Process Development for a Locally Acting SGLT1 ... - ACS Publications

Article Cite This: Org. Process Res. Dev. XXXX, XXX, XXX−XXX

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Process Development for a Locally Acting SGLT1 Inhibitor, LX2761, Utilizing sp3−sp2 Suzuki Coupling of a Benzyl Carbonate Lauren E. Sirois,† Matthew M. Zhao, Ngiap-Kie Lim,† Mark S. Bednarz,‡ Bryce A. Harrison,§ and Wenxue Wu* Chemical Development, Lexicon Pharmaceuticals, Inc., 110 Allen Road, Basking Ridge, New Jersey 07920, United States

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ABSTRACT: Process development for the synthesis of a locally acting sodium-dependent glucose cotransporter 1 (SGLT1) inhibitor, LX2761, a drug candidate for the treatment of diabetes mellitus, is described. A convergent route based on a retrosynthetic disconnection of the central diarylmethane linkage to a benzyl carbonate and a hindered arylboronic ester was designed and developed. The syntheses of these fully elaborated, late-stage crystalline intermediates are discussed, along with optimization of their union via an sp3−sp2 Suzuki−Miyaura coupling reaction. The final active pharmaceutical ingredient was isolated in high purity as its L-proline cocrystal. This process was successfully performed on a kilogram scale to support toxicology and preclinical studies. KEYWORDS: dynamic kinetic resolution, anchimeric assistance, alkylzinc, sp3−sp2 Suzuki, cocrystal

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INTRODUCTION Recently, inhibitors of sodium-dependent glucose cotransporters have attracted considerable attention for the treatment of diabetes mellitus.1 Several sodium-dependent glucose cotransporter 2 (SGLT2)-selective inhibitors, which lower blood glucose levels by inhibiting reabsorption of glucose in the kidneys, have advanced in the clinic and/or received regulatory approval.2,3 Preclinical and clinical studies of our advanced drug candidate LX4211 (sotagliflozin) (Figure 1), a dual

causing excess urinary glucose excretion, which is thought to be responsible for the increases in urinary tract infections seen with SGLT2 inhibitors.5b Following the promising results with LX2761 in animal models, kilogram quantities were required to support additional preclinical and clinical studies. LX2761 shares the stereochemically rich thioxyloside motif with LX4211, and the medicinal chemistry synthetic route was based on a similar approach. Key intermediate 5 was synthesized in a straightforward manner from the same morpholinoamide 1 (Scheme 1).4,5 The subsequent synthetic steps proved to be more challenging, however. The Heck coupling approach required expensive methyl 3-butenoate (6) as the coupling partner, microwave heating, purging of the undesired branched isomer 7a, and olefin hydrogenation. Global saponification followed by amidation with amine 8 using coupling reagent HATU gave LX2761. The high affinity of LX2761 for aqueous media and the challenges of removing coupling-reagent-related byproducts necessitated preparative HPLC purification to obtain LX2761 with acceptable purity. As the project advanced into development phases, a more convergent and practical synthesis was required. Our revised retrosynthetic analysis dissected the target at the central diarylmethane linkage into two fragments of nearly equal complexity (Scheme 2). While numerous methods for constructing diarylmethane motifs have been reported, including several used for the syntheses of various SGLT2 inhibitors,2c the reaction conditions for many of them (e.g., Friedel−Crafts acylation or 1,2-addition followed by reduction)6 were not expected to be compatible with fully functionalized substrates such as 9 and 10 and would require truncated intermediates or inefficient protecting group manipulations. With the goal of joining two fully functionalized fragments of LX2761, a less

Figure 1. SGLT1/2 dual inhibitor LX4211 and locally acting SGLT1 inhibitor LX2761.

inhibitor of SGLT2 (expressed primarily in the kidneys) and SGLT1 (expressed primarily in the intestine), have shown potential benefits of reducing or delaying glucose absorption in the digestive tract.4 By exploiting this latter strategy, Lexicon’s discovery efforts yielded a potent SGLT1-selective inhibitor, LX2761 (Figure 1), that is locally acting in the gastrointestinal system without appreciable systemic exposure.5 As the pharmacological activity of LX2761 is largely independent of kidney function, it has potential for the treatment of diabetic patients with renal impairment. It also has the advantage of not © XXXX American Chemical Society

Received: October 2, 2018

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DOI: 10.1021/acs.oprd.8b00325 Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development

