4-Toluenesulfonic acid: an environmentally benign catalyst for Nazarov cyclizations

An efficient metal-free catalytic protocol for the electrocyclization of α-alkoxydienones to cyclopentenones (Nazarov reaction) in near to quantitative yields is described. The key parameters are the use of inexpensive 4-toluenesulfonic acid in 5 mol % at room temperature in acetonitrile or under solvent-free conditions. The versatility of the transformation is demonstrated with unpolarized dienones with good regioselectivities and excellent yields.