Structure–activity relationship studies on a series of cyclohexylpiperazines bearing a phanylacetamide as ligands of the human melanocortin-4 receptor Joseph Pontillo, a Joe A. Tran, a Nicole S. White, a Melissa Arellano, a Beth A. Fleck, b Dragan Marinkovic, a Fabio C. Tucci, a John Saunders, a Alan C. Foster c and Chen Chen a, * a Department of Medicinal Chemistry, Neurocrine Biosciences Inc., 12790 El Camino Real, San Diego, CA 92130, USA b Department of Pharmacology, Neurocrine Biosciences Inc., 12790 El Camino Real, San Diego, CA 92130, USA c Department of Neuroscience, Neurocrine Biosciences Inc., 12790 El Camino Real, San Diego, CA 92130, USA Received 30 June 2005; revised 17 August 2005; accepted 17 August 2005 Available online 23 September 2005 Abstract—Synthesis and structure–activity relationship studies of a series of cyclohexylpiperazines bearing an amide side chain as ligands of the MC4 receptor are discussed. Compounds such as 11i from this series are potent agonists (EC 50 = 33 nM, IA = 96%). Ó 2005 Elsevier Ltd. All rights reserved. The melanocortin-4 receptor (MC4R) has been associat- ed with regulation of food intake and energy homeosta- sis; therefore, MC4R agonists and antagonists could be potentially used for treatment of obesity and cachexia. 1 Several small molecule MC4R agonists 2 and antago- nists 3 from different chemical classes have been discov- ered since the report of the first potent and selective agonist cyclohexylpiperidine 1 in 2002 (Fig. 1). 4 Very recently, cyclohexylpiperazines such as 2a and b, with EC 50 values of 1.4 and 1.0 lM, respectively, 5 have been reported as MC4R agonists. 6 These data suggest that the extra 1-triazolemethyl group of 2b, which is designed to mimic that of compound 1, has little effect in interac- tion with the receptor. The phenylpiperazine 2c, howev- er, has an EC 50 value of only 30 lM in the same assay, suggesting the cyclohexyl group has stronger interaction with the receptor than does the phenyl ring, and that this interaction does not require the p-electron density of an aromatic ring. Previously we reported that benzylamines such as 2d (EC 50 = 4.7 nM) and 3 (K i = 1.8 nM) are potent and selective agonists and antagonists of the MC4R, 7,8 dem- onstrating that a basic side chain at the ortho-position of the phenyl group has profound impact on potency in comparison with 2c. Based on some initial hypotheses and in combination with computational modeling, we were able to identify a phenylacetamido side chain on the cyclohexyl ring for improved binding affinity at the MC4R. 9 For example, 4 has a K i value of 8.8 nM. We decided to investigate whether replacement of the tria- zole in cyclohexylpiperazine 2b with a phenylacetamide moiety would result in improvement of its potency as a MC4 agonist. The synthesis of the cyclohexylpiperazines 11 is de- scribed in Scheme 1. Reduction of the nitrile 5, obtained from the corresponding cyclohexanone and piperazine via a Strecker reaction, with LiAlH 4 in ether, followed by acylation with phenylacetyl chloride under basic con- ditions (Et 3 N/CH 2 Cl 2 ) afforded the amide 6. Debenzyla- tion of 6 under hydrogenolysis conditions (HCOONH 4 / Pd–C/MeOH/reflux) gave the amine 7, which was coupled with N-Boc-(4-Cl)phenylalanine (EDC/HBTU/ CH 2 Cl 2 ) to provide the intermediate 8. Deprotection of 8 with TFA afforded 9, which was coupled with various carboxylic acids to give the desired compound 10. When N-Boc-protected amino acids were used in the coupling, further deprotection with TFA afforded the final compound 11. The carbamate 12 was prepared by reaction of 9 with isobutyl chloroformate in the pres- ence of triethylamine. All final compounds were purified using an HPLC system equipped with a mass detector as 0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.bmcl.2005.08.061 Keywords: Melanocortin-4; Receptor; Agonist; Cyclohexylpiperazine; Synthesis. * Corresponding author. Tel.: +1 858 617 7600; fax: +1 858 617 7967; e-mail: cchen@neurocrine.com Bioorganic & Medicinal Chemistry Letters 15 (2005) 5237–5240