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Two Approaches to the Chemical Development and Large-Scale Preparation of a Pyrimidyl Tetrazole Intermediate Peter Mullens,*,† Ed Cleator,*,† Mark McLaughlin,*,‡ Brian Bishop,† John Edwards,† Adrian Goodyear,† Teresa Andreani,‡ Yan Jin,‡ Jongrock Kong,‡ Hongmei Li,‡ Michael Williams,‡ and Michael Zacuto‡ †

Department of Process & Analytical Chemistry, Merck, Sharp and Dohme Ltd., Hertford Road, Hoddesdon, EN11 9BU, U.K. Department of Process & Analytical Chemistry, Merck Research Laboratories, Merck & Co., Inc., Rahway, New Jersey 07065, United States

‡

S Supporting Information *

ABSTRACT: Two new routes to a pyrimidyl tetrazole intermediate are described. The first-generation route featured an ironcatalyzed cross-coupling between 4-butenylmagnesium bromide and a 4-chloropyrimidine derivative to afford an alkene-bearing pyrimidine intermediate. A subsequent intramolecular Heck cyclization afforded the desired bicyclic core, which was subsequently converted to the corresponding carboxylic acid via hydroboration and oxidation. This route was rapidly defined and used to prepare the initial 0.3 kg of the pyrimidyl tetrazole intermediate, which supported early toxicology and clinical studies of a drug candidate. A second-generation, eight-step route to the pyrimidyl tetrazole intermediate was defined and demonstrated on multikilogram scale in a 21% overall yield. The key transformation in this sequence was a copper(I) mediated cyclization of an iodopyrimidine, affording the bicyclic core of the target in quantitative yield. Due to the larger scale involved for the secondgeneration approach, significant process safety evaluation was undertaken for a number of steps in this route, and the highlights of these studies are presented.

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INTRODUCTION As part of an ongoing clinical program, a practical synthesis of a pyrimidyl tetrazole intermediate 1 (Figure 1) was required.

from previous projects. Specifically, given the inherent properties of tetrazole, the probability of conducting a regioselective C−N bond formation by reacting a preformed NH-tetrazole heterocycle with a 2-halopyrimidine was considered low. Consequently, it was decided to build the tetrazole ring via condensation of a 2-aminopyrimidine with trimethylorthoformate followed by a cycloaddition reaction with trimethylsilylazide.1 Strategically, it made sense to introduce the tetrazole as late as possible in the synthesis due to safety concerns regarding the handling of low molecular weight derivatives of this nitrogen rich and potentially energetic heterocycle. Hence, the first disconnection (Scheme 1) revealed the pyrimidine amino acid intermediate 2. It was envisaged that the carboxylic acid functionality in 2 could be derived from an exocyclic olefin 3 via a hydroboration/ oxidation sequence. In turn, the fused bicyclic structure of 3

Figure 1. Pyrimidyl tetrazole intermediate 1.

The original synthesis of 1 was unselective and low yielding and was quickly superseded as bulk drug requirements increased. In order to meet project timeline demands, the process research group rapidly defined a first generation route that was capable of delivering hundreds of grams of the key tetrazole intermediate 1. Concurrent with this initial route development, the project team recognized the opportunity for a better long-term route to intermediate 1, targeting improved selectivity and brevity of synthesis when compared to previous iterations. Ultimately this work resulted in an improved second generation route to 1. Described herein, we report the key aspects of the first generation route, followed by a more in depth discussion of the second generation route, together with a detailed safety study to support the reaction conditions required for the formation of the tetrazole functionality present in 1.

Scheme 1. Retrosynthetic Analysis for First Generation Chemistry

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FIRST GENERATION ROUTE Initial retrosynthetic analysis of 1 was guided by literature information and in-house experience with N-aryl tetrazoles © XXXX American Chemical Society

Received: April 18, 2016

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DOI: 10.1021/acs.oprd.6b00136 Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development

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Scheme 2. PMB Protection and Bromination to Intermediate 5

It was discovered that several impurities (4b, 4c, 4d, and 7) still contained an unreacted 4-chloro moiety, which could readily undergo oxidative addition in the presence of the active Pd(0) catalyst. These complexes would be expected to be relatively stable and were shown by spiking studies to poison the desired catalytic cycle. With this information in hand, we sought an effective way to remove these chloro containing impurities by exploiting the electrophilic reactivity of the 4chloro moiety. Our aim was to convert the impurities bearing the 4-chloro functionality into derivatives which could be easily purged prior to the Heck reaction. A screen of readily available amines quickly identified N,N′-dimethylethylenediamine (N,N′DMEDA) as an appropriate nucleophile that quantitatively scavenged all of the remaining species bearing the 4-chloro functionality via an SNAr displacement. The resulting adducts were easily removed from the desired product by washing with aqueous acid. Incorporation of this simple procedure into the workup post Fe-catalyzed coupling yielded 4 in higher purity (Figure 3), and the resulting stream was found to perform reproducibly in the ensuing Heck reaction, allowing the sequence to be telescoped. Attention now switched to the key Heck reaction, initial scouting for appropriate conditions for this transformation made use of high-throughput experimentation techniques in order to screen for an appropriate ligand. The results of this screen indicated that DPE-Phos was a viable ligand for the desired process, giving relatively clean conversion to the bicyclic product(s), while also providing an acceptable ratio of the desired exocyclic 3 versus endocyclic 3a alkene products. However, further development revealed several previously unseen issues which required the initial procedure to be modified in order to guarantee a satisfactory outcome. First, the quality of the incoming crude stream of bromopyrimidine 4 was important and key controls for this were implemented upstream as already highlighted above. Next, control experiments highlighted a background ligandless Heck process catalyzed by Pd(OAc)2 was a competitive side reaction, the major product of which was found to be the undesired endocyclic olefin 3a. The problem was further exacerbated by the low solubility of free DPE-Phos ligand in the reaction solvent, acetonitrile. This caused widely varying performance in the Heck reaction, in terms of selectivity for the exocyclic olefin 3 versus. endocyclic alkene 3a. To address this variability, several adjustments were made to the reaction conditions. It was critical to employ an excess amount of DPE-Phos relative to Pd(OAc)2 (2:1 ratio) in order to suppress the ligand-free background reaction and it was necessary to precomplex these two species in a small volume of CH2Cl2 prior to mixing with the substrate and other reagents in MeCN. Employing these changes ensured an assay yield of 80% and 9:1 ratio of exocyclic 3 vs endocyclic 3a was realized at kilogram-scale. The final product 3 was isolated after workup by crystallization from MeOH in 73% yield (Scheme 4). Although an adequate performance for the Heck cyclization was ultimately achieved for this initial delivery of material, it was recognized that further

