Synthesis and biological evaluation of azobicyclo[3.3.0] octane derivatives as dipeptidyl peptidase 4 inhibitors for the treatment of type 2 diabetes Tang Peng Cho a, * , Yang Fang Long a , Lin Zhi Gang a , Wang Yang a , Lu He Jun a , Shen Guang Yuan a , Fu Jian Hong a , Wang Lin a , Guan Dong Liang a , Zhang Lei a , Luo Jing Jing a , Gong Ai Shen a , She Gao Hong a , Wang Dan a , Feng Ying b , Yan Pang Ke a,b , Leng Ying b , Feng Jun a , Mong Xian Tai a a Shanghai Hengrui Pharmaceuticals Co., Ltd, 279 Wenjing Road, Shanghai 200245, China b State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, 555 Zu Chong Zhi Road, Shanghai 201203, China article info Article history: Received 10 March 2010 Revised 21 April 2010 Accepted 27 April 2010 Available online 18 May 2010 Keywords: Azobicyclo[3.3.0]octane derivatives Dipeptidyl peptidase 4 Oral glucose tolerance test Type 2 diabetes mellitus abstract A series of novel azobicyclo[3.3.0]octane derivatives were synthesized and evaluated as dipeptidyl pep- tidase 4 (DPP-4) inhibitors. The effort resulted in the discovery of inhibitor 2a, which exhibited excellent efficacies in an oral glucose tolerance test. Introduction of methyl group (2j) could prolong the inhibition of serum DPP-4 activity. Ó 2010 Elsevier Ltd. All rights reserved. Inhibition of dipeptidyl peptidase (DPP-4) has been a promising new approach to the treatment of type 2 diabetes mellitus (T2DM) since three drugs have been approved, Sitagliptin (Januvia Ò , MK- 0431), 1 Vildagliptin (Glavus Ò , LAF-237), 2 and Saxagliptin (Ong- lyza™, BMS-477118). 3–5 (Fig. 1). DPP-4 is a key regulatory enzyme and a signaling factor of insulin-stimulating hormones, glucagon-like peptide (GLP-1), and glucose-dependent insulino- tropic polypeptide (GIP). 6–8 Due to rapid degradation of the both incretin hormones by DPP-4, the half-lives of active GLP-1 and GIP are extremely short. 9 Inhibition of plasma DPP-4 enzyme leads to prolong the actions of endogenous GLP-1 and GIP, which ulti- mately decreases blood glucose levels and glucagon levels, and im- proves glucose homeostasis with a low risk of hypoglycemia and potential for disease modification. 10 Bicyclo[3.3.0]octane derivative 1 was previously reported to be a potent DPP-4 inhibitor, 11 we now report the synthesis, biological evaluation and SAR of this series of azobicyclo[3.3.0]octane com- pounds (2) which possess less stereocenters and can be synthe- sized more conveniently. As shown in Scheme 1, Boc-protected cyclopenta[c]pyrrol- 5(1H)-one 3 12 was first deprotected and then treated with various acyl chlorides to afford key intermediates 4a–g, which underwent reductive amination with (S)-1-(2-aminoacetyl)-pyrrolidine-2-car- bonitrile (5) to give 5b substituted compounds 2a–g. 13 The 5a isomer of urea 2a was synthesized as shown in Scheme 2. Ketone 4a was stereoselectively reduced to 5b-alcohol 6, which was mesylated and inverted to 5a-amine 9 by employing potas- sium phthalimide displacement followed by hydrolysis. Compound 9 was further substituted by (S)-1-(2-chloroacetyl)pyrrolidine-2- carbonitrile (10) to afford the desired 5a-substituted isomer 2h. 14 In order to discourage intramolecular cyclization of amine to nitrile so as to improve stability of compounds 2a and 2h, 15 5-methyl derivatives were also synthesized ( Schemes 3 and 4). Reaction of ketone 4a with tosylmethyl isocyanide gave a mixture of diastereomeric carbonitriles 11, which was selectively methyl- ated to give 5a-methyl derivative 12. Acid hydrolysis of the carbo- nitrile 12 followed by Curtis rearrangement afforded the corresponding 5a-methyl-5b-amine 14, which was substituted with 10 to give 5a-methyl isomer 2i. In order to obtain 5b-methyl isomer 2j, another synthetic approach was applied. Wittig methyl- enation of ketone 4a gave methylene 15 12 , which was treated with AgClO 4 and trimethylsilyl cyanide followed by hydrolysis to give desired tertiary 5b-methyl isocyanide 16 in a yield of 18%. 16 Isocy- anide 16 was hydrogenated and alkylated to give 5b-methyl isomer 2j. The stereochemistry of 5-methyl groups for 2i and 2j was determined by 2D NOESY. Enzymatic inhibitions of DPP-4, DPP-8, and DPP-9 of azobicy- clo[3.3.0]octane derivatives were outlined in Table 1. 17 The selectiv- ity of DPP-4 against DPP-8 and DPP-9 is critical as the inhibition of these two enzymes may be associated with profound toxicities. 18 Analog 2a showed an IC 50 of 9 nM against DPP-4. The selectivity ratio 0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.bmcl.2010.04.120 * Corresponding author. Tel.: +86 21 54752877; fax: +86 21 54759072. E-mail address: tangpc@shhrp.com (T.P. Cho). Bioorganic & Medicinal Chemistry Letters 20 (2010) 3565–3568 Contents lists available at ScienceDirect Bioorganic & Medicinal Chemistry Letters journal homepage: www.elsevier.com/locate/bmcl