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26.9.1.1.8.1 Variation 1: Intermolecular Aldol Reaction of Ketones

DOI: 10.1055/sos-SD-026-00987

Marsden, S. P.Science of Synthesis, (2005261074.

The aldol condensation of two carbonyl groups followed by dehydration is one of the simplest strategies for enone formation and has been extensively reviewed.[‌137‌] Intermolecular self-aldol reactions of ketones under acid or base conditions are rarely successful, but the reaction of ketones with aldehydes is a useful process. The simplest class of these reactions involves ketones with only one site available for enolization (by symmetry or substitution) and nonenolizable aldehydes. For example, condensation of 2-acetylthiophene with benzaldehyde promoted by hydrogen chloride gives enone 144,[‌138‌] while the coupling of acetone with cyclopentene-1-carbaldehyde (145) proceeds smoothly under the action of sodium hydroxide to give 146 (Scheme 32).[‌139‌] For nonsymmetrical ketones, selective aldol condensation at the less-hindered position is generally seen under basic conditions. The presence of additional steric hindrance can enhance this selectivity. Aldol condensation of the bicyclic ketone 147 with benzaldehyde gives a single regioisomeric enone 148, attributed to the presence of the β-quaternary center.[‌140‌] Similarly, 4-methylpentan-2-one (149, R1=iPr) gives a single methylene ketone 150 when treated with trioxane as a formaldehyde equivalent under Mannich conditions, whereas a mixture of isomers 150 and 151 is seen with the less hindered undecan-2-one [149, R1=(CH2)7Me].[‌141‌]

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