Efficient Large-Scale Synthesis of BILN 2061, a Potent HCV Protease Inhibitor, by a Convergent Approach Based on Ring-Closing Metathesis Nathan K. Yee,* Vittorio Farina,* Ioannis N. Houpis,§ Nizar Haddad, Rogelio P. Frutos, Fabrice Gallou, Xiao-jun Wang, Xudong Wei, Robert D. Simpson, Xuwu Feng, Victor Fuchs, Yibo Xu, Jonathan Tan, Li Zhang, Jinghua Xu, Lana L. Smith-Keenan, Jana Vitous, Michael D. Ridges, Earl M. Spinelli, and Michael Johnson Department of Chemical DeVelopment, Boehringer Ingelheim Pharmaceuticals, Inc., 900 Ridgebury Road, P.O. Box 368, Ridgefield, Connecticut 06877
Kai Donsbach,* Thomas Nicola, Michael Brenner, Eric Winter, Paul Kreye, and Wendelin Samstag Abteilung Verfahrensentwicklung, Boehringer Ingelheim Pharma GmbH and Company KG, Binger Strasse 173, 55216 Ingelheim, Germany
[email protected];
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[email protected] ReceiVed February 10, 2006
A multistep scalable synthesis of the clinically important hepatitis C virus (HCV) protease inhibitor BILN 2061 (1) is described. The synthesis is highly convergent and consists of two amide bond formations, one etherification, and one ring-closing metathesis (RCM) step, using readily available building blocks 2-5. The optimization of each step is described at length. The main focus of the paper is the study of the RCM step and the description of the main problems faced when scaling up to pilot scale this highly powerful but very challenging synthetic operation. Eventually, the RCM reaction was smoothly scaled up to produce >400 kg of cyclized product.
Introduction In the search for small molecules that could block replication of the hepatitis C virus (HCV), our discovery group has designed a series of peptidomimetics that are potent inhibitors of the HCV NS3 protease, an enzyme that is crucial to viral replication.1 These inhibitors, characterized by three unnatural amino acid residues (P1, P2, P3), which are strung together in a macrocycle and are substituted with a very large hydrophobic heterocyclic moiety at the P2 residue, constitute a formidable challenge for the process chemist who is responsible for designing a practical § Current address: Johnson and Johnson Pharmaceutical Research and Development, Turnhoutseweg 30, B-2340, Beerse, Belgium.
FIGURE 1. Structure of BILN 2061 (1).
10.1021/jo060285j CCC: $33.50 © 2006 American Chemical Society
Published on Web 08/17/2006
J. Org. Chem. 2006, 71, 7133-7145
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FIGURE 2. Retrosynthesis of BILN 2061.4
process that can be scaled to provide multi-kilogram amounts of these substances in a very high degree of purity. A prototype of these HCV protease inhibitors, which recently has been tested in humans, is BILN 2061 (1, Figure 1).2 The new inhibitor has demonstrated antiviral response in HCV patients.3 The molecule features a 15-membered ring bearing a (Z)-1,2-disubstituted alkene subunit, as well as five stereocenters. The obvious disconnections involve scission of two amide bonds and an ether function (Figure 2). Closure of the macrocycle can be effected by a Ru-catalyzed ring-closing metathesis (RCM) reaction,4 a synthetic operation that has been used widely in recent years5 but which still represents a formidable challenge in a manufacturing plant setting because of a variety of factors that we will discuss below. In addition, preparation of the building blocks necessary for such an assembly presents considerable synthetic challenges. In the other papers in the series, we have described practical, (1) (a) Llina`s-Brunet, M.; Bailey, M.; De´ziel, R.; Fazal, G.; Gorys, V.; Goulet, S.; Halmos, T.; Maurice, R.; Poirier, M.; Poupart, M.-A.; Rancourt, J.; Thibeault, D.; Wernic, D.; Lamarre, D. Bioorg. Med. Chem. Lett. 1998, 8, 2719. (b) Llina`s-Brunet, M.; Bailey, M.; Fazal, G.; Ghiro, E.; Gorys, V.; Goulet, S.; Halmos, T.; Maurice, R.; Poirier, M.; Poupart, M.