Article pubs.acs.org/OPRD
Scale-up Synthesis of (R)- and (S)‑N‑(2-Benzoyl-4-chlorophenyl)-1(3,4-dichlorobenzyl)pyrrolidine-2-carboxamide Hydrochloride, A Versatile Reagent for the Preparation of Tailor-Made α- and β‑Amino Acids in an Enantiomerically Pure Form Todd T. Romoff,*,† Andrew B. Palmer,† Noel Mansour,† Christopher J. Creighton,† Toshio Miwa,‡ Yuki Ejima,‡ Hiroki Moriwaki,‡ and Vadim A. Soloshonok*,§,∥ †
Hamari Chemicals USA, San Diego, California 92121, United States Hamari Chemicals Ltd., 1-4-29 Kunijima, Higashi-Yodogawa-ku, Osaka 53300024, Japan § Department of Organic Chemistry I, Faculty of Chemistry, University of the Basque Country UPV/EHU, Paseo Manuel Lardizábal 3, 20018 San Sebastián, Spain ∥ IKERBASQUE, Basque Foundation for Science, Maria Diaz de Haro 3, 48013 Bilbao, Spain ‡
S Supporting Information *
ABSTRACT: Unusual amino acids are of crucial importance to the synthesis of bioactive peptides and new chemical entities. Innovative methodology is always needed for the preparation of enantiomerically pure amino acids that does not rely on tedious resolution procedures. The proline-derived ligands (R)- and (S)-N-(2-benzoyl-4-chlorophenyl)-1-(3,4-dichlorobenzyl)pyrrolidine-2-carboxamide are outstanding, versatile, and recyclable reagents for the synthesis of tailor-made α- and β-amino acids. Here we report initial studies of the scale-up synthesis of the HCl salt of these reagents on the 100 g scale. The results demonstrate an increased efficiency and environmental friendliness of the bench-scale procedure and provides a firm foundation for the manufacture on multikilogram and larger scales.
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INTRODUCTION The current design of new pharmaceuticals includes two characteristic trends: introduction of fluorine, to reduce metabolic oxidation,1 and incorporation of amino acid (AA) residues, to mimic peptide−receptor interactions.2 In particular, tailor-made AAs3 are becoming an increasingly common structural feature of numerous recently marketed drugs.4 Consequently, the need for practical scalable methodology, allowing generalized access to structurally diverse and enantiomerically pure AAs, is at an all-time high.5 Among various generalized approaches,5,6 the asymmetric synthesis of AAs via Ni(II) complexes of AA-derived Schiff bases (Scheme 1) is one of the most versatile7 and of demonstrated practical potential.8 In a typical process, chiral tridentate ligands 1 are reacted with Gly or Ala, in the presence of base and a source of Ni(II) ions, to form Schiff base complexes 2. The homologation of Ni(II) complexes 2 to target tailor-made AAs can be conducted under a variety of conditions, among which most commonly used are alkyl halide alkylations,9 bisalkylations,10 Michael,11 aldol,12 and Mannich13 addition reactions. Since the seminal work by Belokon et al.,14 introducing the proline-derived ligand (R = H) 1, there have been numerous reports15 on various (R = Me, F, Br, Cl) derivatives designed to improve the stereocontrolling properties. One of the most recent and important advances in this area has been the design and development of ligand 3. It was found that the presence of the m-chlorine atom on the o-aminobenzophenone moiety and the p- and m-Cl atoms on the benzyl © XXXX American Chemical Society
group provide for quite efficient parallel displaced type of aromatic interactions between the aromatic rings.16 As the result of these aromatic interactions, the steric environment in the corresponding Ni(II) complexes is highly controlled, leading to virtually complete (de >98%) stereochemical outcome of the homologation reactions. These remarkable stereocontrolling properties of ligand 3 were recently used for the development of the first purely chemical dynamic kinetic resolutions of α-17 and β-amino acids.18 The advantage of this method over other synthetic, enzymatic, and biocatalytic approaches is that racemic AAs 4 and 5 can be used in unprotected form, thus simplifying the overall process and cost of the target enantiomerically pure α-6 and β-7 AAs. One may agree that the exploration of a full technological potential of ligand 3 for production of natural, (R)-configured, and αdeuterated tailor-made AAs will require a convenient scalable synthesis of compound 3 in both enantiomeric forms. In this work, we disclose a scale-up two-step synthesis of ligand 3 which was reliably reproduced on a 100 g scale.
