On the stereochemistry of the aldol-addition

NOE-experiments and STO-3G calculations suggest that E-enol borates exist in an U-conformation $\text{7a}$, whereas the Z-enol borates should prefer the extended conformation $\text{8b}$. These observations have been linked to the stereoselectivity of the aldol addition. The resulting predictions are in line with the stereoselectivity observed on addition of cyclohexenol borates to benzaldehyde.