Letter pubs.acs.org/OrgLett
Asymmetric Synthesis of Akt Kinase Inhibitor Ipatasertib Chong Han,† Scott Savage,† Mohammad Al-Sayah,‡ Herbert Yajima,† Travis Remarchuk,† Reinhard Reents,§ Beat Wirz,§ Hans Iding,§ Stephan Bachmann,§ Serena M. Fantasia,§ Michelangelo Scalone,§ André Hell,§ Pirmin Hidber,§ and Francis Gosselin*,† †
Department of Small Molecule Process Chemistry, ‡Department of Small Molecule Analytical Chemistry and Quality Control, Genentech, Inc., 1 DNA Way, South San Francisco, California 94080, United States § Department of Pharma Technical Development Small Molecules, F. Hoffmann-La Roche AG, Grenzacherstrasse 124, CH-4070 Basel, Switzerland S Supporting Information *
ABSTRACT: A highly efficient asymmetric synthesis of the Akt kinase inhibitor ipatasertib (1) is reported. The bicyclic pyrimidine 2 starting material was prepared via a nitrilase biocatalytic resolution, halogen−metal exchange/anionic cyclization, and a highly diastereoselective biocatalytic ketone reduction as key steps. The route also features a halide activated, Ru-catalyzed asymmetric hydrogenation of a vinylogous carbamic acid to produce α-aryl-β-amino acid 3 in high yield and enantioselectivity. The API was assembled in a convergent manner through a late-stage amidation/deprotection/monohydrochloride salt formation sequence.
T
long-term manufacturing synthesis based on the efficient construction of 2 and 3 and their final assembly into ipatasertib (1). Our second-generation route to the unexpectedly challenging ketone 4 was designed to overcome issues associated with racemization of the C-5-methyl center in the carbonylation− Dieckmann approach we reported previously.3b As depicted in Scheme 1, we started from (R)-triester 5, obtained in two steps from conjugate addition of dimethyl malonate with methyl crotonate followed by lipase resolution.3b Condensation of 5 with formamidine acetate and chlorination using POCl3 gave 4,6-dichloropyrimidine methyl ester 6. Attempted aromatic Finkelstein bromination proved fruitless, since the solubility difference between chloride and bromide salts in organic solvents does not favor the equilibrium toward the 4,6dibromopyrimidine product. More forcing conditions led to halide-mediated SN2 dealkylation and concurrent lactonization to 8 as determined by HPLC-MS analysis.4 Reaction using NaI in acetone at reflux gave the corresponding 4-iodo-6-chloropyrimidine. Attempted iodination using hydroiodic acid led to only formation of ring-opened product derived from lactone 8. We found that reaction with excess NaI in CH3CN in the presence of Me3SiCl as a promoter, or preferably with MeSO3H as a Brønsted acid, allowed full and clean conversion to the desired 4,6diiodopyrimidine 7. Subsequent nucleophilic aromatic substitution with N-Boc-piperazine followed by ester hydrolysis
he serine-threonine protein kinase Akt (protein kinase B) functions as a pivotal junction in the PI3K/Akt/mTOR signaling cascade and regulates cell growth, proliferation, survival, and apoptosis.1 Drug discovery efforts in our laboratories identified GDC-0068 (1, ipatasertib) as a novel and potent inhibitor that targets the ATP-binding cleft of all three isoforms of Akt kinase (Figure 1).2 Ipatasertib is currently
Figure 1. Structure and retrosynthesis of ipatasertib.
