New procedures are described for converting unsaturated pyrrolizidine alkaloids to chemically reactive pyrrolic esters (dehydro-alkaloids), which are probable primary toxic metabolites formed from these alkaloids in vivo. Preparations of dehydro-necines, including dehydroretronecine (DHR) are also described. Dehydrocrotanecine, and four new dehydro-alkaloids, are described for the first time. The methods give superior yields to earlier procedures, do not require a high degree of chemical expertise, and are particularly suitable for making small amounts of compounds for toxicological experiments.