Although the ramifications of non additive

Although the ramifications of non-additive SAR are context-dependent, the knowledge that perturbations in one region of a chemical series are not synergistic with perturbations in another region of the same chemical series is clearly pertinent to any medicinal chemistry effort. In our experience, non-additive SAR is often observed with GPCR targets, possibly due to the inherent mobility of the membrane-spanning α-helices which compose the small molecule binding sites. The detection of non-additive SAR is greatly facilitated by the synthesis of matrix libraries, an observation which has been described in detail elsewhere. All of the aforementioned compounds were synthesized as diastereomeric mixtures because the α-methyl phenoxyacetyl chlorides used in the library synthesis were commercially available only as racemates or were derived from racemic carboxylic CDK4 inhibitor precursors. We were interested in determining the influence of the α-methyl stereocenter on the potency and efficacy of these GHSR agonists, and so we performed a chromatographic separation of compound to afford optically pure diastereomers and , as shown in . The stereochemistry of the α-methyl group of the phenyoxyacetamide sidechain had a dramatic effect on GHSR potency, as illustrated by the 60-fold difference in EC and ⩾40-fold difference in observed between the () and () isomers. The stereochemistry for was assigned via X-ray crystallography, as shown in . describes similar stereochemical effects in the benzylamine series. In the case of the diastereomers of compound , we observed that virtually all GHSR activity resided in , as was ⩾300-fold less active in both functional and binding assays. With , the single diastereomer was 90-fold more potent than in the functional assay, and 70-fold more potent in the binding assay. Combined with the data from , it is clear that the stereochemistry of the α-methyl group of the acetamide sidechain makes a significant contribution to the potency of this chemotype.