in Phase III clinical trials for the treatment of metastatic castration-resistant prostate cancer and triple negative metastatic breast cancer. The active pharmaceutical ingredient (API) is a complex 6,7-dihydro-5H-cyclopenta[d]pyrimidine piperazine amide compound containing three chiral centers from the assembly of chiral bicyclic pyrimidine 2 and chiral α-aryl-βamino acid 3 (Figure 1). As a further challenge to process chemistry, the highly hygroscopic and deliquescent API monohydrochloride salt form was selected for development.3 This letters highlights our efforts to develop a highly efficient © 2017 American Chemical Society
Received: July 21, 2017 Published: August 31, 2017 4806
DOI: 10.1021/acs.orglett.7b02228 Org. Lett. 2017, 19, 4806−4809
Letter
Organic Letters Scheme 1. Asymmetric Synthesis of Ketone 4
Scheme 2. Improved Asymmetric Synthesis of Ketone 4
Me3SiBr7 in CH3CN gave (R)-4,6-dibromopyrimidine 14 that was treated directly with N-Boc-piperazine and Et3N to afford (R)-bromo-piperazine-pyrimidine 15 in 93% yield. Halogen− metal exchange required slow addition of i-PrMgCl over 4 h in 2-MeTHF/toluene at 0−10 °C and concomitant cyclization afforded a persistent primary (R)-enamine 16. Primary enamine 16 proved persistent under a variety of conditions and could be isolated in crude form by quenching with aqueous NH4Cl. We observed a strong pH dependence on the rate of hydrolysis of the metallo-enamine intermediate, since a pH 6 aqueous quench gave preferentially enamine 16, whereas a pH 5 aqueous quench afforded complete hydrolysis to ketone 4. Ultimately, we found that quenching and hydrolysis of 16 with aqueous NaHSO4 gave (R)-ketone 4 in 74% yield without detectable racemization as determined by HPLC analysis of the crude reaction mixture.8 This quench procedure provided a clean phase separation and stable mixture with ≤2% 4 loss to the aqueous layer. The product (R)-ketone 4 was then directly crystallized after a solvent switch to MTBE/heptane in 84% yield without any detectable racemization at the C-5-methyl center as ascertained by chiral HPLC analysis. Preliminary experiments toward the reduction of ketone 4 showed that substrate selectivity provided the undesired (5R,7S)-diastereomer 17 preferentially under a number of conditions (Table 1, entry 1).3b We identified a Ru-catalyzed asymmetric transfer hydrogenation (ATH) that provided a viable solution to the intrinsic diastereoselectivity problem (entry 2). After optimization, the ATH using RuCl(pcymene)[(1R,2R)-Ts-DACH] as the catalyst coupled with an efficient crystallization procedure afforded the desired alcohol 2 in 79% yield with 99.4:0.6 dr after recrystallization.9 In parallel, we identified a ketoreductase enzyme (KRED1) that in conjunction with glucose dehydrogenase (GDH) as a coenzyme, NADP as a cofactor, and D-glucose as a terminal reductant provided excellent diastereoselectivities up to 99:1 for the desired (5R,7R)-2 in 79% isolated yield (entry 3). We then developed a simple solution using a single enzyme KRED2 in
gave 9a. Functional group interconversion, halogen−metal exchange on Weinreb amide 9b using i-PrMgCl in THF at −10 °C, and inverse quench into aqueous NH4Cl allowed smooth formation of the desired (R)-ketone 4 in 82% isolated yield after crystallization from MTBE/heptane.5 Although this approach was used to synthesize multihundred kilogram amounts of 4, we identified a number of drawbacks that limited the long-term sustainability of the route: The biocatalytic resolution to obtain triester (R)-5 required an extended reaction time (>5 d, 20−30% yield) and thus proved unsuitable for late-stage development, and the aromatic Finkelstein halogen exchange required a large excess of NaI to drive reaction equilibrium to 7. Besides cost considerations and poor atom economy, the aromatic Finkelstein conditions created a significant waste disposal issue, since iodide is toxic to freshwater aquatic life at low concentration.6 We surmised that introduction of a nitrile functional group would provide a more robust handle for the overall process, especially under the Finkelstein reaction and the cyclization reaction conditions (Scheme 2). Furthermore, the nitrile would avoid unproductive functional group interconversions including protection−deprotection steps. We found that conjugate addition of dimethyl malonate to crotononitrile 10 proceeded smoothly