Article

Scheme 1. Medicinal Chemistry Synthesis of LX2761

based on knowledge gained from the LX4211 process.4,5 As with the medicinal chemistry route, stereochemically pure morpholinoamide 1 was used to construct this thioxyloside fragment of LX2761. 2-Chloro-4-iodotoluene (13) was selected as a suitable raw material allowing for sequential functionalization of the aromatic ring. Treatment of 13 with iPrMgCl gave the aryl Grignard reagent, which upon addition to amide 1, whose hydroxy group had been deprotonated with t-BuMgCl, afforded ketone 14 in 80−90% isolated yield. It was crucial to accurately charge the appropriate amounts of these Grignard reagents to achieve high yields. Specifically, overcharging i-PrMgCl led to its competitive attack on amide 1 to give the isopropyl ketone side product. The synthesis continued with a substrate-controlled stereoselective Luche reduction of ketone 14 in EtOH to afford diol 15 with a 96:4 ratio of diastereomers (Scheme 3). Diol 15 was directly carried forward to the acid-promoted deprotection of the acetonide and concomitant ring expansion to give tetraol 16 as a 1:1 anomeric mixture. The highly polar tetraol 16 was not crystalline and thus telescoped to the acetylation step. The global acetylation features a notable dynamic kinetic resolution of the anomeric mixture of 16 to give the desired tetraacetate anomer 17 with high stereoselectivity (>95:5 in solution, >99:1 isolated) in high isolated yield (72% over three steps from 14). The key for achieving the high stereoselectivity was slow addition of acetic anhydride so that the rate of acetylation was lower than the rate of anomeric equilibration of 16. The R configuration of the anomeric center was confirmed by NMR studies. Treatment of tetraacetate 17 with thiourea and boron trifluoride diethyl etherate followed by MeI and i-PrNEt provided thioether 20 with exceedingly high stereoselectivity (>99:1).8 The high level of stereochemical control was attributed to nearly exclusive nucleophilic attack of thiourea from the bottom face of the pyranose moiety via the postulated oxocarbenium ion 18 formed by anchimeric assistance from the neighboring acetate group. The resulting thiourea adduct

Scheme 2. Retrosynthetic Analysis for the Process Route to LX2761

conventional approach featuring a late-stage sp3−sp2 Suzuki coupling of benzylic substrate 9 with arylboronate 10 was investigated (Scheme 2). Admittedly, the proposed Suzuki coupling pushed the limits of contemporaneous literature precedents.7 However, as discussed below, intensive experimentation ultimately yielded a scalable process route to LX2761.

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RESULTS AND DISCUSSION Construction of the Thioxyloside and Elaboration to the Arylboronic Ester. The synthesis of the “right-hand” fragment of LX2761, pinacol boronic ester 10 (Scheme 3), was B



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Scheme 3. Preparation of Fully Elaborated Arylboronate 10

Table 1. Borylation of o-Methylaryl Chloride 20a

19 was then converted to the free thiol and methylated with MeI in situ to give 20. The latter was readily crystallized from i-PrOH/water in 93% yield with >99% purity. The last step in the synthesis of key intermediate 10 entailed borylation of o-methylaryl chloride 20. While borylation of hindered aryl chlorides9 has continued to present a challenge in organic synthesis, Buchwald and co-workers made significant advances by using palladium catalysts with biarylphosphine ligands.10 Initial results showed that 2dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (SPhos) was superior to 2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl (XPhos) with regard to both conversion and minimizing the dehalogenation side product 21 (Table 1). While the reaction was faster at 80 °C than 60 °C, significantly more impurity 21 formed. Further refinement identified that a Pd loading of 2 mol % was required to complete the reaction at 60 °C (Figure 2). Implementing these conditions on a kilogram scale afforded arylboronate 10 in 91% isolated yield with ∼99% purity after treatment with activated carbon and crystallization (Scheme 3). Synthesis of the Fully Elaborated Benzyl Alcohol. The first challenge in the synthesis of the “left-hand” fragment of LX2761, compound 9, was the installation of a carbonylfunctionalized aliphatic chain to an aryl ring. Although compound 24a was available via Friedel−Crafts acylation11 of bromobenzene, the subsequent Wolff−Kishner12 (or Zn/ Hg) reduction was undesirable for scale-up because of safety concerns (Scheme 4a). Other conditions (e.g., Et3SiH/TFA)13 afforded mainly the five-membered-ring lactone side product. The required one-carbon homologation of aryl bromide 24a

HPLC yield (normalized area %) entry

liganda

T (°C)

20

10

21

1 2 3 4

SPhos SPhos XPhos XPhos

60 80 60 80

0 0 53 0

97 94 36 56

3 6 11 44

a Conditions: 1.0 equiv of 20, 2.0 equiv of B2pin2, 3.0 equiv of KOAc, 3 mol % Pd(OAc)2, 6 mol % ligand, dioxane (0.2 M), 18 h.

would pose additional chemoselectivity challenges and decrease the efficiency. Similar arguments precluded an analogous toluene-based approach via 24b. Therefore, commercially available 4-bromobenzyl alcohol (26a) was explored14 as a potential starting material (Scheme 4b). Although sequential hydroboration/sp3−sp2 coupling15 appeared to be an attractive approach, it was only briefly explored because of cost considerations and stability/reactivity/ selectivity issues of β,γ-unsaturated substrates 25. Following reports by Knochel and others16 on Negishi17 couplings using highly active Pd−biarylphosphine systems (some in the presence of active protons), the coupling of C