would be generated via a palladium-catalyzed intramolecular Heck reaction connecting the 5-position of the pyrimidine with the pendant terminal olefin in 4. Disconnection of the 4butenyl chain via an appropriate cross-coupling was proposed and would need to display appropriate selectivity between the 4- and 5-positions of dihalopyrimidine 5. Introduction of a bromo group in the 5-position of the pyrimidine (directed by the para-amino functionality via an electrophilic process) would create the requisite handle for the planned Heck reaction. Last, unmasking the PMB protecting groups leaves 2-amino-4chloropyrimidine 6 as a readily available and inexpensive potential starting material for the proposed synthesis. The synthesis began with protection of 2-amino-4chloropyrimidine 6 using commercially available PMB-Cl and a strong alkoxide base (NaOtBu) in a mixture of THF and DMAc. Standard extractive workup followed by solvent switch gave the product 7 as a solution in MeCN in a 70% assay yield, which was used directly in the ensuing NBS bromination, to give bromopyrimidine 5 in a 98% assay yield. After an aqueous workup and solvent switch to i-PrOH, the product could be isolated by the addition of water, affording 5 as a white solid with 99% LC purity and in 62% overall yield from 2-amino-4chloropyrimidine 6 (Scheme 2). Next, we wished to install the butenyl chain as a synthetic handle for the proposed Heck cyclization. In addition to the potential difficulties engendered by the use of a heterocyclic substrate for a metal-catalyzed cross-coupling, in this case there was also the matter of chemoselectivity between the 4-chloro and 5-bromo functional groups. The commercial availability of 4-butenylmagnesium bromide encouraged consideration of a Fürstner-type Fe-catalyzed cross-coupling to achieve the desired bond formation.2 Pleasingly, it was found that the desired cross-coupling was indeed possible and selective for reaction at the 4-position pyrimidine 5 (Scheme 3). Moreover, the conditions were Scheme 3. Fe-Catalyzed Coupling of Butenylmagnesium Chloride

relatively mild, and the reaction was readily scaled with the yield at kilo-scale reproducing the laboratory yield (73%). However, it was found that impurities generated during this step had a significant deleterious effect on the downstream Heck reaction. If crude alkene 4 was used directly in the Heck step, the reactions tended to stall, and impractical Pd-catalyst loadings were required to give acceptable conversions. A detailed MS-guided analysis of the crude reaction mixture was undertaken to fully understand the nature of the problem (Figure 2). B



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Figure 2. Typical LC profile of Fe-coupling and impurities identified by MS.

Figure 3. Improved LC purity profile of 4 after treatment with N,N′-DMEDA.

Scheme 4. Heck Cyclization Reaction

Scheme 5. Hydroboration/Pinnick Oxidation Sequence

8 from this sequence was 86%. Oxidation of the primary alcohol 8 to the carboxylic acid was executed via a two-stage, one-pot sequence where treatment of alcohol 8 with 1.5 equiv of Dess-Martin reagent in DMAc solvent resulted in clean conversion to the aldehyde and then modified Pinnick conditions gave the acid in 84% overall assay yield (pH controlled at 5−6 by using Na2HPO4 in place of the commonly employed NaH2PO4). Also, limonene was added as the scavenger for the hypochlorite byproduct instead of the more pungent and volatile 2-methyl-2-butene, which helped suppress overoxidation at scale.3 The desired acid 2 was ultimately isolated as the cyclohexylamine salt, which provided excellent impurity rejection. (Scheme 5) The final steps required to convert acid 2 to reach tetrazole acid 1 involved deprotection of the PMB groups and construction of the tetrazole ring. These steps also feature in

optimization of the ligand/catalyst/solvent system would be required if the first generation route was to be employed for future deliveries. However, with the development of the second generation chemistry, further optimization of this Heck reaction was deprioritized. Having secured access to the exocyclic alkene 3, attention was turned to conversion of this functional group into the target carboxylic acid 2 via a hydroboration/oxidation sequence. Overall, this series of transformations was uneventful although several key points are worthy of mention. Hydroboration was conducted using BH3·THF complex, and it was necessary to limit stoichiometry of this reagent to a maximum of 0.4 equiv in order to avoid over-reduction of the pyrimidine ring. Oxidative workup was optimally achieved using an aqueous solution of sodium percarbonate, which was found to be more practical and effective than the alternative sodium perborate reagent studied. Overall yield of the product alcohol C



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Scheme 6. Retrosynthetic Analysis

the second generation route to 1 and details of this chemistry will be discussed below. In summary, the first generation route allowed for a rapid delivery of the tetrazole acid 1 that supported early toxicology and clinical testing of a drug development candidate in timely fashion. Recognizing the trade-off in synthetic understanding vs speed of delivery that is often encountered in early API delivery projects, the project team elected to define an improved approach to the target molecule and this is described below.