when mediated by sodium tert-amylate in THF at 60−70 °C and provided rac-11. Gratifyingly, we identified a nitrilase enzyme that afforded the desired (R)-nitrile 11 in 38− 45% yield and >99.8:0.2 er. Interestingly, we found that enzyme activity was enhanced by high concentrations of sulfate, citrate, and phosphate ions. In contrast, addition of polar organic cosolvents impaired enzyme activity. The resolution was thus performed in aqueous 0.5 M K2SO4/50 mM KH2PO4 buffer at pH 7.2 and 20% substrate concentrations without organic cosolvents. Cyclocondensation of (R)-11 with formamidine acetate in MeOH gave (R)-4,6-dihydroxypyrimidine 12 in 84% yield. A three-step through-process started with chlorination of 12 with POCl3 in toluene and 2,6-lutidine as the base gave (R)4,6-dichloropyrimidine 13, and then bromination with 4807
DOI: 10.1021/acs.orglett.7b02228 Org. Lett. 2017, 19, 4806−4809
Letter
Organic Letters
crystallization upgraded the chiral purity of amino acid 3 to 99.5:0.5 er in a reproducible fashion. Removal of residual Ru from the process stream proved cumbersome and required use of an expensive and insoluble metal scavenger. Thus, over 50 different chiral diphosphines were evaluated as ligands for Ru in the asymmetric hydrogenation of 18. We found that the complex prepared by combination of ruthenium dimer complex [Ru(COD)(TFA)2]2 with atropisomeric diphosphines such as (S)-BINAP (L1)11 or (S)-MeOBIPHEP (L2)12 provided the highest catalytic activity and best enantioselectivities (entries 2−3).13 We opted for the BINAP catalyst system based on slightly higher enantioselectivity observed, ligand availability and cost considerations. During this optimization effort, we found that some batches of vinylogous carbamic acid 18 performed well in the asymmetric hydrogenation while others led to almost no conversion to 3. We determined that a small amount of residual NaCl in the bulk 18 led to catalyst activation under hydrogenation conditions. Using halide-f ree 18, [Ru(TFA)2(S)-BINAP] allowed for S/C = 10 000 with catalyst activation using NaBr as an additive (entry 3).14−16 The optimal ratio to ensure reaction robustness was selected as Ru/NaBr = 1:20 and thus afforded >99% conversion and 98.8:1.2 er. It is noteworthy that when addition of NaBr was omitted we observed only 29% conversion after 12 h with 90:10 er (entry 4).17 Using such low catalyst loadings (10 ppm relative to 18) allowed complete elimination of the scavenging step and resulted in a greatly simplified isolation process where α-aryl-βamino acid 3 was readily crystallized from EtOH/MTBE in 92% yield as the corresponding nonhygroscopic and enantiomerically pure sodium salt 3a.
Table 1. Diastereoselective Ketone Reduction
entry
catalyst
reductant
yield (%)
dra
1 2 3 4
5% Pd/C Rub KRED1c KRED2d
HCO2H HCO2Na D-glucose 2-propanol
63 79 79 86
6.0:94.0 95.8:4.2 >99.0:1.0 >99.5:0.5
a
Determined by HPLC analysis of the crude reaction mixture. b0.2 mol % RuCl(p-cymene)[(1R,2R)-Ts-DACH] in MeTHF. c[4] = 4.0 wt %, KRED1 (1.0 wt %), GDH (2.7 wt %), NADP (2.8 wt %), DMSO/H2O/pH 7 phosphate buffer. d[4] = 20 wt %, KRED2 (3.0 wt %), NADP (0.1 wt %), pH 7 phosphate buffer.
conjunction with 2-propanol as the terminal reductant, and further optimization afforded alcohol 2 in 86% yield and essentially perfect diastereoselectivity (entry 4). The sensitivity of ketone 4 in solution was apparent by generation of ppm levels of highly colored impurities but was overcome by performing the KRED step in a slurry-to-slurry reaction mode at room temperature to maintain robust performance. For longterm purposes, we preferred the biocatalytic asymmetric reduction of ketone 4 since it provided levels of diastereoselectivity that avoided downstream dr upgrades and eliminated operations associated with purging of Ru.
Scheme 3. End-Game Chemistry to Ipatasertib (1)
Table 2. Ru Catalyzed Asymmetric Hydrogenation
entry
catalyst
additivea
convb (%)
erc
1d 2e 3f 4
[RuCl2((S)-L1)] [Ru(TFA)2((S)-L2)] [Ru(TFA)2((S)-L1)] [Ru(TFA)2((S)-L1)]
LiBF4 NaBr NaBr −
99 (87) >99 (95) >99.5 (92) 29
93.0:7.0 98.0:2.0 98.8:1.2 90:10
a Additive/Ru = 20:1. bIsolated yield in parentheses. cDetermined by chiral HPLC analysis of the crude reaction mixture. dS/C = 1500, EtOH, 30 atm of H2, 60 °C. eS/C = 5000, NaBr/Ru = 20:1, EtOH (12 mL/g), 18 atm of H2, 60 °C. fS/C = 10 000, NaBr/Ru = 20:1, EtOH (12 mL/g), 18 atm of H2, 60 °C.
