Unusual Nazarov cyclization in near-critical water

The Nazarov reaction of trans,trans-dibenzylidene acetone in near-critical water with or without additional carbon dioxide produces 2,3-diphenyl-2-cyclopentenone instead of the conventional Nazarov product, 3,4-disubstituted 2-cyclopentenone. The use of organic solvents and strong mineral acids, generally required for this reaction, is avoided by using water and carbonic acid as a proton source. After an optimization procedure, 38% yield of 2,3-diphenyl-2-cyclopentenone was obtained. The presence of carbon dioxide had a clear positive effect on yield.