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Scheme 7. Preparation of 4-Methoxybenzyl Chloride

With multikilogram quantities of PMB-Cl required for protection of 6 a rigorous process safety assessment was carried out. The key results are summarized below. (1) The reaction exotherm produced upon addition of SOCl2 to the PMB−OH solution was quantified by reaction calorimetry as −172 kJ/mol with an adiabatic temperature rise of 76 K. When this reagent was added over a minimum of 30 min the reaction could be controlled by the rate of addition of thionyl chloride. (2) Differential scanning calorimetry (DSC) analysis of a sample of PMB-Cl concentrated to a neat oil displayed a fast exotherm (60 J/g) with onset above 150 °C. DSC analysis of a solution of PMB-Cl in DCM (1 vol) and DMF (3 vol) showed no significant exothermic activity. (3) Advanced Reactive System Screening Tool (ARSST) analysis of the DCM solution of PMB-Cl after aqueous work up showed no evidence of exothermic decomposition when aged isothermally at 55 °C for 12 h; however, slow gas release was evident (0.3 L gas per L reaction per hour). (4) ARSST heat ramp analysis of a sample concentrated to a neat oil showed significant exothermic decomposition. This decomposition was potentially initiated as low as 75 °C (max rate of temperature rise was 20 K per min at 180 °C). This decomposition was also associated with a very large gas release initiating at ∼100 °C (max rate greater than 30 psi per min at 180 °C). This equates to a gas release of 50 L gas per L reaction/min. (5) In order to further assess this finding ARSST isothermal age at 75 °C for 20 h showed no evidence of this exothermic decomposition, however a slow gas release was noted over the first 4 h which then subsided. GC assay post experiment indicated 98.5% of the material remained. (6) Finally, ARSST heat ramp analysis of a concentrated DCM/DMF solution of PMB-Cl showed a small exotherm at ∼70 °C and significant pressure increase above ∼150 °C. Based on previous work on the stability of PMB-Cl,6 and these findings, the following precautions were implemented for this large-scale demonstration. (i) The final organic stream should have two aqueous potassium carbonate washes to ensure the DCM stream was basic prior to distillation.

SECOND GENERATION ROUTE

It was known from the first generation process chemistry route that the desired target 1 could be prepared from the protected amino acid 2. This amino acid 2 would be generated by hydrolysis and decarboxylation of diester 9 (Scheme 6). The diester in turn is the product of the proposed key step which is the cyclization of the halopyrimidine 10 (X = halogen). Halopyrimidine 10 a product of a selective halogenation which would follow the conjugate addition of a malonate anion to a vinyl pyrimidine such as 11.4 Ultimately the conjugate addition partner protected amino vinyl pyrimidine 11 was anticipated as the product of a Kumada coupling of a suitably protected version of commercially available chloropyrimidine 6, with a vinyl magnesium halide. Proof of concept for this approach was rapidly achieved, and this route was selected as the secondgeneration route to this fragment.

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PREPARATION OF PROTECTED AMINE 7 Bis-protection of amino pyrimidine 6 with 4-methoxybenzyl chloride (PMB-Cl) had already been demonstrated in the first generation process chemistry route. Unexpectedly, no suppliers could be found to supply this material on the scale required within a reasonable time frame. A brief screen of alternative protecting groups ultimately proved fruitless. The starting point for this campaign, therefore, became the large-scale production of PMB-Cl. A review of the literature syntheses of PMB-Cl from 4-methoxybenzyl alcohol (PMB-OH) highlighted numerous plausible reagents for this transformation.5 A brief in-house assessment of chlorination conditions identified thionyl chloride (1.3 equiv) as the most suitable reagent. Triethylamine (1.05 equiv) was found to be the optimal base with dichloromethane (DCM) as the optimal solvent. These conditions, when employed at −10 °C, were found to give a clean profile and near quantitative yield of the desired chloride as determined by GC analysis and comparison with an authentic sample, Scheme 7. D



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vinylpyrimidine 11 from 7 via a Kumada coupling. Following a brief catalyst and solvent screen PdCl2(dppf)·CH2Cl2 adduct at 5 mol % was found to give superior performance to other catalytic systems evaluated.9 A comparison of typical solvents showed toluene at 80 °C gave better conversions when compared with refluxing THF. The vinylmagnesium chloride charge was set at 1.02 equiv to minimize conjugate addition of Grignard reagent to the vinyl pyrimidine product 11. Once a maximum in HPLC assay yield was reached, it was important to cool and quench the reaction to stop decomposition and a reduction in assay yield. Lab-scale experiments typically afforded a 70−75% assay yield after workup. With these conditions in hand the toluene solution of chloropyrimidine 7 (33 kg) was cross coupled with vinylmagnesium chloride in two batches; these were combined for the quench and workup. Assay yield post workup but before final concentration was 68% (22 kg). However, after distillation to 6.5 volumes toluene the assay yield had dropped to 53% (17.2 kg). Lab experiments showed that the product stream was stable at this point, and further stressing the solution at 50 °C showed no further change in the assay. Stirring the product stream with aq HCl, aq NaOH, or methanol caused no further change in product assay after several days. Polymerization of the product was suspected, but the cause remains unknown.10 While this result was disappointing, the 17.2 kg provided by this method was more than enough to satisfy the program needs. Attention was now turned to the malonate conjugate addition to vinylpyrimidine 11. Proof-of-concept for this transformation was rapidly achieved by simple addition of a large excess of diethyl malonate anion, to a solution of 11 in acetonitrile. When DBU was used as base these conditions gave complete conversion of the starting material, affording conjugate addition product 12 as the major product. While other malonate esters also worked in the reaction diethyl malonate was selected for further development and scale up. The conjugate addition was optimized to use 3 equiv diethylmalonate and DBU in a mixture of DMF and toluene. These conditions allowed the solution of 11 to be telescoped directly into the conjugate addition affording 12 in greater than 97% yield, while minimizing dimeric byproduct 12a to less than 3% (Scheme 9). The optimal conditions were transferred to the plant for a large-scale demonstration on 17.2 kg of input. The reaction achieved was found to reach complete conversion to 12 after 18 h at 20 °C. The product was worked up and assayed by HPLC to show the final solution postworkup contained a total of 24.88 kg (100%). The final product solution was