The original conditions for the amidation step used HBTU/ DIEA in CH2Cl2.18,19 Based on the undesirable properties of HBTU and its byproducts, we sought to substitute it for a more innocuous reagent. Deprotection of 2 with HCl in n-PrOH gave 18, and a series of experiments identified propanephosphonic anhydride (T3P) as a superior amidation reagent that provided 19 in 97% assay yield without detectable epimerization of the sensitive β-amino acid 3 chiral center (Scheme 3). Simple aqueous washes were required to remove
The first generation asymmetric hydrogenation of 18 using [RuCl2(S)-BINAP] (S/C = 1500) required the use of LiBF4 to abstract chloride ion from ruthenium and 30 atm of H2 at 60 °C.10 We observed fluctuations in performance of the asymmetric hydrogenation with enantioselectivities ranging from er 99.5:0.5 to 93.0:7.0 (Table 2, entry 1), but the final 4808
DOI: 10.1021/acs.orglett.7b02228 Org. Lett. 2017, 19, 4806−4809
Letter
Organic Letters
(4) McMurry, J. Org. React. 1976, 24, 187. (5) In our hands, 4,6-dichloropyrimidine 6 and its derivatives proved unreactive in halogen−metal exchange reactions to afford the desired ketone 4. Forcing conditions led to decomposition. (6) An ion-exchange iodide recovery procedure was developed as part of the manufacturing process for the HIV protease inhibitor indinavir sulfate (crixivan); see: Venkat, E.; Magliette, R. J.; McKinney, D.; Michaels, A. S. U.S. patent 6,379,556, Apr 30, 2002. (7) Schlosser, M.; Cottet, F. Eur. J. Org. Chem. 2002, 2002, 4181. (8) Racemization of the C-5-methyl chiral center, presumably through a pentadienyl anion intermediate, has been observed in earlier routes to ketone 4; see ref 3b. For a related indanone dialkylation, see: Trost, B. M.; Latimer, L. J. Org. Chem. 1977, 42, 3212. (9) (a) Canivet, J.; Labat, G.; Stoeckli-Evans, H.; Süss-Fink, G. Eur. J. Inorg. Chem. 2005, 2005, 4493. (b) Ohkuma, T.; Noyori, R. In Comprehensive Asymmetric Catalysis, 1st ed.; Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.; Springer Verlag: Berlin, 1999; Chapter 6.1. (c) Ohkuma, T.; Noyori, R. In Comprehensive Asymmetric Catalysis Supplement 1: Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.; Springer Verlag: Berlin, 2004; Supplement to Chapter 6.1. (10) Remarchuk, T.; Babu, S.; Stults, J.; Zanotti-Gerosa, A.; Roseblade, S.; Yang, S.; Huang, P.; Sha, C.; Wang, Y. Org. Process Res. Dev. 2014, 18, 135. (11) Miyashita, A.; Yasuda, A.; Takaya, H.; Toriumi, K.; Ito, T.; Souchi, T.; Noyori, R. J. Am. Chem. Soc. 1980, 102, 7932. (12) (a) Schmid, R.; Cereghetti, M.; Heiser, B.; Schönholzer, P.; Hansen, H. J. Helv. Chim. Acta 1988, 71, 897. (b) Schmid, R.; Foricher, J.; Cereghetti, M.; Schönholzer, P. Helv. Chim. Acta 1991, 74, 370. (13) [Ru(TFA)2(S)-BINAP] was first reported in: Heiser, B.; Broger, E. A.; Crameri, Y. Tetrahedron: Asymmetry 1991, 2, 51. (14) Chloride content was determined by elemental analysis or titration with aqueous AgNO3 with a limit of detection of 0.05%. (15) While a higher performance could be achieved using either HBr or HCl as additives, we selected NaBr in order to avoid formation of potential genotoxic impurities due to reactivity with EtOH. (16) Although the corresponding ethyl ester performed well in the asymmetric hydrogenation (94% conversion in 12 h at S/C = 4000, er = 98.7:1.3), the downstream ester hydrolysis proved problematic due to competing racemization of the sensitive chiral center. (17) Trace amounts of NH4Cl have been shown to improve Rhcatalyzed asymmetric hydrogenation of unprotected β-enamine amide; see: Clausen, A. M.; Dziadul, B.; Cappuccio, K. L.; Kaba, M.; Starbuck, C.; Hsiao, Y.; Dowling, T. M. Org. Process Res. Dev. 2006, 10, 723. (18) Carpino, L. A.; Imazumi, H.; El-Faham, A.; Ferrer, F. J.; Zhang, C.; Lee, Y.; Foxman, B. M.; Henklein, P.; Hanay, C.; Mügge, C.; Wenschuh, H.; Klose, J.; Beyermann, M.; Bienert, M. Angew. Chem., Int. Ed. 2002, 41, 441. (19) Safety concerns associated with handling of triazole-based reagents including HBTU and its reaction byproducts have been noted in: Dunn, P. J.; Hoffmann, W.; Kang, Y.; Mitchell, J. C.; Snowden, M. J. Org. Process Res. Dev. 2005, 9, 956. (b) Patterson, D. E.; Powers, J. D.; LeBlanc, M.; Sharkey, T.; Boehler, E.; Irdam, E.; Osterhout, M. H. Org. Process Res. Dev. 2009, 13, 900.