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bromobutyrate. Indeed, laboratory trials with many of these reagents gave poor results, as determined by iodine titration and/or GC (Table 2, entries 1−5).21 Encouragingly, when TMSCl22 (10 mol %) was added to the reaction mixture with t-BuONa/CuI at ∼60 °C, a high conversion was observed (Table 2, entry 6). Subsequent experiments showed that TMSCl alone is sufficient and that tBuONa and CuI were not necessary (Table 2, entries 8 and 10−12). Additional experiments showed that high concentrations of 29 afforded faster reactions, but the reactions became less clean at very high concentrations (>50% v/v). Surprisingly, the alkylzinc reagent 28 prepared in this manner was ineffective in the Negishi coupling reaction (Table 2, entry 8 vs 9), giving only 20% conversion versus 70% with commercial material. The latter was presumably prepared using Rieke zinc. Thus, we postulated that the presence of lithium chloride, a byproduct in the preparation of Rieke zinc, might be responsible for the observed differences.23 Indeed, adding as little as 0.05 equiv of LiCl to our in-house-prepared zinc reagent 28 greatly accelerated the Negishi coupling reaction, affording 98% conversion versus 10% without LiCl at the same time point (Table 2, entry 11 vs 10).24 In the latter case, the color of the reaction mixture also turned black, an indication of catalyst deactivation. The optimal charge of LiCl was 0.5 equiv, allowing the reaction to reach full conversion in ∼2 h (Table 2, entry 12). While the precise role of LiCl is unclear, its complexation with alkylzinc reagent 28 was quite apparent: LiCl did not dissolve in pure THF but dissolved readily in a THF solution of 28. Although TMSCl activation of zinc dust worked well on a small scale with a magnetic stir bar, on larger scales with mechanical stirring both TMSCl (5−10 mol %) and LiCl (∼0.5 equiv) were added to provide more reliable zincation (Table 2, entry 13).25 Notably, protecting the benzyl alcohol group with a trimethylsilyl group allowed us to reduce the amount of alkylzinc 28 from 1.75 to 1.10 equiv and catalyst loading from 2 to 0.5 mol % (Table 2, entries 10−12). While it is generally preferable to avoid protecting groups, the propensity of ester 27b to oligomerize (via transesterification with the benzyl alcohol group) and chemoselectivity challenges in the downstream amidation stage ultimately rendered it necessary to protect the benzyl alcohol. For improved compatibility with the downstream chemistry, TMS was replaced by a

Figure 2. Effect of catalyst loading on the borylation of aryl chloride 20. Conditions: 1.0 equiv of 20, 1.5 equiv of B2pin2, 3.0 equiv of KOAc, Pd(OAc)2:SPhos ratio = 1:2, dioxane (8 volumes), 60 °C.

alcohol 26a with alkylzinc reagent 28 was attempted. With Pd(OAc)2/SPhos, the reaction proceeded reasonably well, providing 27b in ∼70% yield (Scheme 4c). Other common catalysts such as Pd(OAc)2/PPh3, PdCl2(PPh3)2, and PdCl2· dppf gave inferior results. Slow addition of excess alkylzinc reagent 28 (1.75 equiv) was required to compensate for competing protonation of 28. Since commercial 28 was expensive and available in rather low concentration (0.5 M solution), direct in situ preparation of 28 from ethyl 4bromobutyrate was pursued. Several methods for generating highly active zinc or activating zinc dust by breaking the oxide surface layer have been described in the literature.18 The high cost of Rieke zinc and inconvenience of acid/base washing of zinc dusts persuaded us to explore other options. Several additives have also been reported for in situ activation of zinc dusts by “etching”, such as iodine, 1,2-dibromoethane, alkylmagnesium halides, alkylaluminum halides, trimethylsilyl halides, diisobutylaluminum hydride,19 and strong bases paired with metal iodides (e.g., LHMDS/NiI2 or t-BuOK/CuI).20 Unfortunately, these procedures appeared to be effective only for more active substrates such as aryl, allyl, or benzyl halides or activated alkyl halides as opposed to unactivated ones, such as ethyl 4Scheme 4. Approaches for Installing the Aliphatic Side Chain

D



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Table 2. Optimization of Zincation/Negishi Coupling

zincation to give 28 entry

conditionsa

1 2 3

8

DIBAL-H LHMDS, NiI2 Mg(OEt)2, FeCl3 BrCH2CH2Br (DMI solvent) NaOtBu, CuI NaOtBu, CuI, TMSCl NaOtBu, CuI, TMSCl, LiCl TMSCl

9

conv. (%)b

Scheme 5. Negishi Coupling of Alkylzinc to the Aryl Bromide

Negishi coupling to give 27 R

conditions

conv. (%)c

31 32 2

− − −

− − −

− − −

61

−

−

−

19 100

− −

− −

− −

77

−

−

−

88

H

20

commercial 28

−

H

10

TMSCl

88

TMS

11

TMSCl

88

TMS

12

TMSCl

88

TMS

13

TMSCl, LiCl (0.5 equiv)

