Furthermore, the quarternary salts of Mannich bases are latent sources of α,β -unsaturated carbonyl compounds required for condensation reactions. For example, for a base-catalyzed reaction utilizing but-3-en-2-one, it is better to employ the quarternized Mannich base derived from acetone, from which the unsaturated ketone is generated in situ by the action of a base, than to use the free but-3-en-2-one which is prone to undergo polymerization. The best example of this application is the Robinson annelation in steroid synthesis.

Mannich reaction finds huge applications in alkaloid synthesis. For example, the Robinson's synthesis of tropionone from calcium acetonedicarboxylate, methylamine and succindialdehyde gives the product in 90% yield (Scheme 16). It consists of two Mannich reactions, followed by spontaneous decarboxylation of the dibasic β -keto acid.

Examples: