HETEROCYCLES

Communication | Regular issue | Vol. 81, No. 11, 2010, pp. 2465-2470
Received, 17th August, 2010, Accepted, 21st September, 2010, Published online, 22nd September, 2010.
DOI: 10.3987/COM-10-12046

■ Design and Synthesis of Novel Retinoid Synergists Having a Dibenzodiazepine Skeleton

Kiminori Ohta, Emiko Kawachi, Koichi Shudo, and Hiroyuki Kagechika*

Graduate School of Biomedical Science, Tokyo Medical and Dental University, 2-3-10, Surugadai, Kanda, Chiyoda-ku 101-0062, Japan

Abstract

Based on the structures of potent RXR agonists 2 and 3, novel dibenzodiazepine derivatives 4 – 6, containing two diphenylamine substructures, were designed as RXR modulator candidates and synthesized by utilizing Pd-catalyzed and Cu-promoted diphenylamine-generating reactions as key reactions. These compounds showed retinoid-synergistic activity, enhancing the HL-60 cell differentiation-inducing ability of the RAR agonist Am80.

Retinoids have a broad spectrum of biological activities related to cellular differentiation and proliferation, and are essential for normal embryonic development in vertebrates.1 Their biological responses are mediated by binding to and activation of retinoic acid receptors (RARs),2 which act in the form of heterodimers with another class of retinoid nuclear receptors, retinoid X receptors (RXRs).3 All-trans retinoic acid (ATRA) binds to RARs, and its 9-cis isomer (9-cisRA) binds to both RARs and RXRs (Figure 1).4 Various synthetic retinoids, such as Am80 (1), bind only to RARs with an affinity that correlates well with most retinoidal activities.5 RXRs are transcriptionally silent partners of RARs, and RAR-RXR heterodimers activated by RAR ligands regulate the expression of specific genes.3 We have developed several RXR-specific agonists and reported their retinoid synergistic activities.6 For example, RXR agonists, such as HX600 (2)7 and DA023 (3),8 themselves exhibited no retinoidal activity, but strongly enhanced the potency of ATRA or Am80 (1). Since RXRs form heterodimers with various nuclear receptors, such as vitamin D3 receptor, thyroid hormone receptors and peroxisome proliferator-activated receptors, RXR ligands may modulate the behaviors of the partner receptors, as well as retinoidal activities.3 Therefore, we have focused on the development of RXR agonists and designed novel dibenzodiazepine-based RXR modulator candidates 4 – 6 by combining the structures of 2 and 3 (Figure 1). In this paper, we describe the synthesis and biological activities of the novel dibenzodiazepine derivatives 4 – 6.

The synthesis of compounds 4 and 5 is summarized in Scheme 1. Compound 8, which was prepared from 1,2,3,4-tetrahydro-1,1,4,4-tetramethylnaphthalene (7) according to the literature procedure,9 was reacted with ethyl 4-iodobenzoate in the presence of Pd catalyst to afford diphenylamine 9.10 An o-bromobenzyl group was introduced at the nitrogen atom of the diphenylamine skeleton of 9 with the corresponding benzyl bromide, followed by reduction of the nitro group with Zn in AcOH to afford compound 10. Construction of a dibenzodiazepine skeleton was accomplished by means of an intramolecular Pd-catalyzed amination between the aniline and bromobenzene moieties of 10 to afford compound 11 in quantitative yield.10 Hydrolysis of the ethyl ester group with aqueous 20% KOH solution afforded compound 4. N-Methylation by using NaH as a base and MeI, followed by hydrolysis of the ester group, afforded N-methyl dibenzodiazepine derivative 5 in quantitative yield. Dibenzodiazepinone 6 was synthesized as shown in Scheme 2. Compound 8 was reacted with methyl 2-chlorobenzoate in the presence of excess amount of CuI (Ullmann coupling) to afford diphenylamine derivative 12 in 65% yield.11 N-Methylation by using NaH as a base and MeI, followed by catalytic hydrogenation of the nitro group with Pd/C afforded aniline derivative 13. Compound 14 containing two diphenylamine substructures was prepared from 13 by means of Buchwald-Hartwig amination with ethyl 4-iodobenzoate using Pd-catalyst.10 Lactamization reaction between the diphenylamine and methyl ester moieties of 14 was performed in the presence of excess NaH, followed by hydrolysis of the ethyl ester group with NaOH, which was prepared in situ from excess NaH and H2O added to the reaction mixture, afforded compound 6.

The biological activities of the synthesized compounds were evaluated in terms of the activity to induce differentiation of HL-60 cells into mature granulocytes in the presence or absence of a synthetic retinoid, Am80 (1).12 The differentiated cells were identified by nitro blue tetrazolium (NBT) reduction assay.12 None of the test compounds induced cell differentiation of HL-60 alone (data not shown), while all of them showed moderate synergistic retinoid activity with Am80 (Figure 2). Their potency is weaker than that of 2. Compound 5, bearing a N-methyl group on the diazepine ring, showed more potent synergistic activity than compound 4 without the N-methyl group. Introduction of a carbonyl group into the diazepine skeleton led to a decrease of the activity; i.e. the synergistic activity of compound 6 was weaker than that of 5. These results suggest that polar functional groups on the diazepine skeleton are unfavorable for the expression of retinoid synergistic activity. Further syntheses and structure-activity relationship studies, particularly for compounds with an oxygen and a sulfur atom instead of the N-methyl group of compound 5, are in progress.

