Conformational analysis of the ergot alkaloids ergotamine and ergotaminine

Conformational analyses by 'H NMR and potential-energy calculations are reported for the ergot alkaloids ergotamine and ergotaminine, both as free bases and as the protonated species. In the neutral forms in CDCl,, two strong intramolecular hydrogen bonds fix the molecules in folded conformations, but the protonated species adopt a more extended conformation, with a single intramolecular hydrogen bond. Of the 24 alternative conformations available to ergotamine, the most likely biologically active species in environments with low dielectric constants, e.g., the presumed ergotamine binding site, is the folded, hydrogen-bonded conformation observed for the neutral molecule in CDC13 solution.