Green and catalyst-free synthesis of olsalazine analogs

ABSTRACT A greener protocol for the synthesis of olsalazine analogs is reported. Olsalazine analogs are prepared in high yields by a one-pot reaction of two molecules of mesalazine with benzotriazole-activated aspartic acid and glutamic acid in water under microwave irradiation. GRAPHICAL ABSTRACT A greener protocol for the synthesis of olsalazine analogs is reported. Olsalazine analogs are prepared in high yields by a one-pot reaction of two molecules of mesalazine and aspartic/glutamic benzotriazoles in water under microwave irradiation.


Introduction
The inflammatory bowel disease (IBD) points to a group of inflammatory disorders affecting the colon and small intestine. Crohn's disease and ulcerative colitis are the principal types of IBD. Aminosalicylates are compounds that contain 5-aminosalicylic acid (5-ASA). These drugs are used as a first-line therapy for treating IBD. However, the pathological pattern of IBD is different among patients. Therefore, there is no 'one-size-fits-all' treatment for the IBD. Treatment therapy should be adjusted according to the different needs of the patients (1). Aminosalicylates are effective in treating mild-to-moderate episodes of ulcerative colitis and Crohn's disease, as well as preventing relapses and maintaining remission (2). Different commercial aminosalicylate drugs are available, such as mesalamine 1 (Pentasa®), olsalazine 2 (Dipentum®) sulfasalazine 3 (Azulfidine®), and balsalazide 4 (Colazal®) Figure 1.
Mesalazine 1, also known as mesalamine or 5-ASA, is an anti-inflammatory drug used to treat the IBD but it has a major drawback that it is absorbed rapidly and extensively in the upper intestine before it reaches to the inflamed colonic site. Therefore, a prodrug approach for colonic delivery of 5-ASA has become a rational system of drug delivery in the treatment of IBD. Olsalazine 2 is an anti-inflammatory prodrug which is designed to deliver its active moiety, mesalazine, to the colon while avoiding the adverse effects associated with the use of a sulfapyridine carrier in sulphasalzine 3 (3). However, most of olsalazine synthesis methods are multistep with low overall yield (4-7).
The ideal linkage between the carrier and 5-ASA in mesalazine prodrugs should be dissociated to release the 5-ASA directly at the large intestine. Various N-aromatic acyl amides of amino acids are stable in the upper intestine but are easily hydrolyzed in the large intestine (8)(9)(10). Thus orally administered 5-ASA-amino acid conjugates might be safely delivered to the colon, and act as prodrugs of 5-ASA.
In this study, we report an efficient, greener synthesis of olsalazine analogs (5-ASA-Asp/Glu-5-ASA) with benzotriazole-activated aspartic acid and glutamic acid in water in the absence of any acid/base catalyst. To the best of our knowledge, this is the first catalyst-, auxiliary-free and environmentally benign synthesis of olsalazine analogs which affords near quantitative yields under mild reaction conditions, in short reaction time (2 h) and with simple work up.
Water was used as the reaction medium because it offers several advantages such as: (i) it is cheap, noninflammable, non-toxic and safe for use; (ii) it eliminates the additional efforts required to make the substrates/ reagents dry before use and thus reduces/eliminates the consumption of drying agents, energy and time; (iii) the unique physical and chemical properties of water often increase the reactivity or selectivity unattainable in organic solvents; and (iv) the product may be easily isolated by filtration (18,19).
Treatment of N-protected amino acids 5 by our standard method (20) using thionyl chloride and 1Hbenzotriazole gave the corresponding benzotriazole derivatives 7. The reaction of N-protected amino acyl bisbenzotriazoles 7 with two molecules of 5-ASA 1 in water under microwave irradiation for 2 h afforded the desired conjugates 8 in good yield (75-77%) (Scheme 1, Table 1). To elucidate the range of applicability, 7a (Cbz-L-Asp-diBt) was selected for coupling with two molecules of 5-ASA in organic solvents (EtOH, DMF, THF), tap and distilled water, and also under solvent-free conditions. Tap and distilled water gave essentially identical results, whereas the other solvents gave inferior results to water. We also tried the reaction with different times (5 min to 3 h under microwave irradiation and up to 6 h under conventional heating). Optimization of reaction conditions revealed the best results under microwave heating at 70°C for 2 h using water as a reaction medium. Conventional heating was less efficient ( Table 2).
The enantiomeric purity of compound 8a was confirmed by the HPLC analysis; thus, using a Chiralcel-OD column, 8a showed a single peak (9.13 min), whereas the racemic mixture 8a ′ showed two peaks (9.12 and 9.60 min), one of which had nearly the same retention time as the single peak from 8a. We have also observed duplication of peaks in 13 C NMR of 8a ′ and 8b ′ , whereas in the case of 8a and 8b, extra peaks were not observed.

Conclusion
In summary, we developed a greener protocol for the synthesis of olsalazine analogs without the use of catalysts, synthetic auxiliaries or organic solvent. The heterogeneous reaction runs in water and produces good yields of olsalazine analogs without loss of chirality.

Experimental design
Commercial reagents were purchased from Sigma-Aldrich and were used without purification. Solvents Table 1. Olsalazine analogs.
Product (8) Intermediate (7) Reactant (  Distilled water 75 97 a Crude yield after workup with aq. Na 2 CO 3 and 4N HCl. b Purity of the crude product was checked using the HPLC analysis. were purified by distillation. Melting points were determined on a capillary point apparatus equipped with a digital thermometer. NMR spectra were recorded in DMSO-d 6 on Mercury or Gemini NMR spectrometers operating at 300 MHz for 1 H (with TMS as an internal standard) and 75 MHz for 13 C. Elemental analyses were performed on a Carlo Erba-EA1108 instrument. All microwaveassisted reactions were carried out with a single-mode cavity Discover Microwave Synthesizer. The reaction mixtures were transferred into a 10-mL glass pressure microwave tube equipped with a magnetic stirrer bar. The tube was closed with a silicone septum and the reaction mixture was subjected to microwave irradiation (Discover mode; run time: 60 s; PowerMax-cooling mode).