Lithium diisopropylamide-mediated aldol condensation of ethyl acetate and 3-pyridine aldehyde: batch limitations and advantages of continuous processing

Abstract

Lithium diisopropylamide (LDA)-mediated aldol condensation is a fundamental method for the synthesis of β-hydroxy esters. This strategy was applied in our previous synthesis of the general cap structure of Chidamide (3-(3-pyridyl)acrylic acid) using tert-butyl acetate (TBA) and 3-pyridine aldehyde, where the steric hindrance of the tert-butyl group effectively suppresses enolate self-condensation. However, this steric advantage also limits subsequent functionalization. In contrast, ethyl acetate (EA) offers greater synthetic flexibility due to the favorable leaving ability of the ethoxy group, but suffers from pronounced competing enolate consumption under batch conditions. Herein, we systematically investigated the limitations of batch LDA-mediated aldol condensation between EA and 3-pyridine aldehyde and developed a continuous-flow strategy to overcome these challenges. Under batch conditions at -60 ^oC, the major competing pathways involving ester enolates and 3-pyridine aldehyde were identified, and a comprehensive side reaction network was proposed. Based on this analysis, the use of a sufficient excess of LDA was found to be critical for improving product yield by suppressing proton transfer to ester enolates and ensuring sufficient deprotonation of EA. However, this strategy becomes ineffective at elevated temperatures, as evidenced by the decrease in yield from 90.1% at -60 ^oC to 45.8% at -20 ^oC, with 2.0 equiv of LDA. Then, a continuous-flow system was implemented to maximize LDA participation in EA deprotonation and suppresses enolate self-condensation and decomposition via precise residence time control. Product yields exceeding 90% were achieved at -20 ^oC for representative aldehydes using 1.2 equiv of LDA, together with good scalability.