DIRECTED ALDOL REACTION.
q A specific crossed aldol product can be obtained if one generates the necessary enolate quantitatively and then adds the other carbonyl component. Because the conversion of a carbonyl compound to its enolate is complete when LDA is used all of this carbonyl compound removed and converted to the enolate, so that only one enolate is present. When, subsequently another carbonyl component is added, there is also only one carbonyl component.
q To see which carbonyl compound is needed to supply the enolate and which the carbonyl component, draw a squiggly line between the alpha and beta carbons of the target aldol. The fragment on the carbonyl side of the aldol must supply the enolate, and the fragment on the alcohol side must supply the neutral carbonyl group.