(ii) A sample of the DCM stream was to be concentrated in the lab and subjected to DSC analysis prior to the large scale batch concentration. (iii) A maximum jacket temperature of 20 °C was to be maintained during distillation. (iv) The final product of PMB-Cl was to be held as a solution at 5 °C, until used in subsequent chemistry. With these process safety precautions in place 49.0 kg of PMB−OH was processed without incident, yielding 54.0 kg of PMB-Cl (99%). The reagent was concentrated to an 80 wt % solution in DCM and then diluted with DMF ready for use in the next step. The bis-protection of 6 developed for the first generation route process chemistry route employed sodium tert-butoxide in a mixture of N,N-dimethylacetamide (DMAc) and THF. For the second generation route sodium tert-butoxide in DMF was found to be a convenient substitute. Employing these changes afforded bis 4-methoxybenzyl (PMB) protected pyrimidine 7 in identical yield when using the freshly prepared PMB-Cl. Typically, complete conversion to 7 was achieved within 3 h at 20 °C. Following workup, 7 could be used directly in the next step after a solvent switch into toluene and elution through a pad of silica to remove colored impurities. A single large scale batch starting with 15.8 kg of 6 was processed affording 7 in 73% assay yield as determined by HPLC analysis of the final product stream, Scheme 8. Scheme 8. Bis-PMB Protection of Aminopyrimidine 6

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PREPARATION OF THE CYCLIZATION PRECURSOR 4 Preparation of vinylpyrimidines by cross coupling of the corresponding chloropyrimidine typically involves the use of undesirable tributylethenyltin or expensive vinyl boron reagents and as such is unsuitable for large-scale implementation.7 Another approach reported in the literature involves addition of vinyl lithium to the CN bond of the pyrimidine, the intermediate is then rearomatized with DDQ to afford vinylpyrimidines.8 We elected to evaluate direct cross coupling approaches and a brief screen of alternative vinyl sources identified vinylmagnesium chloride as optimal to prepare Scheme 9. Synthesis of Conjugate Addition Product 12

E



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pyrimidine 14. Some cyclization to 9 was observed with 13, but the conversion was low when compared 14. 13 was therefore discontinued as a potential substrate for copper coupling, and efforts were focused on the iodide 14 (Scheme 12). A series of experiments was conducted to evaluate ligand, solvent, base, and catalyst loading and reaction temperature (Table 1). The initial cyclization conditions evaluated with iodide 14 employed CuI, L-proline (L1), and Cs2CO3 in DMSO at 45 °C, which afforded 6% conversion to 9 before the reaction stalled (Table 1, Entry 1). A mixture of THF and DMAc improved the level conversion to 17% after overnight at 65 °C (Entry 2). When L-proline was substituted for 2-picolinic acid (L2) at 80 °C in 1,4-dioxane 51% conversion was achieved (Entry 3). This result established L2 as a superior ligand for this transformation, and as such all further development efforts used L2. Increasing the catalyst loading and Cs2CO3 charge afforded 100% conversion in 16 h at 80 °C to a 19:1 mixture of desired 9 and the decarboxylated mono ester 9a (Entry 4). Although formation of 9a was not considered an issue as the next step in the sequence was hydrolysis decarboxylation, it was thought that conditions which led to decarboxylation of 9 may also lead to decarboxylation of 14 to the corresponding monoester 14a, rendering it unreactive to cyclization. Switching the solvent to THF at a reduced temperature of 50 °C gave similar results with 99% conversion to 9 in 24 h with no 9a or 14a observed (Entry 5). Reducing the charge of Cs2CO3 to 3 and then 1.5 equiv reduced conversion to 92% and 2% respectively indicating that the charge of Cs2CO3 was a critical parameter (Entries 6 and 7). Next, we decided to further optimize with a starting point of 4 equiv Cs2CO3 (Table 2). First, increasing the reaction temperature to 60 °C was shown to give complete conversion to 9 within 16 h without any generation of the monoester byproduct 9a (Table 2, Entry 1). Early screening had identified a THF−DMAc mixture as a viable solvent combination which presumably increased the base solubility. This combination was re-evaluated and gratifyingly was found to give complete conversion to 9 within 1 h (Entry 2). The robustness of the catalytic system was demonstrated by running the reaction without any stirring; although the reaction was significantly slower taking 26 h to reach complete conversion no reduction in assay yield was observed (Entry 3). With the identified much more active system it was possible to reduce both the CuI and L2 levels to a more acceptable 25 mol % (Entry 4), further reductions in catalyst loading were not investigated. As the process development moved forward the bulk reagents and solvents were evaluated at lab scale. It quickly became apparent that when all of the bulk materials were used poor conversion resulted (Table 3, Entry 2). This was identified to be a consequence of a combination of two factors. First, the amount of water in DMAc differed between the lab (3.7 mg water per mL) and bulk solvent which was anhydrous when samples were measured by Karl Fischer titration. Second, the morphology of the Cs2CO3 differed with the lab material consisting of irregularly shaped pieces with a particle size distribution (psd) between 10 and 130 μm and the bulk material being almost spherical particles with psd between 20 and 40 μm. It was thought that the lab material would have a higher surface area due to its irregularly shaped particles when compared to the bulk material. A combination of lower surface area and base solubility due to less water content may explain the reduced reactivity observed. The solution identified was addition of 1 equiv water to help solublize the Cs2CO3 and

