1- and 3-Thioformylindolizines, a class of stable heterocyclic thioaldehydes, have been synthesised by a novel variation of the Vilsmeier reaction. They have also been obtained, in lower yield, by thionation of 1- and 3-formyl-indolizines, and by solvolysis of 3-ethoxymethylene-3H-indolizinium salts with sodium hydrogen sulphide. U.v. and 1H n.m.r. spectral data of the thioaldehydes are discussed.
