Article pubs.acs.org/OPRD
Development of a Practical Synthesis of a TORC1/2 Inhibitor: A Scalable Application of Memory of Chirality Frederick Hicks,† Yongquan Hou,† Marianne Langston,† Ashley McCarron,† Erin O’Brien,*,† Tatsuya Ito,‡ Chunrong Ma,§ Chris Matthews,§ Colin O’Bryan,§ David Provencal,§ Yuxin Zhao,§ Jie Huang,# Qiang Yang,# Li Heyang,⊥ Matthew Johnson,⊥ Yan Sitang,⊥ and Liu Yuqiang⊥ †
Process Chemistry Research and Development, Millennium: The Takeda Oncology Company, 35 Landsdowne Street, Cambridge, Massachusetts, 02139, United States ‡ Chemical Development Laboratories, CMC Center, Takeda Pharmaceutical Company Limited, 17-85, Jusohonmachi 2-Chome, Yodogawa-Ku, Osaka 532-8686, Japan § Process Chemistry, Takeda California, 10410, United States Science Center Drive, San Diego, California 92121, United States # Chemical Development Department, AMRI, 21 Corporate Circle, Albany, New York 12203, United States ⊥ Asymchem Life Science, Tianjin 71 seventh Avenue TEDA, Tianjin, 300457, P.R. China S Supporting Information *
ABSTRACT: Progression toward a scalable synthesis of TORC1/2 inhibitor bulk drug, culminating in the first GMP manufacturing campaign, is described. Process research and development was needed to obtain the prerequisite stereocenter in high enantiomeric excess for kilogram-scale production. Through route selection, a six-linear step synthesis was developed which afforded the API in 20% overall yield. Development included an application of memory of chirality (MOC) to install a quaternary chiral center with near complete retention, a reductive cyclization to form a piperazinone core, and a palladium-catalyzed C−C bond-forming step.
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separation as the final step was inefficient. Long-term advancement of the program would clearly benefit from finding an alternative means of installing this quaternary carbon center in high enantiopurity without relying on chiral SFC separation.
INTRODUCTION 1 is an orally available, selective, and potent ATP-competitive small molecule inhibitor of the mammalian target of rapamycin (mTOR) kinase. mTOR belongs to the phosphatidylinositol 3kinase (PI3K) and AKT/PKB (also known as protein kinase B) signaling pathway that inhibits cell proliferation and tumor growth in numerous human tumor xenograft models.1 1 inhibits mTOR by strongly binding both of the mTORassociated signaling complexes, TORC1 and TORC2. TORC1 stimulates cell growth and proliferation, whereas TORC2 has been shown to be involved in the regulation of cytoskeleton functions and is thought to mediate cell survival.2 As part of our ongoing focus in cancer research, we were interested in 1 as a potential drug candidate.3 Herein we describe the development route used to produce GMP supplies of 1.