concentrated to a low volume in toluene for use in the next step. Proof of concept of the key cyclization to diester 9 from conjugate addition product 12 was achieved using a bromination/palladium mediated cyclization sequence. Bromination of 12 with N-bromosuccinimide (NBS) in acetonitrile was found to be regioselective for the 5-position of the pyrimidine ring affording 13. As development of the cyclization progressed, it became clear that the iodo analogue 14 was a more useful precursor than the bromide 13. Iodo pyrimidine 14 was readily accessible under similar conditions using Niodosuccinnimide (NIS) with the addition of an acidic modifier such as acetic acid or trifluoroacetic acid (TFA) at 0 °C, the desired product 14 could be obtained in excellent yield. For the large-scale demonstration the low volume toluene solution of 12 was added to a mixture of NIS and TFA in acetonitrile at a temperature below 5 °C. Complete conversion to iodide 14 took less than 2 h, and after aqueous workup and a solvent switch into toluene the stream was diluted with THF ready for the key cyclization step. Final assay of the product solution by HPLC showed the stream contained 28.7 kg of cyclization precursor, iodide 14 in 93% yield, Scheme 10. Scheme 10. Synthesis of Cyclization Precursor 14

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INTRAMOLECULAR CYCLIZATION TO DIESTER 9 A number of methods are described in the literature for the αarylation of malonates. Radical based Mn(OAc)3-mediated cross-dehydrogenative coupling removes the need for an aryl halogenation step. This method has however been shown to be unsuitable for a five-membered ring formation in high yield.11 Another potential approach, displacement of hypervalent iodine species has been shown to be a metal-free alternative for formation of the key bond. This method was considered to be non atom-economical during triage and was thus deemed impractical for large scale implementation.12 Conversely both Pd- and Cu-catalyzed α arylations have been well-studied and were chosen to be the best choice for further investigations of the key cyclization reaction, Scheme 11.13−15 Scheme 11. Proposed Intramolecular Cyclization of 13 or 14

While Pd-catalyzed conversion of both bromide 13 and iodide 14 to desired cyclized product 9 could be demonstrated with a respectable yield of 66% was obtained, competitive proto-dehalogenation to form 12 was observed as the major byproduct. On the other hand, Cu-catalyzed cyclizations were looking increasingly promising, and as such this reaction was chosen for further development, with details provided below. Initial investigation into copper mediated cyclizations was conducted on both the bromo pyrimidine 13 and the iodo F



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Scheme 12. Copper Mediated Cyclization of Iodo Pyrimidine 14

Table 1. Investigating the Effect of CuI, Ligand, and Cs2CO3 Stoichiometry, Solvent Composition, and Reaction Temperature entry a

1 2a 3b 4c 5c 6c 7c a

equiv CuI

ligand (equiv)

0.2 0.2 0.2 0.5 0.5 0.5 0.5

L1 L1 L2 L2 L2 L2 L2

equiv Cs2CO3

solvent (volumes)

temp (°C)

time (h)

% conversion (% assay yield)

4 4 3 4 4 3 1.5

DMSO (5) 5:1 THF-DMAc (12) 1,4-dioxane (10) 1,4-dioxane (10) THF (10) THF (10) THF (10)

45 65 80 80 50 50 50

1 18 40 16 24 24 24

6 17 51 100 (93) 99 92 2

(0.4) (0.4) (0.4) (1.0) (1.0) (1.0) (1.0)

50 mg scale. b100 mg scale. c1 g scale.

Table 2. Screening the Solvent System and Catalyst Loading

a

entry

equiv CuI

equiv L2

equiv Cs2CO3

solvent (volumes)

temp (°C)

time (h)

% conversion (% assay yield)

1a 2a 3b 4c

0.5 0.5 0.5 0.25

1 1 1 0.25

4 4 4 4

THF (10) 1:1 THF-DMAc (14) 1:1 THF-DMAc (14) 1:1 THF-DMAc (14)

60 60 60 60

16 1 26 3.5

100 (99) 100 100 (100) 100 (100)

570 mg scale. b824 mg scale, unstirred reaction. c15 g scale overhead.

reactivity (Entries 6 and 7) compared with the 1:1 solvent mixture (Entry 4). Finally the bulk stream of iodopyrimidine 14 contained 3 equiv of toluene which was shown not to be an issue (Entry 8). With the reaction performing robustly and in excellent yield, we elected to implement the copper-mediated cyclization for our large-scale demonstration. Thus, the toluene/THF solution containing 28.7 kg of 14 from the previous step was added to a mixture of CuI, L2, and Cs2CO3 in THF and DMAc and heated to 60 °C for 1 h. Following filtration of the inorganics and concentration to remove THF a DMAc solution of 9 resulted which was assayed by HPLC at 23.0 kg (quantitative) suitable for use in the next step (Scheme 13).