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RESULTS AND DISCUSSION The preliminary19 reaction protocols developed for preparation of Pro-derived ligands of type 1 are presented in Scheme 2. The first step, the benzylation, commonly utilizes Bn-Cl and KOH or NaOMe as bases. The common feature of the second step, Received: February 16, 2017
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DOI: 10.1021/acs.oprd.7b00055 Org. Process Res. Dev. XXXX, XXX, XXX−XXX
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Scheme 1. General Application of N-Benzyl (S)-Pro Derived Tridentate Ligands 1 and 3 for the Asymmetric Synthesis of Tailor-Made α- and β-AAs
separating and isolating the product from the large amount of potassium chloride formed in the reaction. Traditionally, the carboxylate of the amino acid is utilized which is much less nucleophilic than the α-amine. Once the product benzylamine is formed, the mixture is acidified to consume all of the residual base; we found that a 10% excess of acid followed by slight pH adjustment if necessary served well to reproducibly provide the product in the desired zwitterionic form. 2-Propanol with 85% potassium hydroxide pellets as the base was found to be an ideal system for this reaction as it sufficiently solubilizes the potassium salt of proline and can be used under ordinary atmosphere. Lower alcohols (methanol, ethanol) form significant colored impurities when treated with potassium hydroxide as a result of their propensity to oxidize and undergo aldol condensation reactions and so are not suitable for this reaction. While most of our experiments used the KOH/iPrOH conditions, we did briefly survey other conditions, and Table 1 displays a qualitative summary of the different solvent/base/temperature systems that were investigated for the development of the conditions for the first step. Table 1. Solvents, Bases, and Temperatures Used in the Experiments Investigating the Alkylation Step Shown in Scheme 3a
Scheme 2. Literature19 Routes to the Preparation of Unsubstituted Ligand (S)-11
solvents
bases
temperatures
THF iPrAc MTBE DCM DMF NMP i-PrOH MeOH 50:50 MTBE:H2Ob 50:50 DCM:H2Ob EtOH
DBU iPr2NEt Et3N KOH K3PO4 K2HPO4 K2CO3 NaHCO3 NaOMe
20 °C 40 °C 60 °C
a
The items in bold correspond to the combination that gave the best results based on conversion to and ease of isolation of product. The amounts of solvent and base used in each experiment relative to Lproline were 10 volumes (mL/g) and 2.1 mol equivalents, respectively, and the reaction was allowed to proceed for 18 h. b5 volumes (mL/g) of each solvent in the mixtures were used.
the amide bond formation, involves the intermediate formation of a mixed anhydride to activate the proline carboxyl group. While these literature methods claim kilogram scalability, the application of SOCl2 affords about 30% reproducible yields,19a and the others are relatively expensive. We report here significantly improved methods and procedures for each of the two steps for the synthesis of 3. Step 1. The challenges of selectively alkylating a zwitterionic substance (Scheme 3) such as an amino acid are 3-fold: first, selective alkylation on nitrogen; second, forming the zwitterionic product under appropriate pH conditions; third,
However, the product 12 has only moderate solubility in 2propanol, and a cosolvent must be added after acidification in order to dissolve 12 away from the precipitated potassium chloride. We previously reported17 a version of this synthesis that utilized 3 volumes of chloroform as the cosolvent. While effective, we sought to replace the chloroform with a nonchlorinated solvent as many manufacturing facilities are minimizing the use of chlorinated solvents for environmental reasons. At the conclusion of the first step, a suspension of the product 12 and the byproduct KCl in 2-propanol is obtained. We surveyed a number of cosolvents to optimize the separation of 12 from KCl: To 5 mmol of 12 was added 5 mL of 2propanol and 5, 7.5, or 10 mL of cosolvent, and the samples were agitated for 1 h. A visual comparison was carried out to compare the result with a soluble control consisting of pure 12 dissolved in 3:1 chloroform−2-propanol. Table 2 describes the results of the solvent screen which demonstrated that 1.5 volumes of methanol was the ideal cosolvent.
