Process Safety Considerations for the Supply of a High-Energy

Article pubs.acs.org/OPRD

Cite This: Org. Process Res. Dev. 2019, 23, 1178−1190

Process Safety Considerations for the Supply of a High-Energy Oxadiazole IDO1-Selective Inhibitor Theodore A. Martinot,*,†,⊥ Michael Ardolino,† Lu Chen,∥ Yu-hong Lam,§ Chaomin Li,†,# Matthew L. Maddess,† Daniel Muzzio,‡ Ji Qi,‡ Josep Saurí,† Zhiguo J. Song,‡ Lushi Tan,‡ Thomas Vickery,‡ Jingjun Yin,‡ and Ralph Zhao‡ †

Process Research & Development, MRL, Merck & Co., Inc., Boston, Massachusetts 02115, United States Process Research & Development, MRL, Merck & Co., Inc., Rahway, New Jersey 07065, United States § Modeling and Informatics, MRL, Merck & Co., Inc., Rahway, New Jersey 07065, United States ∥ WuXi AppTec, 288 Fute Zhong Road, Waigaoqiao Free Trade Zone, Shanghai 200131, China

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S Supporting Information *

ABSTRACT: The development of a stereospecific synthesis of a IDO1-selective inhibitor is described. The synthetic strategy toward enabling early discovery efforts along with additional findings pertaining to process safety that limited scalability are outlined. A convergent approach that supported the synthesis of material suitable for early preclinical and/or good laboratory practice toxicology studies and avoided the formation of key high-energy intermediates is summarized. KEYWORDS: process safety, oxadiazole, benzocyclobutylamine, IDO1

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relationship (SAR) around both sides of the oxadiazole.7 The following discussion features strategic elements surrounding the supply of 2, including route planning with regard to availability of the building blocks and safety as well as the impact of the selection of these building blocks or intermediates on the delivery timeline.

INTRODUCTION Continued interest in immuno-oncology has been spurred by the demonstrated durable responses observed in patients treated with anti-PD1 monoclonal antibodies (mAbs) (e.g., pembrolizumab), among other therapies.1 Indoleamine-2,3dioxygenase-1 (IDO1) has been shown to play an important role in immunomodulation.2 Moreover, IDO1 is overexpressed in multiple tumor types (including melanoma, colon, and ovarian), and this mechanism is believed to contribute to immunosuppression, which in turn may promote tolerance to the cancer.3,4 The combination of IDO1 inhibitors and antiPD1 mAbs has been shown to be synergistic in efficacy models,5 and multiple clinical trials are currently evaluating the efficacy of this combination as a means of improving response rates while maintaining durability. Notably, epacadostat (INCB24360, 1) (Figure 1)6 has been developed to this end and is currently being evaluated in

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RESULTS AND DISCUSSION Initial Route to 2 and Preliminary Evaluation. Initial access to 2 (Scheme 1) started from benzocyclobutyl carboxylic acid 3, which was initially available in small quantities and was converted to the amine using a Curtius rearrangement.7 Initial efforts to support SAR studies for this target (IDO1) carried the racemic carbamate (rac-4) forward with resolution of the final inhibitor (e.g., 2), typically using supercritical fluid chromatography with a chiral stationary phase (“chiral SFC”). However, once the configuration of the optimal benzocyclobutylamine building block was determined to be S, this intermediate was amenable to early resolution, which facilitated downstream operations by avoiding resolution of each new compound. The racemic N-Boc amine was resolved using chiral SFC to afford 4. After deprotection to afford the benzocyclobutylamine hydrochloride salt (5·HCl), hydroxyamidine 7 was formed by condensation with commercially available imidoyl chloride 6.8 The hydroxyamidine was protected as the cyclic carbamate by reaction with carbonyldiimidazole (CDI), and aminooxadiazole 8 was oxidized to the nitrooxadiazole 9 using hydrogen peroxide in trifluoroacetic acid (TFA) or sulfuric acid.9 The final product was obtained by condensation of the ethanolamine side chain

Figure 1. Structure of epacadostat (1) and 2.

multiple clinical trials in combination with anti-PD1 mAb therapy (pembrolizumab, nivolumab, etc.). Additional IDO1 inhibitors have been developed and reported in the literature.7 Recently we sought a synthetic approach to structures such as 2 that would enable the discovery effort around this target (IDO1) and facilitate the construction of a structure−activity © 2019 American Chemical Society