Table 3. Investigating the Effect of Water Content and Cs2CO3 Morphologya entry 1b 2 3 4 5 6 7 8c,d

solvent system 1:1 THFDMAc 1:1 THFDMAc 1:1 THFDMAc 1:1 THFDMAc 1:1 THFDMAc THF DMAc 1:1 THFDMAc

Cs2CO3 source

equiv water

time (h)

lab

0

3.5

bulk

0

19

72

bulk

0

3

31

bulk

1

3

99

bulk

10

3

54

bulk bulk bulk

1 1 1

3 3 3

2 51 99 (98)

% conversion (% assay yield) 100

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SYNTHESIS OF THE PENULTIMATE AMINE 15

Hydrolysis and decarboxylation of diester 9 to produce the mono acid 2 was straightforward. A DMAc solution of 9 could simply be treated with 2 M NaOH at 40 °C. This afforded an initial 98:2 mixture of the mono-Na salt 2a and bis-Na salt 2b. Neutralisation with conc. HCl converted the remaining 2% 2b

a

Standard conditions: 500 mg scale, CuI (0.25 equiv), L2 (0.25 equiv), Cs2CO3 (4 equiv), 60 °C. Lab Cs2CO3 psd irregular 10−130 μm. Bulk Cs2CO3 psd almost spherical 20−40 μm. Anhydrous solvent. b DMAc contained 3.7 mg/mL water ∼1 equiv. c39 g scale, overhead stirred. dInput stream of 14 contained 3 equiv of toluene.

Scheme 13. Large-Scale Demonstration of Intramolecular Cyclization

refresh the surface from any potential deposition of CsI (Entry 4); this change gave comparable performance with the lab control (Entry 1), although further increasing the water charge to 10 equiv gave reduced conversion (Entry 5). With this solution in place we re-evaluated THF and DMAc in isolation with 1 equiv of water and observed significantly reduced G



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Scheme 14. Hydrolysis and Decarboxylation of Diester 9

Scheme 15. PMB Deprotection

amine 2 (Scheme 15). In subsequent deliveries the 15·2TFA has been successfully employed without the need to perform the salt break.

to 2 affording the desired monoacid in a quantitative overall yield (Scheme 14). The sodium salt of 2 could be extracted into aqueous media; this enabled rejection of some of the organic soluble impurities carried from earlier steps by a simple wash sequence. After acidification and re-extraction into IPAc, 2 was isolated by crystallization from IPAc-hexane in 91% yield. This process was employed to prepare 17.0 kg of monoacid 2. The Monoacid 2 is the first isolated solid in a six-step sequence, and as such it was gratifying that the crystallization afforded highquality acid 2 directly. In the first generation route the deprotection of the PMB groups was achieved using a mixture of TFA and methanesulfonic acid (MSA) or trifluoromethanesulfonic acid (TfOH). Conversion to amino pyrimidine 15 was found to be complete within 1 h at 20 °C when either 2 equiv of MSA or TfOH were used in 4 volumes of TFA. The challenge though was the isolation of 15 from the acidic mixtures. The solution identified was a temporary protection of the carboxylic acid as the corresponding methyl ester to allow extraction followed by hydrolysis back to 15 after the PMB deprotection. For the second generation route a more direct solution was identified. Deprotection was found to occur in TFA (4 volumes) at 50 °C without another stoichiometric acid, and the bis TFA salt of 15 was crystallized directly by concentration and addition of toluene. Due to the exothermic nature of the process reaction calorimetry was carried out to determine safe working parameters. It was shown that there was a 19 K adiabatic temperature rise on dissolution and that caution should be taken on heating to 50 °C, due to exothermic behavior at this temperature (∼90% of heat output was given over 2 h at 50 °C). On 17 kg scale dissolution of the starting material 2 into cold TFA at 0 °C increased control over the process and led to a temperature increase to 14 °C. Subsequent warming to 50 °C over 1 h (Tmax = 51.3 °C) and aging for 16 h at 50 °C gave complete conversion to 15. Following a solvent switch into toluene the bis TFA salt of 15 (15·2TFA) was isolated by filtration in 98% yield. While 15·2TFA was found to perform well in the conversion to tetrazole 1, concerns about employing this material on scale without adequate gas phase analysis for hydrazoic acid led us to pursue a salt break to generate the free base. 15·2TFA was slurried in 2-propanol and treated with 3 equiv of 10 M NaOH. Upon dissolution 5 M HCl was used to take the material back to pH 4.5 and free base 15 was obtained by filtration. In the large scale demonstration a corrected 6.37 kg of amine 15 was isolated in 88% yield from PMB protected

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FORMATION OF TETRAZOLE 1 The main safety concerns associated with the formation of tetrazole 1 derived from the use of trimethylsilylazide (TMSazide) which can lead to the formation and subsequent condensation of shock-sensitive hydrazoic acid (HN3) in the vessel headspace. The fact that 15 contains a carboxylic acid moiety provided an additional cause of concern. In the first generation route tetrazole 1 was formed by addition of TMSazide (1.8 equiv) over 2 h to a solution of 15, triethylorthoformate (1.8 equiv) and trimethylsilyl trifluoroacetate (TMS-TFA, 1.8 equiv) in THF at 20 °C. This mode of operation had been shown to achieve a maximum headspace hydrazoic acid concentration of 0.5% (v/v) with a headspace nitrogen sweep at 1 L per minute per L reaction volume which is well below the lower decomposition limit for hydrazoic acid of 10% (v/v).16 This approach was deemed suitable for scale up after further development and safety evaluation. Reducing the TMS-azide charge from 1.8 to 1.2 equiv was possible while maintaining the assay yield at 92%, thus significantly reducing the inventory of excess TMS-azide (Scheme 16). Scheme 16. Formation of Tetrazole 1

A mass balance assessment was carried out by assaying for the tetrazole 1 product and using ion chromatography to analyze for residual azide. This analysis demonstrated that all the charges of azide could be accounted for. The second concern was the stability of the final tetrazole 1. DSC analysis of dried 1 indicated a very large exotherm (1040 J/g) initiating from a melt at 135 °C. An isothermal age of the solid at 70 °C for 22 h did not produce any change to the exotherm size or profile once ramping resumed, and although this indicates the material should be stable at temperatures above 50 °C, with the potential size of the exotherm in mind we elected to limit H