-A.; Rancourt, J.; Thibeault, D.; Wernic, D.; Lamarre, D. Bioorg. Med. Chem. Lett. 2000, 10, 2267. (c) Poupart, M.-A.; Cameron, D. R.; Chabot, C.; Ghiro, E.; Goudreau, N.; Goulet, S.; Poirier, M.; Tsantrizos, Y. S. J. Org. Chem. 2001, 66, 4743. (d) Tsantrizos, Y.; Bolger, G.; Bonneau, P.; Cameron, D. R.; Goudreau, N.; Kukolj, G.; LaPlante, S. R.; Llina`s-Brunet, M.; Nar, H.; Lamarre, D. Angew. Chem., Int. Ed. 2003, 42, 1356. (e) Goudreau, N.; Cameron, D. R.; Bonneau, P.; Gorys, V.; Plouffe, C.; Poirier, M.; Lamarre, D.; Llina`s-Brunet, M. J. Med. Chem. 2004, 47, 123. (f) Rancourt, J.; Cameron, D. R.; Gorys, V.; Lamarre, D.; Poirier, M.; Thibeault, D.; Llina`sBrunet, M. J. Med. Chem. 2004, 47, 2511. (g) Goudreau, N.; Brochu, C.; Cameron, D. R.; Duceppe, J.-S.; Faucher, A.-M.; Ferland, J.-M.; GrandMaıˆtre, C.; Poirier, M.; Simoneau, B.; Tsantrizos, Y. S. J. Org. Chem. 2004, 69, 6185. (2) Llina`s-Brunet, M.; Bailey, M. D.; Bolger, G.; Brochu, C.; Faucher, A.-M.; Ferland, J. M.; Garneau, M.; Ghiro, E.; Gorys, V.; Grand-Maıˆtre, C.; Halmos, T.; Lapeyre-Paquette, N.; Liard, F.; Poirier, M.; Rhe´aume, M.; Tsantrizos, Y. S.; Lamarre, D. J. Med. Chem. 2004, 47, 1605. (3) Lamarre, D.; Anderson, P. C.; Bailey, M.; Beaulieu, P.; Bolger, G.; Bonneau, P.; Boes, M.; Cameron, D. R.; Cartier, M.; Cordingley, M. G.; Faucher, A.-M.; Goudreau, N.; Kawai, S. H.; Kukolj, G.; Lagace´, L.; LaPlante, S. R.; Narjes, H.; Poupart, M.-A.; Rancourt, J.; Sentjens, R. E.; St. George, R.; Simoneau, B.; Steinmann, G.; Thibeault, D.; Tsantrizos, Y. S.; Weldon, S. M.; Yong, C.-L.; Llina`s-Brunet, M. Nature 2003, 426, 186. (4) (a) Faucher, A.-M.; Bailey, M. D.; Beaulieu, P. L.; Brochu, C.; Duceppe, J.-S.; Ferland, J.-M.; Ghiro, E.; Gorys, V.; Halmos, T.; Kawai, S. H.; Poirier, M.; Simoneau, B.; Tsantrizos, Y. S.; Llina`s-Brunet, M. Org. Lett. 2004, 6, 2901. (b) Poirier, M.; Aubry, N.; Boucher, C.; Ferland, J.M.; LaPlante, S.; Tsantrizos, Y. S. J. Org. Chem. 2005, 70, 10765. (5) Grubbs, R. H., Ed. Handbook of Metathesis; Wiley-VCH: Weinheim, Germany, 2003.
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large-scale approaches to building blocks 2,6 3,7 and 4,8 whereas 5 is commercially available in large quantities. In the final paper of this series, we now describe two related strategies to the assembly of this challenging target and report the details of our optimization studies leading to the smooth synthesis of BILN 2061 in multikilogram amounts. Results and Discussion Strategic Considerations. The initial goal of the program was to streamline the highly convergent discovery synthesis so we could rapidly prepare initial quantities of 1 for safety studies in animals and for formulation work. To make the synthesis more convergent, we decided to prepare quinoline subunit 4 and attempt to couple it directly with the peptidic core. This is a departure from the discovery synthesis that used a less functionalized version of 4 and introduced the thiazole ring through a multistep sequence.4 The challenges encountered with the scale-up of this assembly were associated with defining the purification strategy along the way, that is, the strategic placement of crystallizations to obtain pure BILN 2061 without resorting to chromatography. The scale-up of the RCM step also was associated with considerable challenges because of the high cost of the Ru catalysts, the large volumes involved, and the long reaction times. Finally, it was imperative to track the Ru content of all post-RCM intermediates and ensure that the final target was free from Ru contamination (0.05% should be monitored. (10) Anderson, N. G. Practical Process Research and DeVelopment; Academic Press: San Diego, CA, 2000. (11) Hughes, D. L. Org. React. 1992, 42, 2. The role of HOBT as a Mitsunobu nucleaophile is, as far as we know, unreported. In our case, product (i) was obtained from 7 and characterized by 1H NMR and mass spectroscopy.
