Article pubs.acs.org/OPRD
Practical Synthesis of A Macrocyclic HCV Protease Inhibitor: A HighYielding Macrolactam Formation Zhiguo J. Song,* David M. Tellers, Peter G. Dormer, Daniel Zewge, Jacob M. Janey, Andrew Nolting, Dietrich Steinhuebel, Steven Oliver, Paul N. Devine, and David M. Tschaen Department of Process Chemistry, Merck Research Laboratory, P.O. Box 2000, Rahway, New Jersey 07065, United States S Supporting Information *
ABSTRACT: A practical synthesis of a macrocyclic HCV protease inhibitor, MK-1220, is described. The key features are a new synthesis of the trisubstituted isoquinoline, Sonogashira fragment coupling, and a high-yielding, 18-membered macrolactam formation.
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INTRODUCTION Heptatis C virus (HCV) is a leading cause of chronic liver disease and liver transplant operations. HCV protease inhibitors have emerged as a class of new drugs against HCV, as evidenced by the recent FDA approval of boceprevir and telaprevir in 2011.1 Intensive efforts are still in place to discover improved drugs in this area.2,3 MK-1220 was identified as a drug development candidate in Merck as a potent HCV protease inhibitor with a potentially superior therapeutic profile.2a To support the development of this drug candidate and potential commercial production, we needed to develop a scalable, cost-effective, and environmentally friendly synthesis of this macrocyclic compound that would allow large-scale production in a timely and cost-effective manner. This synthesis poses significant challenges due to the complexity of the compound, particularly the 18-membered ring, and potential high-volume drug requirements. The original route was designed to allow access of structural diversity and not suitable for large-scale production.2a This report discloses the development of a robust route that is suitable for preparation of this compound on large scale.
different from the original synthesis used by the discovery group in which the macrocyclization was based on an RCM reaction of the diene 6. We focused on the current strategy for a number of reasons: the inherent low cost for the amide formation to construct the macrocycle, easy access of the akyne intermediate for the linker fragment, and the anticipated high yield of Sonogashira coupling between the alkyne linker and the aromatic ring.4,5
Preparation of Isoquinoline 3. A quick evaluation of the original synthesis of the isoquinoline revealed that it was not suitable for large-scale preparation due to poor regioselectivity and the low yield of the key high-temperature cyclization step of the isocyanate intermediate 7 to form the 1-hydroxy isoquinoline 8 (Scheme 1).2b Survey of the literature revealed that the challenge of preparing this particular isoquinoline was in achieving high regioselectivity for the cyclization.6 This was further confirmed by the results from our exploration of other potential routes. As shown in Scheme 1, compound 9 is commercially available but under typical bromination conditions only gave the wrong regioisomer 10 as the major product. In addition, 11 can be prepared from readily available 3-bromo-4-methoxy benzoic acid but failed to undergo the desired cyclization to form 8 under various Friedel−Crafts reaction conditions; only trace amount of the oxazole 12 was observed, presumably due to electron deficiency of the substituted benzene ring. However, we did observe that 14 (Scheme 2) underwent regioselective formylation under Rieche
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RESULTS AND DISCUSSION The key structural features and retrosynthetic analysis of MK1220 are outlined in Figure 1. After disconnection at the amide bond between the macrocycle 1 and the side chain 2, the macrocycle 1 can be envisioned to be assembled from the properly functionalized isoquinoline 3, 4-hydroxy-L-proline, the alkyne linker piece 4, and cyclohexylglycine 5. As reported previously in the process development of a related compound MK-7009,4 the challenge for developing the macrocycle synthesis lies on the following three fronts: most efficient methods for preparation of the individual fragments, efficient fragment coupling, and macrocyclic ring formation. In the current case, we envisioned that the macrocyle could be closed via amide formation, similar to that for MK-7009. The key connection between the linker and the isoquinoline can be achieved via a Sonogashira coupling, and the ether bond between isoquinoline 3 and the hydroxyl proline can be achieved via SNAr displacement. This strategy is significantly © 2014 American Chemical Society
Received: November 24, 2013 Published: February 25, 2014 423
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Figure 1. MK-1220 retrosynthetic analysis.