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Scheme 17. Overall Second Generation Synthetic Scheme

operating and drying temperatures to a maximum of 25 °C. Furthermore, due to the size of the exotherm, dropweight testing on 1 was conducted by the BAM fallhammer method at 30 J on 20 mg samples and was found to be negative. A positive result at 2 J would have indicated the material was potentially too unstable to safely handle. With the safety evaluation complete, the optimized conditions were performed on 3.6 kg scale. After an initial overnight age 6% starting amine 15 still remained after 20 h at 25 °C, so an additional 0.1 equiv charge of TMS-azide was made and after a further 3 h age only 1.3% of starting material remained with the assay calculated as 89% yield for the product 1. A 130 wt % CUNO R53SP carbon treatment was required to reduce the main impurities to less than 1 HPLC area % each. Crystallization of 1 was effected by solvent switching into ethyl acetate and cooling to 0 °C. When demonstrated this protocol afforded 3.12 kg of 1 in 71% isolated yield and excellent purity (99 wt %, 99.3 area %).

reaction, unless noted otherwise. Reaction monitoring and product purity was analyzed by reverse phase HPLC on an Agilent 1100 series instrument or by GC on an HP 6890 series instrument. HPLC assay yields were determined by comparison with purified samples obtained by chromatography or recrystallization. Isolated yields refer to yields corrected for wt % purity on the basis of HPLC/GC assays against purified standards of the starting material and product. NMR spectra were recorded on a Bruker Avance DPX, DRX 400, or Biospin (1H NMR at 400/500 MHz, 13C NMR at 100/126 MHz) spectrometer. 1H NMR data are reported as follows: chemical shifts are reported in ppm with the solvent resonance resulting from incomplete deuteration as the internal standard (CDCl3: 7.26, d6-DMSO: 2.50), multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, br = broad, m = multiplet, or combinations thereof), integration, and coupling constants. 13C NMR data are reported as follows: chemical shifts are reported in ppm with the solvent resonance as the internal standard (CDCl3: 77.1, d6-DMSO: 39.52). High-resolution mass spectrometry was performed on a Micromass LCT. Azide ion measurements were made by ion chromatography on a Dionex ICS-3000 instrument. Analytical data was obtained on representative samples or materials purified by chromatography or crystallization. Second Generation Route. 4-Methoxybenzyl Chloride (PMB-Cl). To a solution of 98% 4-methoxybenzyl alcohol (24.5 kg, 174 mol) and triethylamine (18.46 kg, 182 mol, 1.05 equiv) in DCM (178.5 kg) cooled to −10 °C was added thionyl chloride (27.0 kg, 226 mol, 1.3 equiv) over 70 min, maintaining the internal temperature below 5 °C. The mixture was aged at 0 °C for 3 h until GC analysis showed >97% conversion. A solution of 1 M hydrochloric acid (245 L, 245 mol, 1.41 equiv) was charged to the mixture, maintaining an internal temperature below 10 °C. The layers were separated, and the DCM layer was washed twice with 1 M aqueous potassium carbonate (245 L). The second aqueous wash had a pH > 9. The batch was repeated, and the combined organic layers were assayed by GC for PMB-Cl (54.0 kg, 345 mol, 99% yield). The combined DCM organics were concentrated under reduced pressure to a 79.6 wt % solution in residual DCM with jacket temperature at

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SUMMARY In summary a practical, scalable, and high-yielding second generation synthesis of target tetrazole 1 has been developed, starting from commercially available raw materials. Key safety issues in the preparation of 4-methoxybenzyl chloride, usage of TMS-azide, and stability of the target tetrazole have been addressed. Tetrazole 1 was prepared in 21% overall yield in eight chemical transformations (Scheme 17). With only three isolated solid intermediates en route to 1, most of the sequence was telescoped which significantly reduced cycle times without compromising final product quality. This work also extended the scope of copper-catalyzed α-arylation of malonates to an intramolecular example.

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EXPERIMENTAL SECTION General. Starting materials were obtained from commercial suppliers and were used without further purification. All reactions were performed under an atmosphere of nitrogen unless noted otherwise. All reactors were glass-lined steel vessels. Temperatures refer to the internal temperature of the I



