Enolization of substituted acetic acids using dialkylboron halides followed by aldolization with aldehydes and ketones provides the corresponding β-hydroxyacids in very good yields and anti selectivities. The reaction, sensitive to sterics on the acid, provides the β-hydroxy acids in particularly excellent yields and excellent anti-selectivties for acids with electron-deficent α-substituents.<br/>Based on the proposed Zimmerman-Traxler TS for aldolization of enolates, aldolization of acid enolates should not be stereoselective. We therefore believe that the observed solvent, temperature, electronic and steric effects may be hinting at an increased carbanion character of these enolates. This belief is exemplified by the boron mediated aldolization of  glycolic acids and alkoxy acids. While alkoxy acids do not enolize under any condition, phenoxy acids undergo enolization and aldolization to provide anti selective aldols. The enolization of glycolic acid with three equivalents of dialkyl boron reagent also provided the corresponding aldol. Thus, we can conclude that substituents that allow delocalization of the 'carbanion' lead to succesful aldolization.

Enolate or carbanion: Exploring the carbanion character in boron enolates towards the direct aldolization of glycolic acid

ACS Spring 2022

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