Scheme 3. N-Alkylation of D-proline with the Requisite Benzyl Chloride
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the final isolation of 12.) Quantitative tracking of proline is not possible with ordinary detection methods, but we are able to track both the formation of 12 and the disappearance of benzyl chloride relative to the internal standard. The maximum concentration of 12 occurs by 4 h; the standard reaction time is 4.5 h. At this point, with the reaction mixture still at 40 °C, we carefully neutralize the reaction mixture with concentrated hydrochloric acid, again exothermic. The amount of conc. HCl is calculated to equalize the number of moles of total chloride with total potassium, and then we add 1.1 times this amount. Once again, the reactor system allows addition in one portion with no more than a 15 °C increase in temperature. We perform an IPC at this point and check the apparent pH with wet pH paper. If the pH is greater than 6, an additional 10% of the conc. HCl volume is added, and this generally brings the pH into the necessary 4−6 range. In addition, there is a visual indication of the appropriate pH for isolating 12 in the correct zwitterionic form, with a mild red color indicating the pH is still too high. Since even more potassium chloride is formed in this neutralization, it is essential that strong stirring is maintained. Once the conc. HCl addition is complete and the final pH adjustment is made, we add the methanol cosolvent (1.5 volumes). The mixture is allowed to stir overnight (16−24 h) at 40 °C allowing 12 to dissolve preferentially. At this point, the insoluble KCl is removed by filtration and washed with a 60:40 mixture of methanol/2-propanol. HPLC analysis of the KCl precipitate generally shows less than 5% of the theoretical yield of 12 contained. The precipitate can be dried and weighed if desired and is generally within 10% of the theoretical amount of KCl. Evaporation of the filtrate gives a crude solid, which is triturated with acetonitrile to furnish the product in final form; typical yields are 85−90%. This trituration also serves to remove the naphthalene internal standard (if present) and any 1,2-dichloro-4-(isopropoxymethyl)benzene, the ether formed by the reaction of 2-propanol with 3,4-dichlorobenzyl chloride. A small amount of unreacted proline is generally present, approximately 3% of the total mass based on NMR integration, but has no observable effect on the subsequent reaction. Step 2. Scheme 4 shows the formation of an amide bond between 12 and 2-amino-5-chlorobenzophenone (13), the second and final step in the synthesis. Once again, the zwitterionic nature of the starting material complicates the activation of the carboxylic acid and subsequent amide formation. Moreover, the amine 13 is considerably electron-deficient and requires a highly activated electrophile to effect complete reaction. We previously reported17 the use of methanesulfonyl chloride and 1-methylimidazole/DMAP to form the acid chloride/mixed anhydride, but this proved to be problematic on scale-up. Even though proline has a low
Table 2. A List of the Nonchlorinated Cosolvents Screened for the Alkylation Step Shown in Scheme 3.a
cosolventsa investigated in the solubility of 12 b
THF ethyl acetateb methanol
b
acetone ethanolb 2-propanolb,d
b
toluene CPMEb,c diethyl etherb
volumes 1 volume 1.5 volumes 2 volumes
a
All experiments were performed using 5 mmol of 12 and 5 mL of 2propanol in addition to the cosolvent listed. bInsoluble at 2 volumes. c Cyclopentyl methyl ether. dFor these experiments, additional volumes of 2-propanol were added to the 5 mL already present.
The alkylation of proline with 3,4-dichlorobenzyl chloride is exothermic, and we made it a priority to understand and control this exotherm in a reproducible manner. We set the initial reactant concentration as 1 M proline (8.7 volumes) in 2propanol containing 2.1 equiv of 85% potassium hydroxide. The reaction vessel needs to be of sufficient size to account for up to 2 volumes of additional cosolvent to be added. Experiments carried out in round-bottom flasks using heating mantles at scales up to 2 mol showed that the temperature increase was variable and somewhat unpredictable and depended greatly on the rate of addition of the chloride. We then chose to carry out the reaction in a fully jacketed and wellinsulated 1 L reactor that was a good model for scale-up into 50 and 100 L reactors: the Chemglass ChemRxnHub system 1000 mL jacketed reactor. At our working concentration of 1 M proline, the prototypical reaction reported here was carried out on a 0.4 mol scale (115 g proline). Reproducible temperature control was maintained by setting the jacket circulating fluid to a temperature of 40 °C. This reactor system is a powerful heat sink and provides such good control of exotherms that we could add the chloride in one portion and the internal reaction temperature did not exceed 45 °C. The formation of potassium chloride is visually apparent shortly after the addition of benzyl chloride, and therefore strong stirring is required throughout the process. For initial testing purposes, the progress of the reaction is most easily monitored by the addition of a small amount of naphthalene (0.75% w/v) for use as an HPLC internal standard. (The naphthalene is easily removed during
Scheme 4. Formation of the Amide Using Phosphorus Pentachloride
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Scheme 5. Formation of Decarbonylated Proline in the Presence of PCl5
