The conventional pinacol coupling of carbonyl compounds was carried out by using metals or metal compounds as mediators under condition of anhydrous organic solvents. Previously, it was reported that the pinacol coupling of carbonyl compounds was achieved by zinc under strong base or weak acid condition. Now, a convenient, efficient pinacol coupling of aldehydes mediated by zinc in good yields has been carried out in ammonia water in the presence of a little crown ether. Adding a small amount of crown ether was highly effective to enhance the yields of pinacol from 63% to 84%. Nevertheless, the yield of pinacol was also strongly affected by the steric environment surrounding the carbonyl group. The pinacol coupling of aromatic aldehydes has been achieved with high yields under such a condition. However, the diastereose-lectivities of pinacols were not satisfying. Unfortunately, the ketones and most aliphatic aldehydes appeared inert under the same reaction conditions.

Key words: aqueous media, pinacol coupling, crown ether, zinc