Development of peptidomimetic ligands of Pro-Leu-Gly-NH(2) as allosteric modulators of the dopamine D(2) receptor.

A variety of stable, small-molecule peptidomimetic ligands have been developed to elucidate the mechanism by which the neuropeptide Pro-Leu-Gly-NH(2) (PLG) modulates dopaminergic neurotransmission. Photoaffinity labeling ligands based upon PLG peptidomimetics have been used to establish that PLG binds to the D(2) dopamine receptor at a site that is different from the orthosteric site, thus making PLG and its peptidomimetics allosteric modulators of the dopamine receptor. Through the design, synthesis and pharmacological evaluation of conformationally constrained peptidomimetics containing lactam, bicyclic, and spiro-bicyclic scaffolds, support was provided for the hypothesis that the bioactive conformation of PLG is a type II ß-turn. In addition, studies with peptidomimetics designed to mimic either a type VI ß-turn or polyproline II helix conformation yielded molecules that were able to modulate dopamine receptors because of their ability to place the carboxamide NH(2) pharmacophore in the same topological space as that seen in the type II ß-turn. Extensive studies with the spiro-bicyclic PLG peptidomimetics also established that both positive and negative modes of modulation were possible for the same series of peptidomimetics simply as a result of minor differences in the stereochemistry about the bridgehead carbon within the scaffold. This information was used to transform existing positive modulators into negative modulators, which demonstrated that small structural changes in the spiro-bicyclic dopamine receptor modulators are capable of causing major changes in the modulatory activity of PLG peptidomimetics.

Transformation of Pro-Leu-Gly-NH2 peptidomimetic positive allosteric modulators of the dopamine D2 receptor into negative modulators.

The synthesis of dimethyl derivatives of 5.6.5 spiro bicyclic lactam Pro-Leu-Gly-NH(2) peptidomimetics was carried out to test the hypothesis that by placing methyl groups on the ß-methylene carbon of the thiazolidine ring steric bulk would be introduced into the topological space that the ß-methylene carbon is believed to occupy in the negative allosteric modulators of the dopamine D(2) receptor. With such a modification, a positive allosteric modulator would be converted into a negative allosteric modulator. This hypothesis was shown to be correct as 3a and 4a where found to be negative allosteric modulators, whereas their unmethylated derivatives were positive allosteric modulators of the dopamine D(2) receptor.

Room-temperature Pd-catalyzed amidation of aryl bromides using tert-butyl carbamate.

The scope of Pd-catalyzed synthesis of N-Boc-protected anilines from aryl bromides and commercially available tert-butyl carbamate is described. For the first time, this process can be conducted at room temperature (17-22 degrees C) using a combination of Pd(2)dba(3).CHCl(3) and a monodentate ligand, tert-butyl X-Phos. Use of sodium tert-butoxide is crucial to the success of the reaction, which proceeds in 43-83% yield.

Allosteric modulation of the dopamine D2 receptor by Pro-Leu-Gly-NH2 peptidomimetics constrained in either a polyproline II helix or a type II beta-turn conformation.

Type II beta-turn mimics and polyproline II helix mimics based upon diastereoisomeric 5.6.5 spiro bicyclic scaffolds have provided two pairs of Pro-Leu-Gly-NH(2) (PLG) peptidomimetics with contrasting dopamine receptor modulating activities. Compounds 1a and 3a were found to be positive allosteric modulators of the dopamine receptor, while the corresponding diastereoisomeric compounds 1b and 3b provided the first PLG peptidomimetics with the ability to decrease the binding of agonists to the dopamine receptor. The positive allosteric modulating activity of 3a supported the hypothesis that a polyproline II helix conformation is the bioactive conformation for the PLG analogue Pro-Pro-Pro-NH(2). The results also show that a change in the bridgehead chirality of the 5.6.5 scaffold brings about opposite effects in terms of the modulation of the dopamine receptor.