90

−

Pd(OAc)2 (2 mol %), SPhos (4 mol %) Pd(OAc)2 (2 mol %), SPhos (4 mol %) Pd(OAc)2 (0.5 mol %), SPhos (1.0 mol %) Pd(OAc)2 (0.5 mol %), SPhos (1.0 mol %), LiCl (0.05 equiv) Pd(OAc)2 (0.5 mol %), SPhos (1.0 mol %), LiCl (0.5 equiv) −

4 5 6 7

SPhos:Pd ratio from 2:1 to 4:1. Remarkably, when the SPhos:Pd ratio was increased to 10:1, complete conversion was achievable in 10−15 h at 65 °C even with 0.01 mol % Pd. A palladium catalyst loading of 0.10 mol % was chosen to ensure process robustness. After quenching of excess alkylzinc 28 with ethanol, the reaction mixture was concentrated under reduced pressure. The residue was diluted with toluene, and after aqueous workup, the crude product 27d solution was concentrated and flushed with toluene to remove residual ethyl butyrate side product (bp 121 °C). This avoided the formation of the corresponding butyramide side product in the amidation step, which not only consumed valuable amine 8 but also interfered with the crystallization of the desired product 9a. Although direct amidation of crude ester 27d would be more efficient than hydrolysis of the ester followed by coupling with amine 8, negligible amounts of 9a were observed after aging for 20 h at 70 °C in THF, MeTHF, or ACN or without a solvent. Evidently, the sterically hindered amine 8 was not competent in direct amidation of the ethyl ester. Therefore, crude ester 27d was diluted with methanol and treated with 1.5 equiv of 30% NaOH at 30−40 °C for 1−2 h to give carboxylic acid 11 as an oil, which was carried forward to the amidation step without further purification (Scheme 5). For the synthesis of amine 8, the initial route investigated was a peptide coupling of N,N-dimethylethylenediamine (31) with Boc-protected α-aminoisobutyric acid (AiB-OH) 30 (Scheme 6). Both conventional (DCC, CDI) and processfriendly “green” propylphosphonic anhydride26 coupling rereagents were evaluated,27 ultimately leading to the selection of CDI. The reaction was facile and clean, but purging the imidazole byproduct was problematic. Since washing the organic layer with aqueous acid was not feasible because of the

70 10 98

99 −

a

The reactions were conducted with zinc dust in the presence of various additives from room temperature to 65 °C in THF, unless otherwise noted. bThe conversions for zincation were determined by iodine titration or GC of the aliquots quenched with acetic acid. cThe conversions for Negishi coupling were determined by HPLC.

tetrahydropyranyl (THP) protecting group. This was readily accomplished by treatment of 26a with 1.1 equiv of dihydropyran and 2 mol % pyridinium p-toluenesulfonate (PPTS) in THF at 50−60 °C. The reaction mixture was then used directly in the Negishi coupling without quenching, workup, or isolation (Scheme 5). For the Negishi coupling, the order of addition proved to be quite important. The palladium catalyst was readily deactivated by alkylzinc 28 in the absence of aryl bromide 26c: the reaction mixture turned dark rapidly with severely degraded catalyst activity after 2 h at room temperature. Therefore, a THF solution of alkylzinc 28 was slowly added to a mixture of the catalyst and the aryl bromide. This controlled addition of alkylzinc 28 is also an intrinsically safer process because it moderates the rate of the highly exothermic reaction. The catalyst loading was further reduced to 0.1 mol % by scavenging residual oxygen in the reaction system with a small amount of 28 prior to catalyst charge while increasing

Scheme 6. Initial Synthesis of Amine 8

E



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Scheme 7. Alternative Synthesis of Amine 8

Scheme 8. Preparation of Benzyl Alcohol 9b

and on scale-up, which posed an unacceptable risk for manufacturing. Fortunately, it was discovered that a high ratio of mixed anhydride/symmetrical anhydride was not necessary for high conversion of the amidation. Even with 50% symmetrical anhydride, the amidation still went to completion upon aging at slightly higher temperature (35 °C). Reaction monitoring by HPLC showed that amidation of the symmetrical anhydride was faster than that of the mixed anhydride. Shortly after addition of amine 8 at −25 °C, the levels of carboxylic acid 11 roughly matched the levels expected from the symmetrical anhydride. In most other cases, the amidation would stall at this stage. However, in this case, carboxylic acid 11 was still gradually converted into the desired product 9a. These results can be rationalized as follows. There were interconversions among mixed anhydride 33, symmetric anhydride 34, and pivalic anhydride during the activation of carboxylic acid 11 with pivaloyl chloride (Figure 3). Mixed