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20 °C and then diluted with DMF (71 kg) to give 138.8 kg of PMB-Cl solution (38.9 wt %) and stored at 5 °C for use in the next step. Analytical data matched that of a commercially obtained sample. 4-Chloro-N,N-bis(4-methoxybenzyl)pyrimidin-2-amine (7). 4-Chloropyrimidin-2-amine (6, 15.84 kg, 122 mol) was added to a 38.9 wt % DCM/DMF solution of PMB-Cl (129.6 kg, 50.4 kg by assay, 257 mol, 2.1 equiv) prepared in the previous step, and the resulting mixture was cooled to −10 °C. A solution of NaOtBu (28.8 kg, 293 mol, 2.4 equiv) in DMF (78 kg) was cooled to 0 °C and then added to the mixture of 6 and PMB-Cl mixture over 45 min maintaining the temperature below 10 °C. After 80 min at 0 °C, a further charge of 38.9% PMB-Cl (5.1 kg, 2.0 kg assay, 10.2 mol, 0.08 equiv) and NaOtBu (1.5 kg, 15.6 mol, 0.128 equiv) was made to drive complete conversion. After a further 2 h age methyl tert-butylether (MTBE, 105.8 kg, 143 L) was added followed by water (143 kg) maintaining the temperature below 20 °C. The temperature was then adjusted to 20 °C; the lower aqueous layer was separated, and the organic layer was washed twice with 10% aq. LiCl (15.8 kg of LiCl in 142.2 kg of water). The organic layer was concentrated to a residue under reduced pressure maintaining temperature below 40 °C. Toluene (45 kg) was charged, and the batch was again concentrated to a residue before being diluted with toluene (100 kg) and passed through a pad of silica gel (40 kg). The silica pad was eluted with toluene (173.2 kg) and the combined organics concentrated under reduced pressure to a 186.2 kg (207 L) solution. HPLC assay indicated 33.0 kg of 7 (89.2 mol, 73%) was present in the toluene solution. 1H NMR (400, MHz, CDCl3): δ 8.21 (d, J = 5.1 Hz, 1H), 7.20 (d, J = 8.2 Hz, 4H), 6.87 (d, J = 8.6 Hz, 4H), 6.57 (d, J = 5.1 Hz, 1H), 4.76 (s, 4H), 3.82 (s, 6H). 13C NMR (100 MHz, CDCl3): δ 162.2, 161.2, 159.0, 158.9, 129.7, 129.2, 113.9, 109.2, 55.3, 48.2. HRMS (ES): [M + H]+ calcd for C20H21ClN3O2: 370.1322; found 370.1308. N,N-Bis(4-methoxybenzyl)-4-vinylpyrimidin-2-amine (11). To a 17.7 wt % solution of 7 in toluene (93.1 kg, 16.5 kg assay for 7, 44.6 mol) was charged PdCl2(dppf)·CH2Cl2 (1.82 kg, 2.23 mol, 0.05 equiv). The resulting mixture was thoroughly degassed by three vacuum/nitrogen cycles followed by subsurface sparging for 10 min with nitrogen before heating to 80 °C. A 1.9 M solution of vinylmagnesium chloride in THF (26.1 kg, 23.9L, 45.5 mol, 1.02 equiv) was then added over 105 min maintaining the temperature between 80 and 85 °C. After a further 20 min HPLC indicated 88% conversion, and the batch was cooled to 20 °C and assayed at 66% yield. The batch was repeated and assayed at 74% yield. The reaction mixtures were combined and quenched into a solution of 5% acetic acid (200 L) over 1 h maintaining the temperature 99% conversion, and J



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20 °C. The bis-TFA salt of 15 was collected by filtration and washed with toluene (17.3 kg, 20 L) and dried at 50 °C under vacuum to afford 15.937 kg of the bis TFA salt of 15. The bis TFA salt of 15 was then slurried in 2-propanol (47.5 kg, 60.5 L) and treated with 10 M sodium hydroxide (16 L, 160 mol, 4.0 equiv), the resulting mixture was aged at rt for 30 min before 5 M hydrochloric acid (20.8 L, 104 mol, 2.6 equiv) was added carefully maintaining internal temperature below 25 °C to obtain a final pH of 4.5 and crystallize 15. Water (9.0 kg, 9L) was added to adjust the 2-propanol−water ratio to 3:2 and the slurry was cooled to 5 °C and aged for 16 h. The solid was collected by filtration, washed with water (30 kg, 30L) and dried at 50 °C under vacuum with nitrogen sweep to afford 15 in a corrected 88% yield (6.365 kg corrected for 93.6 wt %, 35.5 mol). 1H NMR (400 MHz, d6-DMSO): δ 13.5−11.4, (br, 1H), 8.12 (s, 1H), 6.60 (s, 2H), 3.88 (t, J = 7.4 Hz, 1H), 2.81−2.63 (m, 2H), 2.29−2.10 (m, 2H). 13C NMR (100 MHz, d6DMSO): δ 175.2, 174.8, 163.2, 153.4, 120.5, 45.0, 32.4, 25.9. HRMS (ES): [M + H]+ calcd for C8H10N3O2: 180.0773; found 180.0780. 2-(1H-Tetrazol-1-yl)-6,7-dihydro-5H-cyclopenta[d]pyrimidine-5-carboxylic Acid (1). To a mixture of 93.6 wt % 15 (3.6 kg, 3.37 kg assay, 18.8 mol) in tetrahydrofuran (THF, 32 kg, 36 L) at 20 °C was added triethylorthoformate (5.02 kg, 33.9 mol, 1.8 equiv) followed by trimethylsilyl trifluoroacetate (6.31 kg, 33.9 mol, 1.8 equiv) over 7 min. The mixture was warmed to 25 °C, and a vessel headpace nitrogen sweep of ∼50L/min was initiated, with the vessel exhaust directed to a scrubber containing aq. NaOH. Trimethylsilylazide (2.60 kg, 22.6 mol, 1.2 equiv) was charged over 2 h maintaining the temperature at 25 °C and the mixture aged at 25 °C for 20 h giving 96% conversion. A further charge of trimethylsilylazide (217 g, 1.88 mol, 0.1 equiv) was made, and after a further 2 h at 25 °C conversion had reached 98.3%. The solution was then passed through a CUNO carbon filtration system containing 4.7 kg of CUNO R53SP grade carbon; the carbon was washed with THF (262 kg, 294 L) and the filtrates combined and assayed for 3.64 kg (15.7 mol, 83%) The organics were then carefully concentrated under reduced pressure to ∼15 L with jacket temperature at 25 °C, and then ethyl acetate (42 kg, 47 L) was added and the batch concentrated to 15 L to complete the solvent switch and crystallize 1. The slurry was cooled to 0 °C, aged for 3 h, and filtered. The cake was washed with cold ethyl acetate (9 kg, 10 L) and then dried at 20 °C under vacuum with a nitrogen sweep to afford 3.12 kg (98.9 wt %, 13.3 mol, 71%) of 1. (Analysis of the distillates and mother liquors determined the levels of azide ion to be

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