residual 3 and phosphonate byproducts from T3P, and piperazine amide 19 was crystallized from toluene/heptane in 82% isolated yield. Removal of the Boc group in 19 with HCl in n-PrOH gave dihydrochloride 20 that was treated with Amberlite FPA51 polymer or titrated with aqueous NaOH to afford, after a solvent switch to EtOAc, the crystalline monohydrochloride 21 as the corresponding EtOAc solvate. Myriad attempts to crystallize ipatasertib were met with failure and afforded variable mixtures of amorphous and crystalline materials. We identified a simple solution to this problem by dissolving 21 in water, spray-drying the resulting solution, and obtained amorphous ipatasertib API (1) in high yield, with unusually high bulk density (0.45 g/cm3) and excellent material properties for downstream pharmaceutical processing. In summary, by leveraging a combination of careful synthesis design and efficient use of chemo- and biocatalysis, we have identified and developed a sustainable and highly efficient asymmetric synthesis of ipatasertib that has been demonstrated on the multihundred kilogram scale and exhibits the desired attributes of a good pharmaceutical manufacturing process.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b02228. Experimental procedures and characterization (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Chong Han: 0000-0002-2863-3921 Francis Gosselin: 0000-0001-9812-4180 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We thank Dr. Anne Kraft, Kathrin Schneider, Gino Semadeni, Jannik Martin, Patrick Stocker, Patrick Meier, and colleagues in the analytical laboratories of Genentech and F. Hoffmann-La Roche AG for technical contributions and support.
■
REFERENCES
(1) (a) Cheng, J. Q.; Lindsley, C. W.; Cheng, G. Z.; Yang, H.; Nicosia, S. V. Oncogene 2005, 24, 7482. (b) Manning, B. D.; Cantley, L. C. Cell 2007, 129, 1261. (c) Lin, K.; Lin, J.; Wu, W.-I.; Ballard, J.; Lee, B. B.; Gloor, S. L.; Vigers, G. P. A.; Morales, T. H.; Friedman, L. S.; Skelton, N. S.; Brandhuber, B. J. Sci. Signaling 2012, 5, ra37. (2) Blake, J. F.; Xu, R.; Bencsik, J. R.; Xiao, D.; Kallan, N. C.; Schlachter, S.; Mitchell, I. S.; Spencer, K. L.; Banka, A. L.; Wallace, E. M.; Gloor, S. L.; Martinson, M.; Woessner, R. D.; Vigers, G. P. A.; Brandhuber, B. J.; Liang, J.; Safina, B. S.; Li, J.; Zhang, B.; Chabot, C.; Do, S.; Lee, L.; Oeh, J.; Sampath, D.; Lee, B. B.; Lin, K.; Liederer, B. M.; Skelton, N. J. J. Med. Chem. 2012, 55, 8110. (3) For preliminary communications, see: (a) Lane, J. W.; Spencer, K. L.; Shakya, S. R.; Kallan, N. C.; Stengel, P. J.; Remarchuk, T. Org. Process Res. Dev. 2014, 18, 1641. (b) Remarchuk, T.; St-Jean, F.; Carrera, D.; Savage, S.; Yajima, H.; Wong, B.; Babu, S.; Deese, A.; Stults, J.; Dong, M. W.; Askin, D.; Lane, J. W.; Spencer, K. L. Org. Process Res. Dev. 2014, 18, 1652. 4809
DOI: 10.1021/acs.orglett.7b02228 Org. Lett. 2017, 19, 4806−4809