dimethylamine group in the product, multiple water washes and back extractions were employed. Removal of the Boc protecting group by treatment with HCl (4 N in dioxane) in acetonitrile afforded the bis(HCl) salt of 8 as a hygroscopic solid. The latter was treated with powdered K2CO3 in acetonitrile to give free-base 8 as an oil after the inorganic salts were filtered off and the filtrate was concentrated. This streamlined process obviated the difficulties of aqueous workup of highly polar and water-soluble amine 8, affording the product in 94% yield on a kilogram scale. A more efficient route was later explored via direct reaction of methyl ester of α-aminoisobutyric acid hydrochloride (AiBOMe·HCl, 32) with 31 (Scheme 7). Although the risks of dimerization/oligomerization of 32 were expected to be low because of the steric bulk next to the amino group, an excess of 31 (∼3 equiv) was used. The reaction was carried out at 100 °C without a solvent for 7 h and afforded the desired product 8 cleanly. After direct treatment of the reaction mixture with solid potassium hydroxide, excess 31 was removed by codistillation with toluene to afford 8 in nearly quantitative yield. This new process obviated the use of dichloromethane and CDI28 as well as the tedious aqueous workup in the initial process. With both carboxylic acid 11 and amine 8 in hand, amidation29 of 11 via its pivalic acid mixed anhydride was evaluated. This approach offered easy purging of the pivalic acid byproduct into the aqueous phase under basic conditions. The sterically hindered α-amino group in 8 was expected to enhance the regioselectivity. However, initial results showed the formation of significant amounts of the undesired symmetrical anhydride of 11. The ratio of the mixed anhydride to symmetrical anhydride varied depending on the reaction conditions such as base, solvent, temperature, stoichiometry, order of addition, addition rate, and aging time. Adding a solution of carboxylic acid 11 in THF/toluene to pivaloyl chloride (1.1 equiv) at low temperature (−25 °C) helped to improve the ratio of the mixed anhydride to symmetrical anhydride (Scheme 8). However, the ratio decreased with time

Figure 3. Postulated mechanism for the amidation reaction.

anhydrides 33 and symmetric anhydride 34 rapidly reacted with amine 8, but the bulky pivalic anhydride did not because of the combined steric hindrance. On the other hand, carboxylate 11 released from amidation of symmetric anhydride 34 was competent in reacting with pivalic anhydride to regenerate mixed anhydride 33, which was then converted to the desired product 9a. The key is the steric hindrance of the amine that prevents it from reacting with pivalic anhydride. After quenching with aqueous NaOH followed by workup, the product (9a) was crystallized from a mixture of MTBE and n-heptane. Among all of the solvents screened, MTBE was F



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uniquely effective in promoting the crystallization. This process was successfully performed on a 3 kg scale batch, furnishing 9a in 90% overall yield from 4-bromobenzyl alcohol 26a in four steps. The removal of the THP protecting group by treatment with 35% aqueous HCl (1.2 equiv) in methanol was facile because of the formation of THP methyl acetal. The resulting hydroxyamide, 9b, was found to be very water-soluble and difficult to extract from aqueous media even after saturating it with NaCl. Therefore, a more practical process was developed to overcome this challenge. The reaction was quenched with 50% w/w NaOH, and the small amount of water was then removed by concentration and flushing with methanol followed by THF. The precipitated sodium chloride was removed by filtration, and the product was isolated in 95% yield with 99% purity by crystallization from a mixture of THF and n-heptane. With both substrates in hand, the next critical step was their union via an sp3−sp2 Suzuki−Miyaura reaction.30 While not as widespread as sp2−sp2 couplings, it has gained in popularity thanks to advances in boron chemistry (e.g., trifluoroborate salts) and ligand technology.31 An increasing number of examples of constructing diarylmethane motifs via Suzuki couplings of benzyl halides have been described in the scientific literature.32 However, reports of benzyloxy substrates,33 such as carbonates, acetates, and phosphates, were more limited at the time of this work. Strategically, we needed to identify a substrate with a competent leaving group that is compatible with the amine− diamide side chain, which precluded halides or similarly active substrates (Scheme 9). Considering the noticeably greater reactivity of benzyl carbonates34 over benzyl acetates, carbonate 9c was selected for investigation.35

Scheme 10. Synthesis of Carbonate 9c via Methyl Chloroformate

Considering the heightened risk in scale-up, a more robust alternative was required. N-Heterocyclic carbene (NHC)-catalyzed transesterification of esters and, in limited examples, carbonates reported by Nolan and co-workers36 prompted us to investigate milder and less hazardous dimethyl carbonate (DMC)37 for the carbonate formation. Surprisingly, with catalytic potassium tert-butoxide as the base, similar conversions were observed with or without an NHC catalyst such as IMes·HCl or ICy·HBF4. Additionally, powdered molecular sieves used to remove generated methanol were replaced by distillation, a much more practical option for production (Figure 4). It was also found that cheaper NaOMe performed similarly to potassium tertbutoxide.

