15.20 : C–C 键断裂：逆羟醛反应

Recall that the aldol addition reaction is unfavorable for ketones under basic conditions.

The retro-aldol reaction is the exact reverse of the aldol addition, where the β-hydroxy ketone in the presence of an aqueous base is preferably cleaved into two ketone molecules as the retro-aldol products.

The reaction mechanism consists of three distinct steps.

In step one, the deprotonation of the β-hydroxy group generates an alkoxide ion intermediate.  

In the following step, the alkoxide's carbon–carbon bond is cleaved to form an enolate ion and a ketone molecule.

In the final step, the protonation of the enolate ion produces the second ketone molecule.

Similarly, the β-hydroxy aldehyde undergoes the retro-aldol reaction yielding the corresponding carbonyl  precursors.