Aldol self-condensation, unwanted

Aldehydes are an exception. You can make lithium enolates from some aldehydes such as j-PrCHO, but generally self-condensation is too fast, so unwanted aldol selfcondensation products are produced during the formation of the lithium enolate. To make specific enolates of aldehydes we need to use another type of derivative see later. 

Aluminum oxide has been one of the most frequently used reagents for self-condensation of ketones and aldehydes. However applications of aluminum-mediated aldol reactions to cross-coupling have appeared only recently in the literature. Aluminum reagents, in common with other Lewis acids, form aldol product chelates so that unwanted side reactions such as dehydration or secondary condensation may be avoided. ... 

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