was carried out using EDC/HOBT as reagents in dichloromethane. A slight excess (5%) of the less expensive 7 was used, to ensure consumption of the more precious 6. Consumption of 6 was achieved after 1 h at rt, and the solution yield of 8 was found to be 95% (by a quantitative HPLC assay). Removal of EDC urea and excess 7 by aqueous washes was not a problem. Unfortunately, this operation only partially removed HOBT. We found that HOBT reacts as a Mitsunobu nucleophile11 by forming an adduct with 7. Thus, the procedure was modified to include washing the ethyl acetate (EtOAc) extracts of 8 with a dilute sodium hydroxide (NaOH) solution. The resulting solution was analyzed, and the specification of 98% (92-95% HPLC yield for the transformation), which, in a typical run, was satisfactory for the following RCM reaction. The low boiling point of dichloromethane was ideal for the next step, because the RCM in a variety of solvents could be attempted without isolation of 11, that is, by simple evaporative solvent replacement (for example, toluene). RCM Reaction. A number of catalysts for this transformation have been described recently in the literature.5,12 Our initial screen focused on four of the most established Ru catalysts, which are illustrated in Chart 1. Catalyst 17, also referred to as “first-generation Grubbs catalyst” (1G), represents the original Grubbs catalyst design,13 whereas 18 (1H) represents a catalyst prototype first introduced by Hoveyda et al., that is, a catalyst bearing only one phosphane ligand, which is endowed with improved robustness and recyclability.14 Species 19 (2G) and 20 (2H) represent secondgeneration catalysts, that is, catalysts that have been made more active in the RCM reactions by the imidazolium ligands.15 (12) Grubbs, R. H. Tetrahedron 2004, 60, 7117. (13) Schwab, P.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1996, 118, 100. (14) Kingsbury, J. S.; Harrity, J. P. A.; Bonitatebus, P. J., Jr.; Hoveyda, A. H. J. Am. Chem. Soc. 1999, 121, 791. (15) (a) Scholl, M.; Trnka, T. M.; Morgan, J. P.; Grubbs, R. H. Tetrahedron Lett. 1999, 40, 2247. (b) Weskamp, T.; Kohl, F. J.; Hieringer, W.; Gleich, D.; Herrmann, W. A. Angew. Chem., Int. Ed. 1999, 38, 2416. (c) Huang, J.; Stevens, E. D.; Nolan, S. P.; Petersen, J. L. J. Am. Chem. Soc. 1999, 121, 2674. (d) Garber, S. B.; Kingsbury, J. S.; Gray, B. L.; Hoveyda, A. H. J. Am. Chem. Soc. 2000, 122, 8168.
Large-Scale Synthesis of BILN 2061
Early results had shown that the reaction produces only cisisomers, presumably because of the higher thermodynamic stability of cis vs trans.16 On the other hand, variable amounts of epimerization at one of the cyclopropane carbons were observed with catalyst 17 (Scheme 2). The exact proportion of the RCM product 12 was difficult to control and reproduce. Control experiments suggest that free PCy3, liberated by the catalyst during the metathesis, may be the culprit for this isomerization. The 1H catalyst 18 (5 mol % load) did not produce any (15% 22. Examination of the quality of starting material 11 showed the presence of 2-3% unacylated 10, which had not been removed sufficiently by acidic washes and was responsible for the extensive isomerization of 11. Small amounts (2%) of 21 also were produced. Thus, epimerization appears to take place on 11 prior to the RCM. Continued subjection of these mixtures to RCM conditions led to the disappearance of 21, which was quantitatively transformed into 22. At this point, no further epimerization took place. Although the mechanism of this epimerization has not been clarified yet, it is interesting that vinylcyclopropanes have been described to undergo RCM reaction without the complications we encountered,17 whereas alkene migration and Z/E isomerizations during RCM are commonplace.18 Ru(II) compounds are capable of isomerizing alkenes by Ru-H addition/elimination sequences.19 Further studies aimed at understanding the origin of this novel epimerization are ongoing and will be reported.20 In general, 1H catalyst 18 led to the cleanest RCM product. Typically, >90% yields (HPLC quantitation) were obtained. There were four major problems with the use of 18 and the scale-up of the RCM. First, the catalyst load needed to reach reaction completion was quite high and depended on the degree of purity of the diene. Typically, crude 11 required 5-7 mol % 18 to reach completion in refluxing dichloromethane. A charcoal/silica pad filtration produced better quality 11, which required only 2-3 mol % 18 to undergo RCM. It was very difficult to complete the reaction with lower catalyst loads. In addition, it proved impossible to recycle any of the catalyst. Active catalyst left after the RCM was evidenced by our attempts to concentrate the RCM solution for work-up (vide infra), but this was not enough to warrant catalyst recovery. (16) Poirier, M.; Aubry, N.; Boucher, C.; Ferland, J.