intrinsic propensity for epimerization,20 we preferred to find a single reagent that would serve to activate the carboxylic acid without additional base. This had the potential of also directly providing the product as the hydrochloride salt in the course of the reaction. After a number of experiments utilizing thionyl chloride or phosphorus(V) chloride, we settled on the latter reagent as it gave more consistent and reproducible results. Initial experiments using dichloromethane as the solvent showed that we could indeed prepare the desired product 3 using one equivalent of PCl5 and no added base; however, this choice of solvent has a number of disadvantages. First, dichloromethane solubilizes the activated intermediate quite well, causing the activation to take place quickly, and therefore this step had to be carried out at −10 °C in order to control the exotherm. Second, if the activation is allowed to proceed longer than 10 min, significant formation of a decarbonylated alkylated proline,21 as reported by Bates and Rapoport and shown in Scheme 5, is observed. Third, while the product HCl salt does precipitate from the reaction mixture, the initial recovered yield is only 45%, with additional product recoverable from the filtrate. Fourth, dichloromethane is not a modern process-friendly solvent. We sought to replace dichloromethane as solvent for this amide coupling but preferred to continue using PCl5 as the activating agent. A survey of the literature for solvents compatible22 with PCl5 suggested that chlorobenzene might be a suitable substitute. This proved to be the case. As expected, PCl5 activation of 3 was slower in chlorobenzene than in dichloromethane, and we found that the reaction could be carried out at 10 °C initially, followed by warming to 20 °C. Control of the exotherm is further achieved because chlorobenzene has a heat capacity 1.5 times that of dichloromethane. Only a trace of the decarbonylated side product is seen even after activation for up to 90 min. The activation is monitored by quenching a sample of the reaction mixture with methanol and analyzing by HPLC. Complete activation requires 30 min. The bright yellow 13 is added at 20 °C, and the progress of the coupling is observed as the yellow color is quenched. Within 20 min the product HCl salt begins to precipitate, and at 90 min residual active phosphorus compounds are quenched with minimal methanol. After stirring for an additional 30−60 min, the precipitated 3 is collected by filtration, washed with chlorobenzene and acetone, and dried to give a 60−65% yield of product of 86−88% purity by mass, based on a concentration curve of recrystallized 3 (vide infra), with the major impurity being unreacted 12. An additional 20%
of product can be recovered from the filtrate to bring the total yield close to 80%. Crude 3, a flocculent powder, can be upgraded in quality in two stages: First, a simple wash with 2 volumes of methanol gives material that assays at 93−97% purity by mass. Second, the upgraded material can be recrystallized from 10−12 volumes of boiling methanol to increase the purity to >99% by mass and provide the product in a more easily handled form. Analysis of the crude, upgraded, and recrystallized materials by chiral HPLC shows no detectable epimerization. A use test was carried out to compare the efficacy of formation of the glycine− Ni(II) complex 14 from 3, and the results are shown in Table 3. The best quality of 14 was obtained using either upgraded or recrystallized grades of 3. Table 3. Use Test Comparison of Crude vs Recrystallized 3 in the Formation of the Glycine−Ni(II) Ligand Complex 14
quality of 3
puritya of 3
yieldb of 14
puritya of 14
eec of 14
crude upgraded recrystallized
86−88% 93−97% >99%
95% 95% 100%
95% >99% >99%
>99% >99% >99%
a
Mass percent. bBased on initial mass assay of 3. cBased on chiral HPLC.
In conclusion, a practical advancement of the synthesis of ligand 3 from bench scale to small reactor scale was achieved. Improvements were made in process efficiency, environmental friendliness, and yield. While there is still room for further fine optimization of the process, we have established a strong foundation for progression to multikilogram and even larger manufacturing scales.
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EXPERIMENTAL SECTION General Methods. Melting points are uncorrected. All solvents and reagents were obtained from commercial sources and were used without further purification. Temperatures of
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A single 3 mL aliquot was usually sufficient to bring the apparent pH into the range of 4−6. Methanol (600 mL) was added to the reaction mixture to selectively dissolve the product and was allowed to stir at 350 rpm for 16 h at 39 °C. The reaction mixture was transferred to a medium porosity fritted funnel to collect the KCl precipitate. An additional 250 mL of 3:2 methanol/2-propanol was used to wash out the reactor and then to wash the precipitate. The solid was collected and dried in a vacuum oven at 45 °C (60.6 g), which is 97% of the theoretical amount of potassium chloride. HPLC (method 1) of this material showed it to contain 99% pure by mass. Additional crops were recovered as desired from the mother liquor. mp 233−235 °C (dec.). 1H NMR (400 MHz, methanold4) δ 1.53−1.60 (m, 1H), 1.83−1.88 (m, 1H), 2.13 (broad s, 1H), 2.33−2.41 (m, 1H), 3.54 (m, 1H), 4.28−4.33 (m, 3H), 7.35−7.78 (m, 11H). 13C NMR (100 MHz, methanol-d4) δ 24.0, 29.6, 56,2, 58.0, 68.4, 127.2, 129.8, 131.1, 131.3, 132.0, 132.5, 132.6, 133.0, 134.2, 134.3, 134.5, 134.8, 135.0, 135.6, 138.1, 167.3, 195.9. MS(ESI/pos): 487.0/489.0 (M + H)+. [α]20D = +44° (c = 1, MeOH). Chiral HPLC (method 3): retention time 22.9 min. Both crude and recrystallized material showed a single enantiomer (limit of detection:
