Synthesis of Side-Chain Locked Analogs of 1α,25-Dihydroxyvitamin

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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Synthesis of Side-Chain Locked Analogs of 1α,25-Dihydroxyvitamin D3 Bearing a C17 Methyl Group Rita Sigüeiro,*,† Miguel A. Maestro,‡ and Antonio Mouriño† †

Departamento de Química Orgánica, Laboratorio de Investigación Ignacio Ribas, Universidad de Santiago de Compostela, Avda das Ciencias s/n, 15782 Santiago de Compostela, Spain ‡ Departamento de Química-CICA, Universidad de A Coruña, Campus da Zapateira s/n, 15701 A Coruña, Spain S Supporting Information *

ABSTRACT: A convergent synthesis of side-chain locked vitamin D analogs 3 and 4, which bind strongly in silico to the vitamin D receptor (VDR), is described. The synthetic approach features an SN2′-syn displacement of carbamates by cuprates to set the challenging quaternary stereogenic center at C17 and a Pdcatalyzed construction of the triene system in the presence of a diyne moiety.

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t is now well established that 1α,25-dihydroxyvitamin D3 [1α,25(OH)2D3, 1,25D, calcitriol, Figure 1], the most potent hormone known and the biologically active form of vitamin D3, is involved in various biological processes, including regulation of mineral homeostasis, cell proliferation and differentiation, and immune functions through interactions with the vitamin D receptor (VDR), a member of the nuclear receptor superfamily.1 The antitumor properties of 1,25D has suggested its use in the treatment of hyperproliferative diseases, but the required pharmacological doses induce toxic hypercalcemia.2 Despite the large number of analogs of the natural hormone synthesized,3 only a few vitamin D derivatives have found clinical applications.4 For example, calcipotriol5 and oxacalcitriol,6 two 1,25D-analogs modified at the side chain, which are rapidly metabolized before exerting toxic calcemic effects, have been successfully used for topical treatment of psoriasis. Structural features of highly active 1,25D-analogs that exert less calcemic activity in comparison with the natural hormone by unknown mechanisms include the lack of the 19-methylene group,7 unsaturation at the side chain or D-ring,8 14-epiconfiguration,9 3-epi-configuration,10 short nonhydroxylated side chains,11 and carboranic side chains.12 Crystallographic13 and theoretical studies14 on 1,25D and highly active analogs have shown that ligand−VDR hydrogen bonding and van der Waal interactions are required for transactivation, but no detailed mechanism or biologically active conformer has been proposed to rationally design 1,25D analogs with selective biological functions for clinical purposes.15 In the search for understanding how specific structural modifications of 1,25D can induce selective biological activity of therapeutic potential, especially high antiproliferative-prodifferentiating action and low or negligible calcemic effects, we undertook a program directed toward the design and synthesis of 1,25D analogs with partially locked side chains.16 The potent biological activity of analog 216c that imparts © XXXX American Chemical Society

Figure 1. Structures of the natural hormone 1,25D (1), diyne 2, and target analogs 3 and 4. Docking structure of 3 and 4, superimposed with 1,25D in the VDR-ligand binding pocket.

conformational rigidity through a diyne locked side chain, and the observation of a spatial volume around C17 in the binding pocket for further functionalization,17 led us to design 3 and 4 as the first representative 1,25D analogs bearing a methyl group attached to the hindered C17 stereogenic center. Docking calculations show that the C17-methy increases the affinity for the VDR through van der Waals interactions with hVal300 and hLeu313. The CF3 group was designed on the basis that fluorination increases cell prodifferentiation and resistance to metabolic degradation.18 The synthetic plan is summarized in Scheme 1. The triene system arises by a stereoselective Pd-catalyzed ring closure on enoltriflate 6 and subsequent Suzuki−Miyaura coupling with alkenyl-boronic ester 5 following procedures previously developed in this laboratory.19 The C25-OH moiety would be introduced at the end of the synthesis, allowing isotopic labeling. The diyne 5 would derive from alkene 7a, which in Received: March 15, 2018

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DOI: 10.1021/acs.orglett.8b00849 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

at C17 as the result of competitive SN2′-syn and SN2′-anti pathways. Gratifyingly, one-pot deprotonation of 8a (n-BuLi) and consecutive additions of PhNCO, CuCN, and MeLi afforded the desired alkene 7a (51% yield) together with carbamate 12a (34%), which was recycled back into the reaction to give 7a (28%) (79% combined yield). Treatment of alcohol 8b under the same reaction conditions provided the alkene 7a, in lower yield (31%). We considered that the stereochemistry of the C17-methyl group is the result of a slow intramolecular SN2′syn process through the less hindered face. With alkene 7a in hand, we proceeded with the synthesis of the upper fragment 5 (Scheme 3). Initial attempts to generate

Scheme 1. Retrosynthetic Analysis of Target Analogs 3 and 4

Scheme 3. Synthesis of the Upper Fragment 5

turn would be prepared from known16d allylic alcohol 8a. The major synthetic challenge is the stereoselective installation of the C17-methyl group at the sterically hindered C17 position by a Cu-assisted SN2′-syn alkylation of an ester derived from allylic alcohol 8a.20−23 To study the generation of the C17-quaternary stereocenter by the SN2′-syn process, we synthesized the known allylic alcohol 8a and 8b from ketone 9 (Scheme 2).16d Breit’s orthodiphenylphosphanylbenzoate (o-DPPB)24 and phenyl-carbamate25 were independently used as the reagent directing leaving groups. Scheme 2. Generation of the C17-Quaternary Stereocenter

ketone 15 under Wacker oxidation26 (PdCl2, CuCl, DMF/ H2O, O2, 0 °C or rt) resulted in the recovery of starting material, likely because of steric hindrance of the double bond. Higher temperatures led to decomposition. Ketone 15 was then prepared in 92% yield by a three-step sequence involving epoxidation (m-CPBA), opening of the resulting epoxide 13 (LiAlH4), and Dess-Martin oxidation. Exposure of 15 to NaHMDS and trapping of the resulting enolate with N-(2pyridyl)-triflimide afforded enoltriflate 16 in 83% yield. Treatment of 16 with NaHMDS gave alkyne 17a, which, upon deprotonation (n-hexyllithium) and reaction with iodine, delivered iodide 17b in 91% yield (two steps). Stephens− Castro cross-coupling27 between trimethylsilylacetylene and iodoalkyne 17b in the presence of piperidine provided, after separation from the homocoupled product, the desired diyne 18a in 62% yield. Deprotection with aqueous 48% HF solution in CH3CN/CH2Cl2 afforded alcohol 18b, which upon oxidation furnished ketone 19 in 93% yield (two steps). Wittig chemistry between ketone 19 and ylide Ph3PCHBr,28 generated from (Ph3PCH2Br)Br and KOt-Bu in toluene,17 led to vinyl bromide 20 in 55% yield. Boronate 5 was synthesized by Sato’s method.29 Metalation of 20 with tert-butyllithium and reaction with triisopropyl boronate, followed by transester-

Allylic alcohols 8a and 8b were respectively treated with ortho-diphenylphosphanylbenzoic acid (10) in the presence of N,N′-dicyclohexylcarbodiimide and dimethylaminopyridine to provide the corresponding esters 11a (89%) and 11b (98%). Complexation of 11a with CuBr·SMe2 followed by addition of MeMgBr in Et2O resulted in the formation of a 1:1 mixture of alkenes 7a and 7b in 33% combined yield. Attempts to improve the stereoselectivity of the allylic substitution employing the (Z)-ester 11b provided a 7:3 mixture of alkenes 7a and 7b in 67% combined yield. We explain the observed stereochemistry B

