Function of Enolate Geometry

From a historical standpoint, the Zimmerman-Traxler study on the stereochemical aspects of the Ivanov reaction is of considerable significance (22). Their investigation revealed that the magnesium enediolate 19a, upon condensation with benzaldehyde (Et2 O, reflux 5 hr), afforded the erythro and threo acids 20E and 20T, respectively, in a 24 76 ratio (eq. [13]). In the analysis of plausible transition states, both chair A and boat B geometries were considered. Zimmerman and Traxler concluded that the major diastereomer 20T could 

One of the first important studies to address the implications of enolate geometry on aldol product stereochemistry was reported by Dubois and Fellmann (eq. [14]) (5c). The condensation of the (Z)- 

An additional important observation that (Z)-enolates exhibit erythro diastereoselection was made by Dubois and Fellmann (5b). Their investigation demonstrated that the magnesium enolate 24a (20°C, Et2 0) condensed with benzaldehyde under kinetic conditions to give exclusively the erythro diastereomer 25E (R3 = Ph, E T 95 5), and upon prolonged equilibration afforded the isomeric threo adduct (T E 95 5) (eq. [ 15]). Heathcock has reported 

100 0) could be ascribed to a crossover from a chair-preferred (tran- 

The studies just cited demonstrate several important design features pertaining to the objective of achieving highly stereoselective aldol condensations with Group I-II metal enolates. 

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