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Retrosynthesis. Retrosynthetic analysis for 1 is shown in Scheme 2. Our efforts were focused on developing a scalable route for installation of the quaternary center that would avoid chiral separation at the end of the synthesis. We planned to take advantage of a Suzuki coupling between chiral aryl chloride 7 and commercially available 4-cyclopropyl-urea-pinacol boronate 9.4,5 This choice at the end of the synthetic sequence, however, highlighted the need for low catalyst loading or an efficient palladium-scavenging step. Further retrosynthesis revealed lactam 10, which we planned to alkylate with a derivative of 2,2-difluoropropanol. Disconnection of the quaternary morpholine-carboxylic ester simplified the structure to a diastereomeric morpholine salt ((S)-12·(+)-CSA) and commercially available 2,4-dichloro-5-nitropyrimidine (11). Our initial plan was to obtain 12(S)·(+)-CSA via resolution of methyl morpholine carboxylic acid methyl ester.6 Detachment of the quaternary methyl substituent provides commercially available racemic 20. This retrosynthetic analysis takes advantage of a relatively early resolution step to install the key chiral center of the molecule, a
DISCOVERY SYNTHESIS
The Discovery synthesis of 1 began with commercially available morpholine-3-carboxylic acid (2) (Scheme 1). Aromatic substitution of 5-amino-2,4-dichloropyrimidine (3) with 2, followed by an in situ cyclization, generated the piperazinone core (4). Alkylation with chloroacetone, followed by treatment with DAST afforded the difluoropropyl moiety (6). Installation of the pivotal quaternary center was achieved via methylation of 6 to yield 7 as the racemate. A moderate yielding Suzuki coupling with boronate 8 generated the biaryl bond and completed a convergent synthesis of racemic 1. The desired enantiomer was obtained via chiral SFC chromatography. While acceptable from a Discovery chemistry standpoint, chiral © XXXX American Chemical Society
RESULTS AND DISCUSSION
Received: November 14, 2012
A
dx.doi.org/10.1021/op300330f | Org. Process Res. Dev. XXXX, XXX, XXX−XXX
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Scheme 1. Discovery synthetic route to 1
stage development, the procedure involved long processing times and took many recycles to afford a highest yield of 40%. To meet supply needs for later stages of development, it was deemed necessary to explore other options for synthesizing 12(S). (S)-2-Methylserine hydrochloride salt 13 (Scheme 3) was commercially available as a single isomer and was a logical
Scheme 2. Retrosynthetic analysis
Scheme 3. α-Methylserine annulation with bromoethylsulfonium salt
starting material for optically pure 12. Literature precedent encouraged us to investigate synthesizing the morpholine utilizing a commercially available bromoethylsulfonium salt 15 (Scheme 3).7 This annulation of tosyl-protected 2-methylserine (14) afforded the product in 72% yield after chromatography. While concise and effective, it was not further developed as process issues remained; most notably purification was difficult due to contamination with Ph2S, a byproduct of the sulfonium salt. In addition, the tosyl protecting group was removed with phenol and hydrogen bromide which formed phenyl 4methylbenzenesulfonate (PhOTs), a byproduct that was difficult to remove. Other conditions explored failed to remove the sulfonate. Protection groups such as N-tert-butoxycarbonyl (Boc) and N-benzyl were investigated in addition to the HCl salt of 13, but in these cases a mixture of unidentified polar compounds was obtained, and no desired product was observed.
significant improvement of the Discovery synthesis, which utilized chiral separation as the final step. Preparation of Morpholine 12-(S)·(+)-CSA. Resolution with (1S)-(+)-camphorsulfonic acid, used to provide early-stage supplies of (S)-α-methylmorpholine 12, has been proven on kilogram scale in Takeda laboratories. While effective for earlyB
dx.doi.org/10.1021/op300330f | Org. Process Res. Dev. XXXX, XXX, XXX−XXX
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Arguably, the most routine procedure for morpholine synthesis is ring closure of N-benzyl protected serine 17 with 2-chloroacetyl chloride, followed by morpholinone reduction with borane (Scheme 4).8 At the onset of our research it was
Scheme 6. Memory of chirality with morpholine 21
Scheme 4. Morpholine synthesis from serine
Table 1. Effects of base, temperature and solvent on the methylation of 21 unclear if procedures precedented for serine would work for the more hindered α-methylated derivative. Acylation of benzyl protected 2-methylserine (22) with 2-chloroacetyl chloride and 2-chloroacetyl bromide were successful, but these products (23) failed to cyclize to the desired morpholinone (24) and resulted in unreacted starting material or hydrolysis to 22 (Scheme 5).
Run
Basea
Solvent (10 vol)
Temp
ee (%)b
Conv (%)b
1 2 3 4 5 6
NaHMDSc NaHMDS KHMDSe KHMDS KHMDS LiHMDSf
THF THF THF toluene THF THF
−78 °C −50 °C −78 °C −78 °C 25 °C −78 °C
99 (S)d 74 (S) 97 (S) 88 (S) 35 (S) 77 (S)
>99 >99 >99 95 22 >99
a
Reactions were run using 1.3 equiv of base. bDetermined by GC analysis. cSodium hexamethyldisilazide. d100-g scale. ePotassium hexamethyldisilazide. fLithium hexamethyldisilazide.
