It is proposed, based upon comparisons with apomorphine, that the rigid pyrroleethylamine moiety of the ergolines is the portion of the molecule responsible for dopamine agonist activity. In support of this hypothesis, bicyclic and tricyclic ergoline partial structures 6, 11, 25, and 35 have been synthesized. In addition, some pyrazole isosters (37, 38, 40, and 45) of these rigid pyrroleethylamines have been made. All of the classes show dopaminergic activity in prolactin inhibition and in lesioned rat turning assays. The most potent drugs, the linear tricyclic pyrazoles 38 (R = Pr) and 40 (R = Pr), are comparable in potency with the highly active ergoline pergolide (41).

The title compound (+/-)-5 (R = Pro) (LY141865) has been resolved into a (-) isomer and a (+) isomer as the D- and L-tartrate salts, respectively. Biological studies have shown that dopamine agonist activity is a property of only the (-) isomer. Crystallographic analysis has proven that the absolute configuration of the active (-) isomer is the same as that of the natural ergolines.

A general method has been developed for removal of the pyrrole ring from the ergolines. Oxidation of various ergolines at the 2,3 double bond gave formamido ketones 10, which afforded the amino ketones 11. These were deaminated to give the ketones 12, from which the carbonyl group was removed by reduction. The resulting depyrroloergolines (9) have, in contrast to the ergolines, little or no dopamine agonist activity in two tests.