DOI: 10.1021/acs.orglett.8b00849 Org. Lett. XXXX, XXX, XXX−XXX

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ification with pinacol, gave the desired pinacol boronate 5 in 78% yield. The completion of the synthesis of target compounds is depicted in Scheme 4. Pd-catalyzed ring closure of the enol-

triflate 6 (1.1 equiv) followed by Suzuki−Miyaura crosscoupling with boronate 5 (1 equiv) in the presence of aqueous K3PO4 in THF proceeded smoothly and stereoselectively to give, after removal of the TMS group, the diyne 22 in 78% yield. Finally, deprotonation of 22 (n-hexyllithium) followed by reaction with acetone delivered, after deprotection with tetrabutylammonium fluoride, the 1,25D analog 3 in 67% yield (17 steps, 7% overall yield). Replacement of acetone by hexafluoroacetone gave analog 4 in 57% yield (17 steps, 6% overall yield). Preliminary results show that compounds 3 and 4 bind strongly to the VDR ligand binding domain in comparison with the natural hormone. Biological studies of these compounds are ongoing and will be published in due course.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b00849.

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REFERENCES

(1) (a) Bouillon, R.; Okamura, W. H.; Norman, A. W. Endocr. Rev. 1995, 16, 200−257. (b) Vitamin D, 2nd ed.; Feldman, D., Pike, F. J. W., Adams, J. S., Eds; Elsevier Science: New York, 2011. (2) (a) Eisman, J. A.; Barkla, D. H.; Tutton, P. J. M. Cancer Res. 1987, 47, 21−25. (b) Deeb, K. K.; Trump, D. L.; Johnson, C. S. Nat. Rev. Cancer 2007, 7, 684−700. (c) Feldman, D.; Krishnan, A. V.; Swami, S.; Giovannucci, E.; Feldman, B. J. Nat. Rev. Cancer 2014, 14, 342−357. (d) Santagata, S.; Thakkar, A.; Ergonul, A.; Wang, B.; Woo, T.; Hu, R.; Harrell, J. C.; McNamara, G.; Schwede, M.; Culhane, A. C.; Kindelberger, D.; Rodig, S.; Richardson, A.; Schnitt, S. J.; Tamimi, R. M.; Ince, T. A. J. Clin. Invest. 2014, 124, 859−870. (3) For synthetic strategies, see: (a) Dai, H.; Posner, G. H. Synthesis 1994, 1994, 1383−1398. (b) Zhu, G. D.; Okamura, W. H. Chem. Rev. 1995, 95, 1877−1952. (c) Chapelon, A. S.; Moraleda, D.; Rodríguez, R.; Ollivier, C.; Santelli, M. Tetrahedron 2007, 63, 11511−11616. (4) (a) Plum, L. A.; DeLuca, H. F. Nat. Rev. Drug Discovery 2010, 9, 941−955. (b) Leyssens, C.; Verlinden, L.; Verstuyf, A. Front Physiol. 