-M.; LaPlante, S.; Tsantrizos, Y. S. J. Org. Chem. 2005, 70, 10765. (17) (a) Yang, Z.-Q.; Geng, X.; Solit, D.; Pratilas, C. A.; Rosen, N.; Danishesky, S. J. J. Am. Chem. Soc. 2004, 126, 7881. (b) Yamamoto, K.; Biswas, K.; Gaul, C.; Danishefsky, S. J. Tetrahedron Lett. 2003, 44, 3297. (c) Barrett, A. G. M.; Hamprecht, D.; James, R. A.; Ohkubo, M.; Procopiou, P. A.; Toledo, M. A.; White, A. J. P.; Williams, D. J. J. Org. Chem. 2001, 66, 2187. (d) Verbicky, C. A.; Zercher, C. K. Tetrahedron Lett. 2000, 41, 8723. (18) (a) Cadot, C.; Dalko, P. I.; Cossy, J. Tetrahedron Lett. 2002, 43, 1839. (b) Lee, C. W.; Grubbs, R. H. Org. Lett. 2000, 2, 2145. (19) (a) Schmidt, B. Eur. J. Org. Chem. 2004, 1865. (b) McGrath, D. V.; Grubbs, R. H. Organometallics 1994, 13, 224. (20) Zeng, X.; Wei, X.; Farina, V.; Napolitano, E.; Zhang, L.; Xu, Y.; Haddad, N.; Yee, N. K.; Grinberg, N.; Shen, S.; Senanayake, C. S. To be submitted to J. Org. Chem.
SCHEME 3. Screen for Ru Catalyst Inactivators and Structure of 23 (MNA)
The second problem associated with 18 was the reaction time. Typically, the RCM took 24 h at reflux, which is an impractically long time in a manufacturing setting. This limitation is due to the slow kinetics using 18 and, presumably, could be overcome only by using more active catalysts. The third problem was the dilution factor. The reaction yield was inversely proportional to the initial concentration of diene. Thus, a 0.01 M solution of 11 led to >90% yield of 12, a 0.030.05 M solution led to 12 in 60-64% yield, and a 0.1 M solution led to 12 in only 45% yield. The balance probably is constituted by oligomers and dimers. In later experiments, the presence of cyclic dimers was elucidated clearly by LC-MS techniques. Thus, the effective molarity of this reaction is such that a concentration of 0.01 M cannot be exceeded, if a high yield is desired. The fourth problem was closely related: the RCM was reversible. This first was evidenced by our attempts to scaleup the reaction. On a small scale, the reaction mixture could be evaporated safely or distilled to a small volume in preparation for work-up and isolation, whereas on a large scale, evaporation led to extensive decomposition. Quantitative experiments showed that when some of the solvent was distilled off, the HPLC yield of 12 dropped with time. Reasoning that ethylene, the coproduct of this metathesis, may be involved in product ring-opening and dimerization, we bubbled nitrogen through the solution prior to and during the distillation. This had little or no effect. We rationalized the results by hypothesizing that, in some cases, an active form of the catalyst is still present at the end of the reaction and this active form is apparently capable of ringopening 12. In other cases, the catalyst accidentally is completely consumed as 11 reacts, and the concentration of the solution to the small volume causes no harm to the product. Therefore, the presence of a still active catalytic species at the end of the RCM was a major hurdle toward the isolation of our product, and it became imperative to devise a means for catalyst inactivation. Extensive screening suggested that mercaptonicotinic acid (MNA, 23) is both capable of inactivating 18 (using 5 mol equiv at 30 °C for 6 h) and of being extracted by aqueous bicarbonate. MNA was discovered by screening potential RCM inhibitors using an easy substrate, diallyl diethyl malonate (Scheme 3). This method was applied to the synthesis of 12, and thereafter, no decomposition was experienced upon distillation. In addition, this provided us with a tool for removing some of the Ru from the product, occasionally in conjunction with other Ru-removing agents described in the literature. In an effort to overcome the low reactivity and low TON associated with catalyst 18, we tested the performance of the more active second-generation catalysts 19 and 20 on the RCM reaction of 11. Some key results are shown in Table 1. The trend that emerges is quite clear: both second generation catalysts are kinetically more active than 18, but they also form J. Org. Chem, Vol. 71, No. 19, 2006 7137
Yee et al. TABLE 1. RCM Reaction of 11 with Catalysts 18-20a entry
catalyst (equiv)
1 2 3 4
18 (5%) 18 (3.5%) 19 (0.5%) 20 (1%)
solvent
temp (°C)
time (h)
HPLC yield 12
% dimer content
CH2Cl2 PhMe PhMe PhMe
40 60 60 55
24 20 4 1
90 90 87 85
