Lieben’s rule

Mixed aldol reactions between two different aldehydes generally give mixtures when each aldehyde can function both as enolate precursor and electrophilic component. In fortunate cases, one of the four possible aldols may predominate. An early generalization (Lieben s rule) stated that the major aldol results from attachment of the carbonyl carbon of the aldehyde with the lesser number of a-substituents to the a-carbon of the aldehyde having the greater number of a-substituents e.g. equation 18). ... 

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