Multicomponent synthesis and anticonvulsant activity of monocyclic 2,6-diketopiperazine derivatives

Maciej Dawidowski 0 Jadwiga Turo 0 0 M. Dawidowski (&) J. Turo Department of Drug Technology and Pharmaceutical Biotechnology, Medical University of Warsaw , Banacha 1 Str., 02-097 Warsaw, Poland In this study, a series of diastereomerically pure monocyclic 2,6-diketopiperazine (2,6-DKP) derivatives were synthesized. The key synthetic step involved a multicomponent Ugi five-center, four-component reaction which was used to generate the convertible tert-butylamidoesters with both good yields and high diastereoselectivity toward the desired bioactive (S,S) absolute configuration. In subsequent steps, selective tertbutyl cleavage by use of BF3 CH3COOH and base-induced intramolecular cyclocondensation gave the final 2,6-DKP derivatives. The relative stereochemistry of the target molecules was confirmed by 1H NMR experiments. The compounds obtained were submitted to in vivo screening in animal models of epilepsy. Some of them displayed good activity in maximal electroshock seizure and 6 Hz tests. - Epilepsy is a major neurological disorder characterized by recurrent, spontaneous seizures. It affects approx. 50 million people (*1 % of the worlds population). Currently, the main treatment for epilepsy is the chronic administration of anticonvulsant drugs (AEDs). Although more than 30 AEDs are available, they provide satisfactory seizure control in only 60 % of patients. Additionally, major concerns of pharmacotherapy of epilepsy include high incidence of severe side effects and drugdrug interactions resulting from enzyme induction. Therefore, there is substantial need for new, more effective and safer AEDs (Kwan and Brodie, 2000; Brodie, 2001). Another essential challenge for epilepsy research is to develop therapeutics that would not only symptomatically suppress seizures, but would also inhibit or reverse progression of the sickness (the so-called disease modifying drugs; Perucca et al., 2007; Bialer and White, 2010). Presently, the compounds at different stages of development belong to various chemical classes and display diverse, often unknown mechanisms of action (Bialer et al., 2013). Most of these agents have been identified initially through in vivo screening in animal models of epilepsy rather than by a mechanistic approach. Although the animal models utilized for screening are associated with certain endpoints, it is generally accepted that they offer a good starting point in the early discovery of new AED candidates (Loscher and Schmidt, 1994; Malawska, 2005; Rogawski, 2006; Smith et al., 2007; Bialer and White, 2010; Banerjee and Sharma, 2012; Mishra and Ganguly, 2012). Recently, we have reported that chiral, bicyclic 2,6diketopiperazines (2,6-DKPs) derived from cyclic amino acids display a broad anticonvulsant activity in various animal models of epilepsy (Dawidowski et al., 2011, 2012a). Among the newly developed agents, compound ADD408003 exhibited a broad spectrum of seizure-suppressing activity. A preliminary structureactivity relationship (SAR) study of close analogs revealed that several factors are responsible for the anticonvulsant activity (Fig. 1): the (S,S) absolute configuration on the stereogenic centers, the presence of imide moiety and the benzene ring adjacent to 2,6-DKP core. Further, neither substitution of the imide nitrogen of ADD408003 with different alkyl and arylalkyl moieties nor expansion of the fused pyrrole chain markedly influenced the antiseizure activity. These findings led us to ask whether the related monocyclic 2,6-DKPs, derived from non-polar L-amino acids other than L-proline or L-homoproline display comparable anticonvulsant activity. The designed compounds fulfill all requirements determined on the basis of the preliminary SAR analysis, i.e., proper stereochemistry, the presence of imide moiety and benzene ring attached to 2,6-DKP scaffold. Further, due to the absence of the fused pyrrolidine or piperidine rings, these agents are less sterically constrained, which might allow for a better fit to the putative receptor(s). Results and discussion The target enantiopure, monocyclic 2,6-DKP derivatives 3ae were synthesized according to the reaction sequences depicted in Scheme 1. In the first step, the Ugi five-center four-component reaction (U-5C-4CR; Demharter et al., 1996) of the appropriate non-polar amino acid (L-valine, L-leucine, L-isoleucine, L-phenylalanine, L-phenylglycine), benzaldehyde, tert-butyl isocyanide, and methanol in the presence of a catalytic amount of iron(III) chloride gave the tertbutylamidoesters 1ae with chemical yields ranging from Fig. 1 Preliminary SAR of anticonvulsant 2,6-DKPs and proposed structural modifications Scheme 1 Synthesis of enantiopure 2,6-DKP derivatives 3ae 41 to 75 %. The reaction proceeded with the formation of the new stereocenter and in all cases, the major diastereomer was (2S,1S)-1, as judged by the 1H NMR analyses of the crude post-reaction mixtures. In general, the degree of diastereoinduction depended on the steric bulkiness of the side chain of the substrate amino acid. The highest diastereomeric ratios were measured for L-valine and L-isoleucine derivatives 1a (dr = 7.3/1) and 1c (dr = 9.0/1), respectively, bearing branched alkyl chains directly adjacent to the position C-2, located close to the newly formed stereocenter. The U-5C-4CR adducts of L-leucine and L-phenylalanine 1b and d, respectively, were formed with a slightly lower diastereoinduction (dr & 5/1 for each). This could be attributed to the lower steric hindrance of a methylene group adjacent to the carbon C-2. A surprisingly small degree of diastereoselectivity was found for the L-phenylglycine derivative 1e (dr = 1.5/1), having a bulky phenyl substituent in the position C-2. The possible explanation for this unexpected observation is the stabilization of the six-membered cyclic Ugi intermediate (Demharter et al., 1996) leading to (2S,1R)-1e by a pipi interaction of the two phenyl rings occupying axial positions. Attempts to quantitatively separate the diastereoisomers of 1ae by column chromatography or fractional recrystallization failed. Therefore, the obtained diastereomeric mixtures were used in the subsequent amide N-detertbutylation. Reaction of (2S,1S)/(2S,1R)-1ae with BF3 CH3COOH at 4555 C provided amidoesters 2ae with the yields range from 55 to 83 %. With the exception of 2e, all diastereomeric mixtures could be efficiently resolved by column chromatography. In the last step, compounds (2S,1S)-2ad were subjected to base-induced intramolecular cyclization. The reaction was accompanied by a notable degree of epimerization at stereogenic centers C-5 of the products 3. Nevertheless, in all cases, the unwanted (3S,5R) isomers could be separated by means of column chromatography (compounds (3S,5R)3a, c, d) or recrystallization (compound (3S,5R)-3b). Intramolecular cyclization of 1.4/1 diastereomeric mixture of (2S,1S)/(2S,1R)-2e gave (3S,5S)-3e and (3S,5R)-3e (a meso compound) in equal proportion. (...truncated)