15.21 : 交叉羟醛反应:概述

A crossed aldol reaction is an aldol condensation reaction between two different carbonyl compounds, at least one of which has an α hydrogen atom, with one compound acting as the nucleophile and the other as the electrophile.

If both carbonyl compounds can enolize, this reaction yields a mixture of products: two formed via self-condensation, and two via crossed-condensation.

In this reaction, preventing self-condensation is key to reaction efficiency by minimizing the number of products formed.

For example, formaldehyde, with no α hydrogen atoms, behaves solely as an electrophile while reacting in a base with another aldehyde that does contain α hydrogen atoms to yield a single crossed aldol product.

Other popular reactions that exhibit a similar phenomenon are Claisen–Schmidt condensation, the directed aldol reaction, and the Reformatsky reaction.