15.30 : 交叉克莱森缩合的 β-二羰基化合物

Claisen condensation between two different esters is termed crossed Claisen condensation.

If both esters contain α hydrogens, then four different types of ꞵ-ketoesters are formed. Of these, two are self-condensation products and the other two are crossed-condensation products.

A successful crossed Claisen condensation requires at least one ester molecule with no α hydrogen or the use of LDA base.

For instance, aryl esters have no α hydrogen and, therefore, cannot form enolates. They give crossed Claisen condensation products with other enolate-forming ester molecules.

Formate esters, in addition to containing no α hydrogen, are highly reactive compared to other esters, making them suitable for crossed Claisen condensation.

If a less reactive, no α-hydrogen-containing ester is used, an excess of it is required for a feasible reaction.

If α-hydrogen-containing esters are used, then a strong base like LDA facilitates a directed crossed Claisen condensation by generating an irreversible ester enolate, followed by its reaction with another ester molecule.