Published: June 22, 2011 r2011 American Chemical Society 4977 dx.doi.org/10.1021/jm200555r | J. Med. Chem. 2011, 54, 4977–4986 ARTICLE pubs.acs.org/jmc Regioselective Suzuki Coupling of Dihaloheteroaromatic Compounds as a Rapid Strategy To Synthesize Potent Rigid Combretastatin Analogues Sewan Theeramunkong, † Antonio Caldarelli, † Alberto Massarotti, Silvio Aprile, Diego Caprioglio, Roberta Zaninetti, Alessia Teruggi, Tracey Pirali, Giorgio Grosa, Gian Cesare Tron,* and Armando A. Genazzani Dipartimento di Scienze Chimiche, Alimentari, Farmaceutiche e Farmacologiche, Universit  a degli Studi del Piemonte Orientale “A. Avogadro”, Via Bovio 6, 28100 Novara, Italy b S Supporting Information ’ INTRODUCTION Combretastatin A-4 (CA-4, 1) is a natural product derived from the African bush willow Combretum caffrum and has been first described over 20 years ago. 1 It is a strong tubulin depoly- merizing agent and therefore inhibits tumor growth and has antivascular effects. 2 Its prodrug (disodium salt water-soluble phosphate derivative) has now entered clinical trials for both solid and liquid tumors. 3 Similarly, a close derivative of combretastatin A4 (AVE8062) 4 is also undergoing clinical trials. A number of trials have been initiated with these drugs (www.clinicaltrials.gov), and both compounds have demonstrated a sufficient safety profile in phase I trials, suggesting that this may be a viable therapeutic strategy. Despite its low molecular weight and simple molecular structure, CA-4 (Figure 1) is one of the most powerful inhibitors of tubulin polymerization known to date. The structure can be divided into three separate components, the two rings (usually termed A and B) and the olefinic bridge. The structureÀactivity relationship (SAR) of CA-4 has been investigated thoroughly by a number of groups and is reasonably well-understood, 5 although full SAR comprehension may be in part hampered by synthetic limitations and by literature bias. In brief, it is thought that the cis- configuration of the olefinic bridge, the presence of the 3,4,5- trimethoxy group on ring A, and the para-methoxy group on ring B are all fundamental for antitubulin activity. Nonetheless, active compounds not obeying this general rule have been described. 6 It is important to note that (i) the cis olefinic bridge is able to undergo rapid cisÀtrans isomerization under the influence of heat, light, and protic media 5 and that (ii) the olefinic bridge represents a weak point for metabolic stability. 7 It is therefore not surprising that a number of studies have attempted to replace the olefinic bridge, in particular with more rigid and metabolically stable structures able to maintain the correct conformation of the two adjacent rings. 5,8 To this end, one of the strategies consists of the replacement of the olefinic bond with five-membered heterocyclic rings (Figure 1). Many of these novel compounds have been shown to be active, and indeed, some of the new compounds have been shown to display an increased potency. A second reason to replace the olefinic bridge is given by the difficulty in adding moieties to the two ring Figure 1. Rationale for the constrainment of combretastatin A-4. Received: January 26, 2011 ABSTRACT: Combretastatin A-4 (CA-4) is a potent tubulin depolymeriz- ing agent able to inhibit tumor growth and with antivascular effects. Although it is in clinical trials, the search for novel analogues that may display better/ different features is still ongoing. In this manuscript we describe the synthesis of novel constrained analogues of CA-4 obtained in only two synthetic steps exploiting a regioselective Suzuki coupling of dihalogenated heteroaromatic and alicyclic compounds. All the compounds synthesized have been eval- uated for cytotoxicity and for their ability to inhibit tubulin assembly. One of them, 38, displayed low nanomolar cytotoxicity and proved to have a pharmacodynamic profile similar to that of CA-4 and a better pharmacoki- netic profile, but most important of all, this synthetic strategy may pave the way for the easy and rapid generation of novel rigid analogues of combretastatins.