2014, 5, 1−18. (5) Binderup, L.; Binderup, E.; Gotdfredsen, W. O. In Vitamin D; Feldman, D., Glorieux, F. H., Pike, J. W., Eds.; Academic Press: New York, 1997; pp 1027−1043. (6) Kubodera, N.; Sato, K.; Nishii, Y. In Vitamin D; Feldman, D.; Glorieux, F. H., Pike, J. W., Eds.; Academic Press: New York, 1997, 1071−1086. (7) (a) Perlman, K. L.; Sicinski, R. R.; Schnoes, H. K.; DeLuca, H. F. Tetrahedron Lett. 1990, 31, 1823−1824. (b) Toyoda, A.; Nagai, H.; Yamada, T.; Moriguchi, Y.; Abe, J.; Tsuchida, T.; Nagasawa, K. Tetrahedron 2009, 65, 10002−10008. (8) (a) Uskokovic, M. R.; Studzinski, G. P.; Reddy, S. G. The 16-ene vitamin D analogs. In Vitamin D; Feldman, D., Glorieux, F. H., Pike, F. J. W., Eds.; Academic Press: New York, 1997; Vol. 62, pp 1045−1070. (b) Posner, G. H.; Wang, Q.; Han, G.; Lee, G. H.; Crawford, K.; Zand, S.; Brem, H.; Peleg, S.; Dolan, P.; Kensler, T. W. J. Med. Chem. 1999, 42, 3425−3435. (9) (a) Verlinden, L.; Verstuyf, A.; van Camp, M.; Marcelis, S.; Sabbe, K.; Zhao, X. Y.; De Clercq, P.; Vandewalle, M.; Bouillon, R. Cancer Res. 2000, 60, 2673−2679. (b) Wu, Y.; Zhao, Y.; Tian, H.; De Clercq, P.; Vandewalle, M.; Berthier, M.; Pellegrino, G.; Maillos, P.; Pascal, J. C. Eur. J. Org. Chem. 2001, 2001, 3779−3788. (10) Molnar, F.; Sigüeiro, R.; Sato, Y.; Araujo, C.; Schuster, I.; Antony, P.; Peluso, J.; Muller, C.; Mouriño, A.; Moras, D.; Rochel, N. PLoS One 2011, 6, e18124. (11) Plum, L. A.; Prahl, J. M.; Ma, X.; Sicinski, R. R.; Gowlugari, S.; Clagett-Dame, M.; DeLuca, H. F. Proc. Natl. Acad. Sci. U. S. A. 2004, 101, 6900−6904. (12) Otero, R.; Seoane, S.; Sigüeiro, R.; Belorusova, A.; Maestro, M. A.; Pérez-Fernández, R.; Rochel, N.; Mouriño, A. Chem. Sci. 2016, 7, 1033−1037. (13) (a) Rochel, N.; Wurtz, J. M.; Mitschler, A.; Klaholz, B.; Moras, D. Mol. Cell 2000, 5, 173−179. (b) Carlberg, C.; Molnar, F.; Mouriño, A. Expert Opin. Ther. Pat. 2012, 22, 417−435. (14) (a) Yamamoto, K.; Abe, D.; Yoshimoto, N.; Choi, M.; Yamagishi, K.; Tokiwa, H.; Shimizu, M.; Makishima, M.; Yamada, S. J. Med. Chem. 2006, 49, 1313−1324. (b) Yamagishi, K.; Tokiwa, H.; Makishima, M.; Yamada, S. J. Steroid Biochem. Mol. Biol. 2010, 121, 63−67. (15) Eduardo-Canosa, S.; Fraga, R.; Sigüeiro, R.; Marco, M.; Rochel, N.; Moras, D.; Mouriño, A. J. Steroid Biochem. Mol. Biol. 2010, 121, 7− 12. (16) (a) Martínez-Pérez, J. A.; Sarandeses, L.; Granja, J.; Palenzuela, J. P.; Mouriñ o, A. Tetrahedron Lett. 1998, 39, 4725−4728. (b) Fernández-Gacio, A.; Vitale, C.; Mouriño, A. J. Org. Chem. 2000, 65, 6978−6983. (c) Pérez-García, S.; Rumbo, A.; Larriba, M. J.; Ordóñez, P.; Muñoz, A.; Mouriño, A. Org. Lett. 2003, 5, 4033−4036. (d) Riveiros, R.; Rumbo, A.; Sarandeses, L. A.; Mouriño, A. J. Org. Chem. 2007, 72, 5477−5485. (e) Hourai, S.; Rodrigues, L. C.; Antony, P.; Reina-San-Martin, B.; Ciesielski, F.; Magnier, B.; Schoonjans, K.; Mouriño, A.; Rochel, N.; Moras, D. Chem. Biol. 2008, 15, 383−392.