Received: March 5, 2019 Published: May 28, 2019 1178

DOI: 10.1021/acs.oprd.9b00105 Org. Process Res. Dev. 2019, 23, 1178−1190

Organic Process Research & Development

Article

Scheme 1. Initial Route to 2

Figure 2. DSC thermogram of 8 (sample size: 5.70 mg).

relative to the number of energetic functional groups in the molecule (oxadiazole, nitro, etc. in 8 and 9).11,12 Differential scanning calorimetry (DSC) was used as a primary means of evaluating and triaging intermediates of concern. Specifically, aminooxadiazole 8 was first analyzed by DSC (Figure 2) and showed a clean melt with an onset at 213.9 °C followed by an exothermic decomposition, which is attributed primarily to decomposition of the oxadiazole and/or the benzocyclobutylamine. The integrated heat for this decomposition, which has an onset of 254 °C, is 1567 J/g, which is considered significant.13 Drop-weight tests were performed on 8 at up to 30 J and were negative (six of six drop tests), indicating that the compound is not shock-sensitive. The high onset of decomposition (both absolute and relative to a reaction temperature of 25 °C), coupled with the apparent

(10) through displacement of the nitrooxadiazole and subsequent deprotection of the carbamate. A significant advantage of the route outlined in Scheme 1 is that intermediate 9 is a versatile building block for rapidly enabling SAR evaluation of the tailpieces, with the nitro group being a suitable leaving group for a variety of nucleophiles (amine, alcohol, thiol, etc.).10 However, the route was also limiting in that the stereogenic and arguably more precious portion of the molecule was installed first. This produced a potential bottleneck, as the availability of benzocyclobutylamine 5 was limited early in the program. Moreover, the safety elements associated with handling compounds 8 and particularly 9 were a concern because of the low carbon count relative to the number of heteroatoms (e.g., in 8) or 1179



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Figure 3. DSC thermogram of 9 (sample size: 3.40 mg).

Scheme 2. Oxidation of 8 to 9

Formation of 9. The conversion of 8 to 9 can be effected by a number of different reaction conditions. “Piranha” (sulfuric acid/hydrogen peroxide16) conditions9 were evaluated but resulted in a complex mixture, and alternative options were therefore considered. After some additional evaluation, it was determined that either TFA17/urea hydrogen peroxide (UHP)18 or TFA/hydrogen peroxide afforded the cleanest reaction profile. Both sets of conditions are believed to proceed through the same reactive species (pertrifluoroacetic acid (PTFA)19−21), and therefore, the selection of conditions was based on more practical considerations. Further development led to the conclusion that the latter set of conditions using hydrogen peroxide was likely easier to operate because hydrogen peroxide can be dosed continuously more easily than UHP (a solid), which in turn means that the individual reaction steps could be better controlled, thus avoiding potential runaway exotherms. The main impurity of the reaction is nitrile oxime 11 (Scheme 2), which is believed to originate from the condensation of the intermediate nitrosooxadiazole with the starting material 8 followed by degradation of the corresponding diazooxadiazole dimer.15 Empirically, only 8, 9, and 11 were observed (by HPLC− MS analysis) over the course of the reaction; therefore, it is believed that the relative rate for the initial oxidation to the

highly crystalline nature of the intermediate (as indicated by the sharp melt) led us to conclude that 8 was an intermediate that could be synthesized and handled safely at intermediate scale (3000 J/g; it is noted that the onset of each these transitions is a best-case scenario, as solutions or impure phases with no melting obstacles may have lower onset temperatures. Drop-weight tests of 9 gave three positive results out of six at 30 J impact (decomposition (charring); no smoke, flame, or audible report) but no positive results at 20 J intensity. Combined, these results constituted a significant safety risk for scale-up. Therefore, a better understanding of the synthetic process leading to the formation of 9 was deemed necessary in order to fully evaluate whether the approach to 2 outlined in Scheme 1 could be safely conducted. 1180



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Figure 4. ARC data showing (a) the pressure rise rate and (b) the self-heat rate as functions of temperature for the oxidation of 8 to 9. Experiments were performed by dissolving 8 in TFA, heating the mixture to 50 °C, and then injecting H2O2 at once (see the annotation) with the ARC in adiabatic tracking mode.