Scheme 5. Failed methyl morpholine synthesis from αmethylserine
of 26 dropped to 74% ee (entry 2). Comparable enantioselectivity was obtained when KHMDS was used as base (entry 3). LiHMDS afforded lower selectivity (entry 6), which could be due to chelation of the lithium cation to the Boc-carbonyl group disrupting a chiral ring conformation.11 This hypothesis was supported by the lower ee observed for KHMDS in toluene, as the potassium cation would be more likely to coordinate to the Boc-carbonyl in less polar solvents (entry 4). Interestingly, when the batch was held at −78 °C with NaHMDS for 1 h prior to quenching with methyl iodide, the resulting ee was comparable to the ee obtained when methyl iodide was added immediately after deprotonation (97% ee vs 99% ee). To further validate the involvement of the Boc group in this mechanism, benzyl morpholine 25 was subjected to the methylation conditions (Scheme 6). For this substrate, both NaHMDS and LiHMDS at −78 °C afforded racemic product 27. Further investigation into the mechanism is warranted. For the plant run (16-kg scale) NaHMDS was chosen, and the product (26) was obtained in 97% ee (S) (Scheme 6). After Boc removal, enantiopurity was upgraded from 97% ee to >99% ee by (+)-CSA salt formation. This procedure afforded 15.6 kg of the 12·CSA salt in 99.5% ee (S) and 70% yield over three steps from 21. Furthermore, the CSA salt of 12-(S) was a solid and had greater stability than 12-(S) as the free base. As such, the CSA salt of 12-(S) was chosen as our GMP starting material. Aromatic substitution of 11 with 12. 2,4-Dichloro-5nitropyrimidine (11) was an attractive starting material for this process due to its low cost and high availability. In addition, nucleophilic substitution reactions of 11 have been reported to be selective toward the formation of the C4 product.13 Aromatic substitution of 11 with morpholine 12-(S)·CSA at −30 °C afforded a 2.5:1 ratio of products in favor of C4 substitution product 28 (Scheme 7, Table 2). This ratio was
As manipulation of the α-methylserine derivative proved challenging, resolution of 12 with (1S)-(+)-CSA was chosen for early development. (S)-21 could be synthesized via the procedure shown in Scheme 4, at a price comparable to that of the racemate.9 It was expected that if enantiopure compound 21 were to be used, the enantiopurity would be lost during enolate formation as part of the methylation process. However, a search of the literature showed that cases of ‘memory of chirality’ (MOC) did exist for amino ester derivatives comparable to our N-Boc-protected morpholine.10,11 To the best of our knowledge, no such examples of MOC had been reported for a morpholine ester such as 21 (Scheme 6). For 21, a dynamically chiral ring orientation may be possible due to the Boc group favoring an equatorial-like position.12 As such, we investigated α-methylation of enantiopure 21, hypothesizing that some of the enantiopurity would be retained. To our advantage, when the reaction was conducted at low temperature (−78 °C), we were able to α-methylate with near complete retention of chirality on a 100-g scale; 26 was obtained in 99% ee when NaHMDS was used as the base in THF (Table 1, entry 1). Maintaining the batch temperature at −78 °C during addition of the base was essential for this process. When the temperature was raised between −40 °C and −50 °C and maintained during base addition, the enantiopurity C
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in a 2.5:1 ratio of C4/C2 which was carried on crude through the next step. Pt/C Nitro Reduction and Ring Closure. Nitro reduction of 28 and subsequent ring closure to lactam 10 (Scheme 7) were adopted from the Discovery route as they were a concise means of forming the core of 1. However, safety studies using the RC-1 found that the procedure (0.50 mol % Pt/C, 3 mol % VO(acac)2, 50 °C, 50 psi H2) was classified as a medium safety risk with a ΔTad of 129 °C over a period of 1 h. For the process to be safe, it was necessary to extend the exotherm over a longer period of time. Reducing the catalyst loading, pressure and temperature, was complicated by the build-up of hydroxyamide intermediate 30 which required longer reaction time to reduce with these conditions, even with the addition of VO(acac)2 as a promoter.14 Careful examination by Thermal Screening Unit (TSu) on a solution enriched with hydroxyamide intermediate (30% 30) showed that it did not present a safety risk. With this finding, it was considered safe to reduce the catalyst (Pt/C) loading from 0.5 mol % to 0.25 mol %, the temperature to 35 °C, and the hydrogen pressure to 25 psi. As such, the exotherm was extended over 9 h and considered safe for scale-up. After filtration of the catalyst, product 10 could be precipitated directly from THF utilizing heptanes as an antisolvent. The product was obtained in high yield with good rejection of the reduced C2 regioisomer (29) to the filtrate. Although the purity was moderate (87% by HPLC area), this was primarily due to a large number of baseline impurities (all 99% ee (S).