Scheme 9. Suzuki Coupling Strategy for the Diarylmethane Linkage

The most straightforward approach to prepare carbonate 9c was via treatment of benzyl alcohol 9b with methyl chloroformate (Scheme 10). The reaction was surprisingly sluggish, often requiring additional charges of methyl chloroformate and pyridine to push the reaction to completion. More alarmingly, various levels of quaternary ammonium salt 36 were observed. The culprit was either residual methyl chloroformate or its degradation product, methyl chloride. In one experiment, 36 was generated in ∼30% yield, most of which formed during overnight aging of the organic layer. Attempts to curtail this side reaction by adding diethylamine or dipropylamine as sacrificial amines were not entirely effective.

Figure 4. Mechanism for the formation of carbonate 9c.

Mechanistically, the reaction mixture is composed of benzyl alcohol 9b, benzyl carbonate 9c, dibenzyl carbonate 37, methanol, DMC, sodium methoxide, and sodium benzyloxide in a rapid equilibrium (Figure 4). Excess DMC favors desired product 9c, while removal of methanol and DMC drives the reaction toward 37. Thus, after the initial distillation, DMC and THF were added, and the mixture was distilled and diluted with DMC and THF until the desired conversion was achieved. Typically, 5 mol % NaOMe was used, affording a G



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delivering the product in 96% yield with ∼96% purity on a 3 kg scale batch. Residual starting material 9b (∼2%) was readily purged in the downstream Suzuki step, and dibenzyl carbonate 37 (2−3%) reacted to yield one molecule of the desired product 35 and benzyl alcohol 9b. Optimization of the sp3−sp2 Suzuki Coupling. With both fully elaborated coupling partners in hand, optimization of the Suzuki coupling step was performed (see Table 3). Under conditions from the literature,33a,b the initial reaction proceeded smoothly in DMF at 80 °C. While the overall transformation was fairly clean, a major side product (5−10%), imidazolinone 38, formed via cyclodehydration of the diamide side chain (Scheme 11). Further screening of the reaction conditions, including solvent, temperature, palladium precatalyst, ligand, and base, was conducted. Since imidazolinone impurity 38 increased with temperature and aging time, the reaction temperature range was limited to 50−70 °C. The liability of the acetate groups to strong bases (e.g., Cs2CO3, K3PO4) and nucleophiles (e.g., H2O, MeOH, EtOH, n-BuOH) also limited the scope of reaction conditions. Key screening results are summarized in Table 3. As reported in the literature,33 several large-bite-angle diphosphine ligands (e.g., DPPF, DPEphos) were active in this transformation. 1,4-bis(diphenylphosphino)pentane (DPPpentane) and 1,4-bis(diphenylphosphino)butane (DPPbutane) showed the most consistent reactivity. The latter was selected for further optimization because of cost considerations. Precatalyst [Pd(allyl)Cl]2 was the most active and chemoselective palladium source at 50 °C, while other common palladium sources (e.g., Pd(OAc)2) required higher temperatures (>70 °C) to achieve an acceptable conversion. Reactions in aqueous cosolvent mixtures (∼20 vol % water) generally gave poor impurity profiles, mostly due to hydrolysis of the substrates and product. Among the anhydrous systems tested, alcohols gave the most promising results. Although

reaction mixture containing desired product 9c (90−95%) along with starting material 9b and dibenzyl carbonate 37 (2− 5% each). Higher NaOMe charges were deleterious to the reaction, converting product 9c and dibenzyl carbonate 37 to sodium benzyloxide 9b-Na when MeOH and DMC were removed by distillation (Figure 5).

Figure 5. Deleterious effect of overcharging NaOMe.

Considering the propensity of the carbonate moiety to hydrolyze and the high aqueous affinity of 9c, an aqueous workup was deemed impractical. Quenching the reaction with acetic acid was convenient and facile, but the isolated product 9c did not perform well in the Suzuki coupling, which was attributed to residual acetic acid. To overcome this complication, Et3N·HCl was used to quench the reaction, affording isolated carbonate 9c that performed well in the Suzuki coupling reaction. Residual Et3N·HCl and Et3N byproducts were removed by filtration and distillation, respectively. Operationally, after the reaction was quenched with Et3N·HCl, THF was added to dissolve the product. The resulting slurry was filtered, and the filtrate was concentrated. The solvent was exchanged to MTBE to crystallize carbonate 9c. n-Heptane was then added to increase the recovery,

Table 3. Screening of Reaction Conditions for the Suzuki Coupling entry a

solvent

ligand

time (h)

10 (area %)d

35 (area %)d

LX2761 (area %)