performed on this reaction following a rapid addition of hydrogen peroxide showed a significant temperature and pressure increase at approximately 50 °C, which was also indicative of a potential runaway exotherm due to PTFA degradation (Figure 4). These data as a whole indicated that care should be taken when this oxidation is performed in order to avoid the thermal decomposition of the PTFA formed. On the basis of the results collected as well as the lack of benefit of performing this oxidation at a significantly higher temperature (Figure 5), the

hydroxylamine must be a rate-determining step (i.e., no appreciable accumulation of the hydroxylamine or nitroso intermediates), and the formation of 11 as a byproduct of the reaction may be inevitable under these reaction conditions unless the mechanism for the formation of 9 itself were to be changed. Indeed, varying the reaction temperature, concentration, hydrogen peroxide addition rate, or stoichiometry did not have a statistically significant effect on the ratio of 11 to 9 at the end of the reaction. While the reaction temperature was shown to have an effect on the reaction conversion, there was no statistically significant variation in the amount of impurity relative to the product under any of the conditions tested. This observation indicates that formation of the impurity under the reaction conditions used may not be easily suppressed. Fortunately, upon completion of the reaction, the reaction mixture is diluted with water, which causes the desired product 9 to nucleate, and most of the impurities (including 11) can be purged through the mother liquor. In addition, reaction calorimetry for the oxidation of 8 to 9 was performed (both in a SuperCRC calorimeter and in a Mettler Toledo EasyMax system equipped with HFCal) to determine safe operating conditions. First, the addition of hydrogen peroxide to TFA was evaluated. From the data collected in the EasyMax, it was determined that the enthalpy associated with the addition of 0.10 mol of hydrogen peroxide to 0.78 mol of TFA and the subsequent formation of PTFA is approximately 1 kJ, which corresponds to an adiabatic temperature rise (ΔTad) of 95% purity of converted product, and 200 g). From 31, the synthesis is completed by diazotization of the hydroxyamidine and in situ formation of imidoyl chloride 32, 1184



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Scheme 8. Homodimerization of 32

at ambient temperature. After analysis of the reaction mixture by HPLC confirmed that the initial condensation was complete, diisopropylethylamine (190 g, 1.47 mol, 1.25 equiv) and CDI (219 g, 1.53 mol, 1.30 equiv) were charged sequentially with stirring at ambient temperature. The resulting solution was stirred for 1 h at ambient temperature. The reaction mixture was then diluted with water (2000 mL, 10 vol), which caused precipitation of the product. The resulting solution was extracted with ethyl acetate (2 × 3000 mL; 2 × 15 vol), and the organic layers were combined. The resulting mixture was washed with 1 M aqueous hydrochloric acid (2 × 1000 mL; 2 × 5 vol), dried over anhydrous sodium sulfate, and concentrated to dryness under reduced pressure. This resulted in 250 g (74% yield) of (S)-3-(4-amino-1,2,5-oxadiazol-3-yl)4-(4-fluorobicyclo[4.2.0]octa-1(6),2,4-trien-7-yl)-1,2,4-oxadiazol-5(4H)-one (8) as a white solid. 1H NMR (300 MHz, DMSO-d6) δ 7.22−7.01 (m, 3H), 6.58 (s, 2H), 5.79 (t, J = 4.0 Hz, 1H), 3.66−3.51 (m, 2H); 19F NMR (282 MHz, DMSOd6) δ −113.1; MS m/z calcd for C12H9FN5O3 (M+H+) 290.1, found 290.1; [α]20 D +32.5 (c 0.22, MeCN). (S)-4-(4-Fluorobicyclo[4.2.0]octa-1(6),2,4-trien-7-yl)3-(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,4-oxadiazol-5(4H)one (9). Compound 8 (55.27 g, 191 mmol, 1.00 equiv) was charged to a 5 L three-neck round-bottom flask. The flask was equipped with an overhead stirrer, reflux condenser, and thermocouple probe and inerted under nitrogen. Trifluoroacetic acid (1105 mL, 20 vol) was charged, and the resulting mixture was stirred and heated with a target internal temperature of 40 °C. A hydrogen peroxide solution (32% w/w, 183 mL, 1911 mmol, 10 equiv) was charged to an addition funnel and added to the reaction mixture dropwise, and a batch temperature below 45 °C was maintained throughout the addition. After the addition was complete, stirring was continued at 40−45 °C until the starting material was no longer consumed as determined by LC−MS analysis (i.e., a steady state was reached). The heating mantle was removed and replaced with an ice/water bath. Water (2.8 L, 50 vol) was added dropwise to the reaction mixture while a batch temperature of

Process Safety Considerations for the Supply of a High-Energy

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