Polyethylene glycol (PEG) as a benign solvent for Knoevenagel condensation

The Knoevenagel condensation of aromatic aldehydes with active methylene compound proceeded efficiently in polyethylene glycol at room temperature with l-proline as catalyst. The yield is high and the products had E-isomer dominantly. Polyethylene glycol containing l-proline could be recycled and reused for several times without noticeably decreasing in productivity.