2 20 2 20 2 20 2 20 2 20 2 20 2 20 18 18 18 18 18

50 14 67 55 68 14 90 0 2 0 92 30 45 0 2 2 64 10 13

49 84 32 43 31 83 10 98 91 0 7 70 54 95 98 98 36 90c 87c

−

1

DMF

DPPpentane

2a

MeCN

DPPpentane

3a

DME

DPPpentane

4a

toluene

DPPpentane

5a

EtOH

DPPpentane

6a

dioxane

DPPpentane

7a

tert-amyl alcohol

DPPpentane

i-PrOH i-PrOH i-PrOH i-PrOH i-PrOH

DPPbutane DPPpentane DPPhexane DPEphos DPPF

8b 9b 10b 11b 12b

− − − 6 98 − 0 50 g, addition of 1−2 equiv of water helped to achieve satisfactory conversion. Preforming the active catalyst complex by stirring [Pd(allyl)Cl]2 and DPPbutane in toluene ([Pd]:ligand = 1:1) before it was added to the reaction mixture appeared to enhance the robustness of the reaction. The resulting off-white precipitate was palladium−allyl−ligand complex 39 (Figure 6)38 on the basis of preliminary HPLC−MS and 31P NMR data.39 Addition of i-PrOH to the catalyst slurry in toluene gave a homogeneous solution, which facilitated its air-free transfer. Isopropanol could also possibly play a role in nucleophilic activation of the Pd(II)−allyl complex to a bisphosphine-ligated Pd(0) species.40 Ultimately, only 0.5 mol % [Pd(allyl)Cl]2 and 1.1 mol % DPPbutane were required under the optimized reaction conditions. A slight excess (1.1 equiv) of carbonate 9c was employed to compensate for its hydrolysis (up to ∼5%) during the reaction. The liability of the product’s acetate groups to hydrolysis prevented the use of an aqueous workup. Therefore, after completion of the reaction, acetone was added to dissolve the product, and the mixture was treated with activated carbon at 40 °C for 2−4 h to help remove the palladium catalyst. Acetone was removed by vacuum distillation to crystallize product 35 in 85% yield with excellent purity (>99 HPLC area %)41 in our initial scale-up (two batches, 1.5 kg total). With penultimate intermediate 35 available as a stable, crystalline solid, the synthesis of LX2761 concluded with removal of the acetate groups using catalytic sodium methoxide in ethanol. At this critical juncture of the process, the rather challenging physicochemical properties of the final product were addressed. While LX2761 was not very soluble in pure water, high losses to the aqueous layers were observed during extraction. Only dichloromethane gave an acceptable recovery for the workup/extraction. Additionally, it was challenging to reproducibly obtain crystalline LX2761 directly from the process stream while providing acceptable impurity purging. These issues prompted a search for a more suitable form of LX2761 such as a salt or cocrystal for isolation and formulation.

Figure 6. Palladium−allyl−DPPbutane complex: (a) reaction conditions; (b) HPLC (220 nm) after 30 min of stirring at room temperature (toluene; MeCN diluent); (c) 31P NMR spectrum overlay (CD2Cl2 diluent) of DPPbutane, DPPbutane monoxide, DPPbutane dioxide, and 30 min catalyst sample.

A screening of approximately 40 pharmaceutically acceptable acids yielded several leads; however, follow-up experiments mostly gave tacky solids that were prone to deliquescence, discouraging further exploration. Screening to discover a cocrystal,42 with a particular focus on amino acids and other common excipients/coformers with hydrogen-bonding capability, was then conducted. Gratifyingly, a well-defined LX2761·L-proline 43 cocrystal (1:1) was identified that exhibited acceptable physicochemical properties. I



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Acid-catalyzed deprotection of the acetonide and concomitant ring expansion afforded six-membered-ring tetraol 16 as a 1:1 anomeric mixture. Dynamic kinetic resolution of 16 by stereoselective acetylation and anomeric equilibration of the tetraol afforded tetraacetate 17 as a single stereoisomer in high yield. Boron trifluoride-promoted thiolation gave triacetate 20 with >99:1 stereoselectivity attributed to neighboring group assistance. Palladium-catalyzed borylation of the hindered aryl chloride 20 furnished arylboronic ester 10. For the synthesis of benzyl carbonate 9c, a highly efficient Negishi coupling reaction of aryl bromide 26c with in situprepared alkylzinc 28 was employed to install the aliphatic side chain. After saponification, carboxylic acid 11 was coupled with amine 8 via its pivalic mixed anhydride. After removal of the THP protecting group, benzyl alcohol 9b was converted to benzyl carbonate 9c using nontoxic dimethyl carbonate. Joining benzyl carbonate 9c and arylboronic ester 10 via the key sp3−sp2 Suzuki coupling afforded the final intermediate 35. Global deprotection followed by cocrystallization with Lproline delivered high-quality LX2761 active pharmaceutical ingredient (API). This synthetic process was successfully performed on a kilogram scale to meet the initial demands of drug development.

Thus, penultimate intermediate 35 was treated with catalytic sodium methoxide in EtOH at 40 °C to give LX2761 as the free base (Scheme 12). Separately, a solution of L-proline in Scheme 12. Conversion to LX2761 L-Proline Cocrystal

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EtOH/water was prepared and charged in portions to the above reaction mixture. Upon addition of MTBE as an antisolvent and cooling, the LX2761·L-proline cocrystal crystallized smoothly. This process was performed successfully on a kilogram scale in 91% yield with >99% purity (HPLC area %).

