For example, two molecules of acetone condense to a single compound mesityl oxide in the presence of an ion-exchange resin:[1] For synthetic uses, this is generally an undesirable, but spontaneous and favored side-reaction of mixed aldol condensation, and special precautions are needed to prevent it.
Therefore, chemists have adopted many ways to prevent this from occurring when performing a crossed aldol reaction.
[3] The deprotonation step using LDA is so fast that the enolate formed never gets a chance to react with any unreacted molecules of cyclohexanone.
Using LDA will not work when attempting to make enolate ion from aldehydes.
One way to get around this is to turn the aldehyde into a silyl enol ether using trimethylsilyl chloride and a base, such as triethylamine, and then perform the aldol condensation.