Stereochemistry Knoevenagel reaction

In the Knoevenagel reaction, the thermodynamically more stable compounds are usually formed. However, a closely related transformation provides access to compounds of opposite stereochemistry. Condensation of phosphonoacetate (40) with aliphatic and aromatic aldehydes in the presence of N-methylmorpholine/TiCU yields product (41) with the thermodynamically more stable ( )-configura-tion. In contrast, titanated (40), obtained from the reaction of the sodium salt of (40) with ClTi(OCHMe2)3 reacts with aldehydes to give preferentially the thermodynamically less stable (Z)-isomer (42). ... 

Pyrazolones (154) and isoxazolones (155) can also be used in the Knoevenagel reaction. Thus condensations with aliphatic aldehydes, aromatic aldehydes and ketones in the presence of ethylenediammon-ium diacetate or other typical catalysts provide the corresponding alkylidene and benzylidene compounds in good yields (for stereochemistry see Section 1.1A new method involves the use of dicyclohexylcarbodiimide at 20 C without an additional catalyst. ... 

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