Yang MG, Masse CE (2001) J. Am. Chem. Soc. 123:10840-10852.
S Su ub bj je ec ct ts s o of f R Re ev vi is si io on n
Control of the stereoselectivity in nucleophilic additions to the carbonyl group.
Facial selectivity. Felkin Anh’s and Cram’s models. Reactivity of TMS enol
ethers towards electrophiles.
130
This mechanism would explain the formation of the final product. However,
this is only a mechanistic proposal and additional experimental evidence would
need to be compiled to confirm its validity.
In Summary
The stereoselectivity of the addition of pinacolone enolsilane 1 to E-alkoxy aldehydes bearing two stereocenters depends on the ability of the metal to form intermediate chelates. Those metals that monocoordinate the carbonyl group form Felkin products and the stereochemistry of these aldols is predicted by the FelkinAnh’s model. For metals chelating both the carbonyl and alkoxy groups, antiFelkin products are obtained. In these cases the cyclic-Cram’s model has to be
used to predict the stereochemical outcome of the reaction. Therefore, nonchelated (Felkin-Ahn) and chelated models (cyclic-Cram) have been successively
applied to understand the stereochemistry of the final reaction products.
A Ad dd di it ti io on na al l C Co om mm me en nt ts s
This problem is based on the work by Evans DA, Allison BD, Yang MG (1999)
Tetrahedron Letters 40:4457-4460 and by the work by Evans DA, Allison BD,
Yang MG, Masse CE (2001) J. Am. Chem. Soc. 123:10840-10852.
S Su ub bj je ec ct ts s o of f R Re ev vi is si io on n
Control of the stereoselectivity in nucleophilic additions to the carbonyl group.
Facial selectivity. Felkin Anh’s and Cram’s models. Reactivity of TMS enol
ethers towards electrophiles.