isoquinoline ring and led to the successful development of a new synthesis of isoquinoline 3. The new synthesis started from readily available 2-bromo-1-fluoro-4-iodobenzene 13. Substitution of the fluorine by a methoxy group occurred readily by using sodium methoxide solution in methanol to form 14 in 90% yield. The crude product was then subjected to Rieche formylation to give the desired substituted benzaldehyde 15 which was conveniently isolated via crystallization from isopropyl acetate and heptane in 89% yield. Oxidation of the aldehyde 15 to the methyl ester 16 with iodine and NaOMe established the correct oxidation state.8 Introduction of the other two carbons for the isoquinoline was accomplished by Heck reaction of N-vinyl phthalimide.9 This reaction was regioselective toward the terminal carbon to give 17:18 in 9:1 ratio based on NMR of the reaction mixture. The desired isomer 17 was isolated in high purity by crystallization from the reaction solvent 2-methyl THF. Other vinyl amides such as Nvinyl pyrrolidinone, N-vinyl acetamide or vinyl acetate, vinyl ethers all underwent the Heck reaction mainly on the internal carbon.10 The key cyclization was carried out by treating intermediate 17 with formamide and sodium methoxide,
Scheme 1. Unsuccessful isoquinoline synthesis
formylation conditions (Cl2CHOMe/TiCl4 in CH2Cl2)7 to give 15. This key reaction established the desired one carbon in the Scheme 2. New isoquinoline synthesis
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Scheme 3. Hydroxy proline coupling
conditions known to convert esters to carboxamides.11 These conditions gave clean, facile conversion to hydroxy isoquinoline 8 in high yield. Alternatively, compound 17 was converted to 8 with ammonia in methanol under pressure, but these conditions gave less reproducible results, due to precipitation of unidentified intermediates. To our knowledge, this is a new sequence of reactions to prepare hydroxy isoquinoline from an ortho halo benzoate derivative; such an approach will likely be applicable in the preparation of other substututed hydroxy isoquinolines. Subsequent treatment of 8 with 2 equiv POCl3 in acetonitrile gave the 1-chloroisoquinoline 3 in 89% overall yield for the two steps. The use of only 2 equiv POCl3 greatly simplifies the workup compared with the common practice of using POCl3 as the solvent. The excess POCl3 was quenched with 2-propanol, and compound 3 was crystallized directly from the reaction by addition of water to the acetonitrile solution. Coupling with CBZ-Hydroxyproline. The CBZ protected 4-hydroxyl-L-proline 19 was installed via SNAr reaction on the 1-chloroisoquinoline 3 under basic conditions (Scheme 3). The slow charge of the potassium tert-butoxide into a mixture of the isoquinoline 3 and hydroxy proline 19 was important to minimize the side reactions derived from the decomposition of CBZ hydroxy proline 19, i.e., rearrangement of CBZ group onto the hydroxyl group to generate side product 21. The acid intermediate 20 was telescoped to ethyl ester 22 by treatment of the ethanol solution of 20 with thionyl chloride. The hydrochloride salt 22 crystallized out of the reaction directly in 77% overall yield for the two steps.2a Preparation of the Alkyne Linker 24. The alkyne linker 24 was prepared first by alkylation of ethyl isobutyrate with propargyl bromide through deprotonation with LDA (Scheme 4). Subsequent LAH reduction of the resulting ester generated the alcohol 4 which was purified by vacuum distillation.12 Carbamate formation between the alcohol and cyclohexylglycine methyl ester 23 mediated by CDI followed by
saponification gave the linker acid 24. Attempted direct coupling of (S)-cyclohexylglycine acid with alcohol 4 resulted in poor yields largely due to extremely low solubility of cyclohexylglycine acid even in DMSO. The linker acid 24 can be isolated as the dicyclohexylamine (DCHA) salt as a crystalline solid. Coupling and Macrocycle Formation. The Sonogashira coupling conditions between the bromoisoquinoline intermediate 22 and the alkyne linker acid 24 DCHA salt were selected after high-throughput screening for a variety of ligands, solvents, and bases (Scheme 5).13 Thus, with palladium allyl chloride, tri-(tert-butyl)phosphine ligand, and diisopropylamine base, the Sonogashira reaction in acetonitrile afforded 25 in 90% yield.14 After the reaction, trifluoroacetic acid was charged to neutralize the excess diisopropylamine, and then the desired product crystallized out along with some diisopropylamine TFA salt. This isolation was important because it removed the phosphine ligand which can poison the subsequent hydrogenation catalyst. The solid intermediate was hydrogenated to reduce the alkyne and to remove the CBZ protecting group to afford intermediate 26, setting the stage for the key macrocycle formation via amide formation. The intermediate 26 did not crystallize after repeated attempts, so it was telescoped to the amide formation step. Thus, a solution of 26 was charged to HATU and Hunig’s base, resulting in the instantaneous formation of lactam 27. The fast lactamization reaction rate coupled with inverse addition of substrate 26 to HATU allowed for the macrolactamization to be run under standard dilution conditions (20 volumes solvent relative to substrate), which was important for scale-up considerations. An impurity can form in the cyclization reaction as high as 20 area % determined by HPLC. LC−MS analysis indicated a molecular weight consistent with the structure 28 shown.
