Scope
Ethyl 2-methylacetoacetate and campholenic aldehyde react in an Aldol condensation. The synthetic procedure is typical for this type of reactions. In the process, in addition to water, an equivalent of ethanol and carbon dioxide are lost in decarboxylation.
Ethyl glyoxylate 2 and diethyl 2-methylglutaconate 1 react to isoprenetricarboxylic acid 3 (isoprene skeleton) with sodium ethoxide. This reaction product is very unstable with initial loss of carbon dioxide and followed by many secondary reactions. This is believed to be due to steric strain resulting from the methyl group and the carboxylic group in the cis-dienoid structure.
Occasionally an aldol condensation is buried in a multistep reaction or in catalytic cycle such as the one sketched below:
In this reaction an alkynal 1 is converted into a cycloalkene 7 with a ruthenium catalyst and the actual condensation takes place with intermediate 3 through 5. Support for the reaction mechanism is based on isotope labeling.
The reaction between menthone and anisaldehyde is complicated due to steric shielding of the ketone group. The solution is use of a strong base such as potassium hydroxide and a very polar solvent such as DMSO in the reaction below :
Due to epimerization through a common enolate ion (intermediate A) the reaction product has (R,R) cis configuration and not (R,S) trans as in the starting material. Because it is only the cis isomer that precipitates from solution this product is formed exclusively.
Read more about this topic: Aldol Condensation
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