Scheme 4. Synthesis of the Target Analogs 3 and 4

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Letter

Experimental procedures and spectroscopic data (1H, 13C NMR, HRMS, IR, [α]) (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Rita Sigüeiro: 0000-0002-5289-2464 Antonio Mouriño: 0000-0001-8922-8033 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Spanish Ministry of Science and Innovation (MSI) and the European Regional Development Fund (Project: SAF2010-15291) and Xunta de Galicia (Project GPC2014/001) for financial support, and CESGA for computing time. R.S. thanks Xunta de Galicia for a Postdoctoral Fellowship (Plan I2C ano 2012, modalidade A). C

DOI: 10.1021/acs.orglett.8b00849 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters (f) Antony, P.; Sigüeiro, R.; Huet, T.; Sato, Y.; Ramalanjaona, N.; Rodrigues, L. C.; Mouriño, A.; Moras, D.; Rochel, N. J. Med. Chem. 2010, 53, 1159−1171. (17) Rochel, N.; Hourai, S.; Pérez-García, X.; Rumbo, A.; Mouriño, A.; Moras, D. Arch. Biochem. Biophys. 2007, 460, 172−176. (18) Eelen, G.; Valle, N.; Sato, Y.; Rochel, N.; Verlinden, L.; De Clercq, P.; Moras, D.; Bouillon, R.; Muñoz, A.; Verstuyf, A. Chem. Biol. 2008, 15, 1029−1034. (19) Gogoi, P.; Sigüeiro, R.; Eduardo, S.; Mouriño, A. Chem. - Eur. J. 2010, 16, 1432−1435. (20) Steroidal numbering was used for discussion. (21) Buschleb, M.; Dorich, S.; Hanessian, S.; Tao, D.; Schenthal, K. B.; Overman, L. E. Angew. Chem., Int. Ed. 2016, 55, 4156−4186. (22) For recent methods to generate steroidal C17-quaternary stereocenters, see: Nawrat, C. C.; Jamison, C. R.; Slutskyy, Y.; MacMillan, D. W. C.; Overman, L. E. J. Am. Chem. Soc. 2015, 137, 11270−11273. (23) For synthetic strategies toward natural products containing contiguous stereogenic quaternary carbon atoms, see: Buschleb, M.; Dorich, S.; Hanessian, S.; Tao, D.; Schenthal, K. B.; Overman, L. E. Angew. Chem., Int. Ed. 2016, 55, 4156−4186. (24) Rousseau, G.; Breit, B. Angew. Chem., Int. Ed. 2011, 50, 2450− 2494. (25) For the use of carbamates as reagent-directing groups in SN2′syn transformations, see: (a) Karlström, A. S. E.; Bäckvall, J. E. CopperMediated Enantioselective Substitution Reactions. In Modern Organocopper Chemistry, Krause, N., Ed.; Wiley-VCH Verlag GmbH, 2002. (b) López-Pérez, B.; Maestro, M. A.; Mouriño, A. Chem. Commun. 2017, 53, 8144−8147. (26) (a) Tsuji, J. Synthesis 1984, 1984, 369. (b) Tsuji, J. Palladium Reagents and Catalysts; John Willey & Sons: New York, 2004. (c) Johnson, W. S.; Bartlett, W. R.; Czeskis, B. A.; Gautier, A.; Lee, C. H.; Lemoine, R.; Leopold, E. J.; Luedtke, G. R.; Bancroft, K. J. Org. Chem. 1999, 64, 9587−9595. (27) Stephens, R. D.; Castro, C. E. J. Org. Chem. 1963, 28, 3313− 3315. (28) Trost, B. M.; Dumas, J.; Villa, M. J. Am. Chem. Soc. 1992, 114, 9836−9845. (29) Hanazawa, T.; Wada, T.; Masuda, T.; Okamoto, S.; Sato, F. Org. Lett. 2001, 3, 3975−3977.

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DOI: 10.1021/acs.orglett.8b00849 Org. Lett. XXXX, XXX, XXX−XXX