Scheme 4. Synthesis of the alkyne linker acid 24
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Scheme 5. Coupling and macrocyclization
Scheme 6. Final coupling and MK-1220 isolation
potassium salt from ethanol in 94% yield by reaction with potassium ethoxide. This salt formation allows for purification of the final product to 98.4% area HPLC purity and was suitable for safety and clinical studies.18
This was thought to be derived from residue CO2 generated in the CBZ hydrogenolysis. The residue CO2 presumably reacted with proline to form the carbamic acid (NCO2H) which could then react with HATU to form 28.15 When the hydrogenation product solution was vigorously degassed and sparged with nitrogen to remove the residue CO2, this impurity was no longer detected. The high yield of this macrolactamization is quite remarkable considering the large 18-membered ring size.16,17 The dimer impurity was typically formed at 2−3% area as determined by HPLC. The ethyl ester was then cleaved to give the acid 1 and isolated as the HCl salt in 80% yield and 99% area HPLC purity. The 1-HCl salt had the unusual property of being insoluble in both dilute hydrochloride acid and in moderately polar organic solvents, which allowed convenient isolation from water. The overall yield from the aryl bromide 22 to this key macrocycle intermediate 1-HCl is 72%, an efficient macrocycle preparation. Final Coupling and MK-1220 Isolation. The side chain 2 tosylate was prepared from the known Boc protected 2-Boc4 by treatment with toluenesulfonic acid in ethyl acetate and isolated from the reaction mixture as a crystalline salt (Scheme 5). It was coupled with the macrocycle acid 1-HCl via EDC-mediated amide formation to give MK-1220 in 91% assay yield. The neutral MK-1220 was amorphous and was crystallized as the
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CONCLUSION A pratical synthesis of the HCV protease inhibitor MK-1220 has been developed and is highlighted by (1) a new synthesis of the chloroisoquinoline core; (2) an efficient fragment coupling based on SNAr reaction of the hydroxyproline with the chloroisoquinoline, Sonogashira coupling of the alkyne linker, and the aryl bromide; (3) a hydrogenation and high-yielding macrolactam formation, enabling the efficient preparation of the macrocycle. This efficient synthesis has been successfully scaled up to make multiple kilograms of MK-1220.
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EXPERIMENTAL SECTION General. The purity of a compound or reaction conversion based on HPLC peak integration may be reported simply as % purity or % conversion unless otherwise noted. The assay yield determined by HPLC was calculated using working standards. The weight purity of isolated compound or crude solution may be analyzed against a working standard and reported as wt %.
