Highly Efficient Darzens Reactions Mediated by Phosphazene Bases under Mild Conditions

The highly basic and poorly nucleophilic phosphazene base P-1-t-Bu promotes the Darzens condensation of alpha-halo esters with aromatic aldehydes affording alpha,beta-epoxy esters in nearly quantitative yields under mild conditions and in short reaction times. The more basic P-4-t-Bu phosphazene was found useful with low reactivity aldehydes. These reactions can be performed in aprotic organic solvents of low polarity, thus minimizing the hydrolysis of alpha,beta-epoxy esters which often accompanies the base-promoted Darzens condensations.