Abstract After deprotonation, saturated five-membered heterocyclic rings give a facile (3+ 2) cycloreversion. This approach is employed in syntheses of ketenes, thioaldehydes, thioketones, thioketenes and a thioquinone at low temperature. These highly reactive double-bond systems are usually trapped in situ using (2+ 2),(2+ 3), or (2+ 4) cycloadditions to give various types of heterocycles.