Scheme 7. Aromatic substitution and reductive cyclization
lower than expected, likely due to the steric bulk of 12-(S). Other inorganic bases explored showed selectivity comparable to that of DIPEA (entries 9−17). Alternative solvents afforded similar results. Reaction temperatures below −50 °C improved the ratio (3.3:1, 28:29), but the reaction rate was slow, and therefore not synthetically viable (entry 6). Although changing the reaction conditions did not significantly improve the regioselectivity, 29 could be purged to the filtrate via crystallization of 28. While lab-scale investigations found that the desired C4 regioisomer (28) could be selectively crystallized away from the C2 regioisomer (29) at this stage, for ease of processing, it was decided to telescope the reaction and carry crude material through the reduction/cyclization step. To our advantage, the reduced C2 isomer was purged to the filtrate during isolation of 10. MTBE was chosen as the solvent for the aromatic substitution reaction for ease of azeotropic distillation with the solvent for the subsequent reaction, THF. The optimized conditions (MTBE (12 vol), DIPEA (2 equiv), −30 to 20 °C) were conducted on a ∼6.6-kg scale three times to afford 28:29 Table 2. Aromatic substitution of 11 with 12 run 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
solvent (vol) EtOAc (14) EtOAc (6.7) EtOAc (14) EtOAc (6.7) EtOAc (6.7) EtOAc (6.7) DCM (6.7) DCM (6.7) DCM (10) DCM (10) DCM (10) THF/H2O MTBE/H2O acetone toluene (15) toluene (15) toluene (15) toluene (15) MTBE (12)
base (equiv) DIPEA (1) DIPEA (2.4) DIPEA (3) DIPEA (2.4) DIPEA (2.4) DIPEA (2.4) DIPEA (2.4) DIPEA (2.4) NaOAc (2.1) NH4OAc (2.1) NBu4OAc (2.1) KHCO3 (1.1) KHCO3 (1.1) K2CO3 (1.1) KOAc (2.1) K2CO3 (2.1) Cs2CO3 (2.1) DIPEA (2.1) DIPEA (2.5)
pyrimidine (1.15 equiv) addn (time)
temp (°C) −5 °C to −5 °C to −5 °C to −20 °C −30 °C −50 °C −20 °C −30 °C −20 °C −30 °C −30 °C −5 °C to −5 °C to −5 °C to −5 °C −5 °C −5 °C −5 °C −30 °C
b
1 min 1 minb 1 minb 5 ha 5 ha 5 ha 2 hc,d 5 hc,d 10 ha,d 10 ha,d 10 ha 1 minb 1 minb 1 minb 1 minb 1 minb 1 minb 1 minb 20 mina
rt rt rt
rt rt rt
time (h)
(28:29) (287 nm)
% conv (287 nm)
24 22 24 20 19 26 22 21 20 19 24 24 24 24 2 2 2 2 12
2.6:1 2.4:1 2.1:1 2.7:1 2.5:1 3.3:1 2.5:1 3.0:1 4.3:1 3.5:1 1.8:1 2.0:1 2.0:1 2.1:1 2.4:1 2.4:1 2.3:1 2.0:1 3.1:1
92 95 100 >99 98 86 stallede 93 93 70 stallede 48 stallede 16 stalled 100 95 100 86 96 91 93 94
a
Added pyrimidine as a solution. bAdded pyrimidine as a solid. cAdded morpholine as a solution. dReverse addition: adding morpholine/base/ solvent mixture to pyrimidine solution. eAfter stalling, reactions were warmed to 25 °C and reached completion, but with decreased selectivity. D