In conclusion, newly designed benzodiazepine derivatives 4 – 6 were efficiently synthesized by utilizing Pd-catalyzed and Cu-promoted diphenylamine-generating reactions as key reactions. Compounds 4 – 6 acted as retinoid synergists, and enhanced the cell differentiation induced by 0.1 nM Am80 (1). Although the potency of these compounds is not great, this efficient synthetic method for novel dibenzodiazepine structure bearing two diphenylamine substructures should be applicable elsewhere in the medicinal-chemical study of retinoids and other nuclear receptor ligands.

References

1. M. B. Sporn, A. B. Roberts, and D. S. Goodman, 'The Retinoids: Biology, Chemistry, and Medicine', 2nd ed., Raven Press, New York, 1994; M. Mark, N. B. Ghyselinck, and P. Chambon, Ann. Rev. Pharmacol. Toxicol., 2006, 46, 451; CrossRef M. Mark, N. B. Ghyselinck, and P. Chambon, Nuc. Rec. Signal., 2009, 7, 1.
2. D. J. Mangelsdorf, C. Thummel, M. Beato, P. Herrlich, G. Schuetz, K. Umesono, B. Blumberg, P. Kastner, M. Mark, P. Chambon, and R. M. Evans, Cell, 1995, 83, 835; P. Kastner, M. Mark, N. Ghyselinck, W. Krezel, V. Dupe, J. M. Grondona, and P. Chambon, Development, 1997, 124, 313; C. K. Glass and M. G. Rosenfeld, Gene Dev., 2000, 14, 121. CrossRef
3. P. Chambon, FASEB J., 1996, 10, 940; G. R. Benoit, M. Flexor, F. Besançon, L. Altucci, A. Rossin, J. Hillion, Z. Barajthy, L. Legres, E. Ségal-Bendirdjian, H. Gronemeyer, and M. Lanotte, Mol. Endocrinol., 2001, 15, 1154. CrossRef
4. P. Germain, P. Chambon, G. Eichele, R. M. Evans, M. A. Lazar, M. Leid, A. R. De Lera, R. Lotan, D. J. Mangelsdorf, and H. Gronemeyer, Pharmacol. Rev., 2006, 58, 712; CrossRef P. Germain, P. Chambon, G. Eichele, R. M. Evans, M. A. Lazar, M. Leid, A. R. De Lera, R. Lotan, D. J. Mangelsdorf, and H. Gronemeyer, Pharmacol. Rev., 2006, 58, 760. CrossRef
5. H. Kagechika, E. Kawachi, Y. Hashimoto, T. Himi, and K. Shudo, J. Med. Chem., 1988, 31, 2182; CrossRef Y. Hashimoto, H. Kagechika, and K. Shudo, Biochem. Biophys. Res. Commun., 1990, 166, 1300; CrossRef K. Shudo, H. Kagechika, N. Yamazaki, M. Igarashi, and C. Tateda, Biol. Pharm. Bull., 2004, 27, 1887; CrossRef M. Ishido and H. Kagechika, Drugs Today, 2007, 43, 563. CrossRef
6. H. Kagechika and K. Shudo, J. Med. Chem., 2005, 48, 5875. CrossRef
7. H. Umemiya, H. Fukasawa, M. Ebisawa, L. Eyrolles, E. Kawachi, G. Eisenmann, H. Gronemeyer, Y. Hashimoto, K. Shudo, and H. Kagechika, J. Med. Chem., 1997, 40, 4222; CrossRef H. Umemiya, H. Kagechika, H. Fukasawa, E. Kawachi, M. Ebisawa, Y. Hashimoto, G. Eisenmann, C. Erb, A. Pornon, P. Chambon, H. Gronemeyer, and K. Shudo, Biochem. Biophys. Res. Commun., 1997, 233, 121. CrossRef
8. K. Ohta, M. Tsuji, E. Kawachi, H. Fukasawa, Y. Hashimoto, K. Shudo, and H. Kagechika, Biol. Pharm. Bull., 1998, 21, 544.
9. L. Eyrolles, H. Kagechika, E. Kawachi, H. Fukasawa, T. Iijima, Y. Matsushima, Y. Hashimoto, and K. Shudo, J. Med. Chem., 1994, 37, 1508. CrossRef
10. J. P. Wolfe, S. Wagaw, J.-F. Marcoux, and S. L. Buchwald, Acc. Chem. Res., 1998, 31, 805; CrossRef J. F. Hartwig, Angew. Chem. Int. Ed., 1998, 37, 2046. CrossRef
11. J. Lindley, Tetrahedron, 1984, 40, 1433; CrossRef J. Hassan, M. Sevignon, C. Gozzi, E. Schulz, and M. Lemaire, Chem. Rev., 2002, 102, 1359. CrossRef
12. K. Ohta, E. Kawachi, N. Inoue, H. Fukasawa, Y. Hashimoto, A. Itai, and H. Kagechika, Chem. Pharm. Bull., 2000, 48, 1504.