Combretoxazolones: Synthesis, Cytotoxicity and Antitumor Activity Nguyen-Hai Nam, a Yong Kim, a Young-Jae You, a Dong-Ho Hong, a Hwan-Mook Kim b and Byung-Zun Ahn a, * a College of Pharmacy, Chungnam National University, Taejon 305-764, Republic of Korea b Korea Research Institute of Bioscience and Biotechnology, Taejon 305-600, Republic of Korea Received 23 July 2001; accepted 17 September 2001 Abstract—Two series of combretoxazolones including 3,4-diaryloxazolones (6) and 4,5-diaryloxazolones (7) were synthesized and evaluated for cytotoxicity and antitumor activity. Both series showed strong cytotoxicities against a variety of tumor cell lines. Compound 6g exhibited a significant antitumor activity in BDF1 mice bearing B16 murine melanoma cells with inhibition rates of 67 and 61% at 100 and 30 mg/kg/day, respectively. # 2001 Elsevier Science Ltd. All rights reserved. Combretastatin A-4 (CA-4, 1), isolated from Com- bretum caffrum, is one of the most potent antimitotic agents and binds to tubulin on the colchicine binding site. 1 It shows strong cytotoxicity against a variety of human cancer cells, including multi-drug resistant cell lines. 2 However, the low water solubility of CA-4 limits its efficacy in vivo therefore, a water-soluble sodium phosphate prodrug of CA-4 (2) was prepared and eval- uated for clinical applications. 3 Recently, the first com- bretastatin A-4 analogues (3, 4) showing potent antitumor activity in vivo were also described. 4 From a series of SAR studies, it was established that the cis- orientation of two phenyl rings is essential to strong cytotoxicity. However, cis-combretastatin analogues are prone to isomerize to trans-forms during storage and administration. The trans-forms of these compounds show dramatic reduction in both antitubulin activity and cytotoxicity. This prompted the syntheses of a number of cis-restricted five-membered heterocyclic analogues of CA-4 5 (5) (Fig. 1). Perusal of these analo- gues revealed that there was an apparent lack of oxazo- lone-type compounds (6, 3,4-diaryloxazolones; 7, 4,5- diaryloxazolones) that can be readily synthesized. In this paper we describe the synthesis and evaluation of cytotoxicity of this compound class, hereafter given a trivial name of combretoxazolone. The combretoxazolones 6 were synthesized as shown in Scheme 1. Acetophenones 8 6 were brominated to give a-bromoacetophenones 9 which were converted to a-hydroxyacetophenones 10 with betaine in moderate yields. Reacting of 10 with respective arylisocyanates and subsequent cyclization by refluxing in acetic acid (AcOH) provided 6 in 60–70% yields. Under the cycli- zation conditions, a 4-methoxybenzyl (PMB) group used to protect a phenol in 8e was removed to afford 6e directly. Reduction of a nitro group in 6f with zinc gave 6g in a good yield. The combretoxazolones 7 were con- structed as shown in Scheme 2. Coupling of 11 with various aryl benzaldehyde Ar 2 CHO 7 by a reported procedure 8 gave a-hydroxyketones 12. Reacting of a-hydroxyketones 12 with PMB-isocyanate and sub- sequent cyclization provided the intermediates 13. The N-PMB group was removed by refluxing in tri- fluoroacetic acid (TFA) for 3 h to give the expected products 7. Under these conditions, a benzyl group used to protect a phenol in 13e was removed to afford 7e directly. Compound 7g was obtained from 7f as descri- bed for 6g. All newly synthesized compounds were fully characterized by spectral methods such as IR, 1 H NMR, 13 C NMR and HRMS. 9 The synthesized combretoxazolones were evaluated 10 against a small panel of tumor cell lines including mur- ine melanoma (B16), human colon tumor (HCT116), human breast tumor (MCF-7), human lung carcinomas (A549) and prostate tumor (PC-3). The results are summarized in Table 1. 0960-894X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0960-894X(01)00622-9 Bioorganic & Medicinal Chemistry Letters 11 (2001) 3073–3076 *Corresponding author. Fax: +82-42-821-6566; e-mail: ahnbj@cnu. ac.kr