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(s, 3H). 13C NMR (100 MHz, CDCl3): δ 165.01, 158.56, 135.90, 127.34, 124.74, 111.66, 94.32, 56.91, 52.64. Anal. Found: C 28.12, H 2.07 calcd for C9H8BrIO3 C 29.14, H 2.17. LC−MS m/z 371, 373 (M + 1). Heck Coupling Product 17.9 N-Vinyl phthalimide (22.7 g, 131 mmol) and methyl 5-bromo-2-iodo-4-methoxybenzoate 16 (45.5 g, 123 mmol), triethylamine (37.3 g, 368 mmol), 2methyl-THF (195 g) were added to a reactor and purged thoroughly by performing three vacuum/nitrogen purge cycles. Palladium acetate (0.275 g, 1.23 mmol) was then charged and the reactor purged again. The batch was heated to reflux (80 °C) and aged for 16 h under nitrogen. The reaction was judged to be complete by HPLC. The batch was cooled to room temperature, the slurry filtered, and the cake washed with acetonitrile (106 g) in three separate washes and then with methanol (108 g). The wet cake was dried in a vacuum oven at 50 °C with a nitrogen sweep to give title compound 17 as white solid, 39.0 g (76.4% yield). Mp 212−214 °C. 1H NMR (400 MHz, CDCl3): δ 8.49 (d, J = 15.0 Hz, 1H), 8.18 (s, 1H), 7.90 (m, 2H), 7.77 (m, 2H), 7.22 (d, J = 15.0 Hz, 1H), 7.01 (s, 1H), 4.02 (s, 3H), 3.91 (s, 3H). 13C NMR (100 MHz, CDCl3): δ 166.41, 166.14, 158.91, 139.77, 136.32, 134.85, 131.77, 123.95, 122.12, 119.64, 119.60, 110.58, 109.39, 56.72, 52.34. Anal. Found: C 54.65, H 3.22, N 3.30 calcd for C19H14BrNO5 C 54.83, H 3.39, N 3.37. LC−MS m/z 416, 418 (M + 1). 7-Bromo-1-hydroxy-6-methoxyisoquinoline 8. In a pressure reactor, Heck reaction product 17 (39.0 g, 94 mmol) methanol (93 g), and formamide (8.44 g, 187 mmol) were charged under nitrogen followed by a sodium methoxide solution (30 wt % in MeOH, 19.4 g, 98 mmol). The vessel was pressurised to 45 psi with nitrogen and then the contents were heated to 85 °C. After 4 h at 85 °C, the batch was cooled to 50 °C, depressurised,and sampled to confirm complete conversion to product. A solution of dilute acetic acid (1.13 g in 20 mL water) and water (100 mL) were added sequentially while maintaining the batch temperature >50 °C. The batch was then cooled to 20 °C over 1 h and filtered. The vessel and filter cake were washed with a mixture of water (40 mL) and methanol (35 mL). The solid was dried in a vacuum oven at 50 °C overnight to give 23.9 g title compound2a with 95 wt % purity, 95% yield (corrected for purity). The main impurity was phthalimide which was rejected at the next step. 7-Bromo-1-chloro-6-methoxyisoquinoline 3. 7-Bromo1-hydroxy-6-methoxyisoquinoline 8 (23.9 g, 95 wt % pure, 89 mmol) and acetonitrile (94 g) and phosphorous oxychloride (28.9 g, 188 mmol) were charged to a reactor under nitrogen. The batch was heated to 75 °C and aged for 2 h. HPLC showed no starting material present. The batch was cooled to 60 °C, and 2-propanol (11.5 g) was added at 60−80 °C. After the exothermic events had subsided, a further charge of 2-propanol (42 g) was made at 60−80 °C. The batch was aged for 20 min at 60 °C after complete addition of IPA. Water (253 g) was added over 20 min and the batch cooled to 20 °C. The batch was aged for 2 h at 20 °C and then filtered. The filter cake was washed with water (51 mL) and then dried in a vacuum oven at 50 °C to give 22.7 g title compound,2a (93% yield). Ester HCl Salt 22. Chloroisoquinoline 3 (20.5 g, 75.4 mmol) and N-Cbz-4-trans-L-hydroxyproline 19 (20.0 g, 75.4 mmol) were dissolved in DMSO (124 g). A solution of 2 M tert-BuOK in DMSO was prepared by dissolving tert-BuOK (22.8 g, 228 mmol) in DMSO (112 g). The 2 M tert-BuOK in DMSO was then added to the reaction mixture over 1 h at
