Chapter 28: Retrosynthetic Analysis

Under acidic or basic conditions, this tautomerization equilibrium can be shifted to favor enol formation, allowing the carbon-carbon double bond and hydroxyl oxygen to participate in electrophilic substitution reactions. The chapter then transitions to enolate ion chemistry, explaining how deprotonation of the α-carbon by strong bases such as lithium diisopropylamide generates stabilized carbanions through resonance delocalization between the α-carbon and carbonyl oxygen. A central theme is regioselectivity in enolate formation, distinguishing between kinetic enolates, which form rapidly at lower temperatures with bulky bases, and thermodynamic enolates, which predominate at higher temperatures and represent the more stable conjugate base. Temperature, solvent polarity, and base identity all modulate which isomer is preferentially generated. The chapter details mechanistic pathways for α-halogenation, α-deuteration, and enolate acylation, emphasizing how stereoelectronic factors dictate observed selectivity and reaction outcomes. Enolate geometry—whether the carbanion adopts an E or Z configuration—directly influences the stereochemistry of products formed in subsequent transformations. The chapter also covers enamine chemistry, where secondary amines condense with carbonyl substrates to form enamines that function as electron-rich equivalents of enolates, reacting with electrophiles under considerably milder conditions. The Stork enamine reaction is presented as a practical alternative to traditional enolate chemistry that circumvents the need for strong bases. Throughout, these concepts are anchored to applications in pharmaceutical synthesis and natural product construction, demonstrating how mastery of enol and enolate reactivity provides access to sophisticated retrosynthetic strategies for assembling complex molecular structures.