Examples Predicting Aldol Stereochemistry

Aldol Reaction with ( )-Enolate Gives Anti I roduct 

Once again, consideration of the chelated, cyclic transition state, known as the Zimmerman-Traxler model, provides the rationale for this diastereo-selectivity. In the most favorable chair-Uke transition state, the aldehyde R group is in an equatorial position. This preferred orientation produces the syn product from the (Z)-enolate and the anti product from the (i )-enolate. Each transition state shown is forming a single enantiomer product attack by the enolate to the opposite face of the aldehyde would give rise to the other enantiomer. 

Zimmerman-Traxler Transition States Explain Aldol Stereoselectivity 

Predict the major products for each of the following reactions. 

The bulky r-butyl group gives rise to a (Z)-enolate and syn aldol product. Esters predominantly form ( )-enolates so the second example produces an anti aldol product. 

---

The correlation of metal enolate geometry and aldol product stereochemistry via diastereomeric chair-preferred transition states has been widely accepted (2,5,6,16). The observations that the steric bulk of the enolate ligand Rj and attendant aldol diastereoselection are directly coupled are consistent with the elaborated Zimmerman model illustrated in Scheme 3 for chair-preferred transition states. For example, for ( )-enoIates, transition state Q is predicted to be destabilized relative to Ci because of the Rj R3 variable steric... 

---