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Alkylation and MSA Salt Formation. With an efficient means to access the piperazinone core of 1 in hand, investigation of the alkylation of 10 to install the difluoropropyl moiety of 7 was initiated. It was discovered that 2,2difluoropropylnosylate 31 (synthesized from difluoropropanol, Scheme 8) was an effective electrophile for lactam alkylation. In
salt while the undesired O-isomer 32 hydrolyzed back to starting material 10 (Scheme 9). Starting material 10 was largely purged in the filtrate, and 99% ee (S)). Pd levels were 99% ee after isolation as the CSA salt) that increased the efficiency of the synthetic sequence. Morpholine CSA salt 12-(S) served as a novel starting material for the production route, while the convergent approach of the Discovery synthesis was retained. Aromatic substitution of 2,4-dichloro-5-nitropyrimidine with 12 was optimized to produce 28, which was carried on through a reductive cyclization step. Alkylation of lactam 10 was improved with the synthesis of a stable 2,2-difluoropropyl nosylate. Optimization of the Suzuki coupling lowered the catalyst loading to 0.2 mol %, affording 1 in 95% yield and high HPLC area % purity. Finally, palladium scavenging, followed by F
dx.doi.org/10.1021/op300330f | Org. Process Res. Dev. XXXX, XXX, XXX−XXX
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mL) over 5 min at −50 °C. The reaction was allowed to warm to 0 °C, and 5% (aq) sodium thiosulfate solution (2 mL) was added at 0 °C. After stirring for 30 min at 0 °C, isopropyl acetate (2 mL) was added at 25 °C. After mixing, the aqueous phase was removed, the organic phase was washed with water (2 mL), and the aqueous layer was removed. The organic layer was washed with saturated sodium chloride solution (2 mL) and the aqueous layer removed. The organic layer was concentrated at 45 °C under reduced pressure. The desired product 27 was obtained as a light-yellow liquid. Chiral purity: 0% ee (Method B). 1H NMR (300 MHz, CDCl3) δ 7.50−7.13 (m, 5H), 4.21−3.96 (m, 2H), 3.91−3.65 (m, 5H), 3.58 (tt, J = 10.7, 5.4 Hz, 1H), 3.40 (d, J = 11.0 Hz, 1H), 2.97−2.84 (m, 1H), 2.44 (dt, J = 12.1, 2.8 Hz, 1H), 1.35 (s, 3H). 13C NMR (75 MHz, CDCl3) δ 19.7, 46.6, 51.6, 54.7, 62.8, 68.2, 75.3, 126.7, 128.1, 128.2, 140.3, 174.7. Synthesis of 2,2-Difluoropropyl 4-Nitrobenzenesulfonate (31). To a reactor was charged 2,2-difluoropropanol (6.0 kg, 62.4 mol) and 2-MeTHF (36 L, 6 vol) at 5 °C. TEA (10.4 L, 74.9 mol) was added over 30 min at 5 °C. A solution of nosyl chloride (13.8 kg, 62.4 mol) in 2-methyl THF (36 L, 6 vol) was added over 2.3 h at 5 °C. The reaction was stirred at 5 °C for 1 h, at which point HPLC analysis indicated that there was 3.2% (specification