EXPERIMENTAL SECTION General. All reagents and materials were used as received. Unless additional GC or GC−MS analysis was necessary, reactions were monitored by reverse-phase HPLC, generally using a C18 or phenyl-hexyl column with water/MeCN or water/MeOH as the mobile phase and trifluoroacetic acid (TFA) as a modifier. Melting point information was collected using a differential scanning calorimeter (peak temperature). NMR spectra were acquired in deuterated solvents. Mass spectrometry data were obtained during LC−MS analysis. Compound purity was assessed by reverse-phase HPLC and/ or 1H NMR analysis. (3-Chloro-4-methylphenyl)((3αS,5R,6S,6αS)-6-hydroxy-2,2-dimethyltetrahydrofuro[2,3-d][1,3]dioxol-5yl)methanone (14). tert-Butylmagnesium chloride (∼1 M in THF, 5.45 L, 1.15 equiv vs 1) was added to a solution of

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CONCLUSIONS A convergent process route to SGLT1 inhibitor LX2761 was developed (Scheme 13). It features a late stage sp3−sp2 Suzuki coupling of benzyl carbonate 9c with sterically hindered arylboronic ester 10. The optimization of this key reaction and the challenges overcome in assembling the two fully functionalized Suzuki coupling partners have been discussed. The synthesis of stereochemically rich arylboronic ester 10 entailed aryl Grignard addition to L-xylose-derived, enantiomerically pure morpholinoamide 1 to give ketone 14. Luche reduction of 14 gave diol 15 with 96:4 diastereoselectivity. Scheme 13. Summary of the Process Route to LX2761

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with water (18 L) at ≤30 °C. The resulting suspension was cooled to 10−15 °C, aged, and filtered. The filter cake was washed sequentially with 2-propanol (3 × 4 L) and n-heptane (4 L) and dried under reduced pressure at 40−45 °C to give 2.11 kg (72% yield over three steps from 14, corrected for 97.1% assay) of tetraacetate 17 as an off-white solid with ∼100% purity (HPLC area %). mp: 181 °C (DSC peak temperature). LC−MS: calcd [M + NH4]+ 460, found m/z 460. 1H NMR (700 MHz, DMSO-d6): δ 1.80 (s, 3H), 1.96 (s, 3H), 2.04 (s, 3H), 2.08 (s, 3H), 2.32 (s, 3H), 4.92 (d, J = 9.7 Hz, 1H), 5.16 (t, J = 9.6 Hz, 1H), 5.21 (dd, J = 9.7, 8.4 Hz, 1H), 5.49 (t, J = 9.6 Hz, 1H), 6.03 (d, J = 8.4 Hz, 1H), 7.23 (dd, J = 7.8, 1.3 Hz, 1H), 7.33 (d, J = 7.8 Hz, 1H), 7.45 (s, 1H). 13C NMR (101 MHz, DMSO-d6): δ 19.4, 20.1, 20.26, 20.31, 20.5, 70.0, 71.82, 71.85, 74.4, 91.1, 126.2, 127.5, 131.0, 133.1, 135.7, 135.9, 168.5, 168.8, 169.1, 169.5. (2S,3S,4R,5S,6R)-2-(3-Chloro-4-methylphenyl)-6(methylthio)tetrahydro-2H-pyran-3,4,5-triyl Triacetate (20). Boron trifluoride diethyl etherate (1.46 L, 2.50 equiv) was added to a mixture of tetraacetate 17 (2.09 kg, 4.71 mol) and thiourea (394 g, 5.18 mol, 1.1 equiv) in ethyl acetate (14 L), and the mixture was aged at 55 °C for 4 h to give thiourea adduct 19. Methanol (4 L) and methyl iodide (358 mL, 1.22 equiv) were added, and then N,N-diisopropylethylamine (3.59 kg, 5.9 equiv) was slowly added below 15 °C. The reaction mixture was aged at 20−25 °C until reaction completion, concentrated to 8−9 L under reduced pressure, and flushed with isopropyl alcohol (IPA) in portions (8−9 L) to a final volume of 14.5 L. Water (11 L) was added, and the suspension was aged at 35 °C for 0.5 h and then at 10 °C for 4 h. The product was collected by filtration and washed sequentially with IPA/water (2:1, 6 L), IPA (3 L), and n-heptane (4 L). Drying under reduced pressure at 40−45 °C gave 1.88 kg (92.7% yield) of aryl chloride 20 as an off-white solid with >99% purity (HPLC area %). mp: 170 °C (DSC peak temperature). LC−MS: calcd [M + NH4]+ 448, found m/z 448. 1H NMR (700 MHz, DMSO-d6): δ 1.79 (s, 3H), 1.96 (s, 3H), 2.05 (s, 3H), 2.13 (s, 3H), 2.32 (s, 3H), 4.73 (d, J = 9.7 Hz, 1H), 4.89 (d, J = 9.9 Hz, 1H), 5.14 (t, J = 9.6 Hz, 1H), 5.18 (t, J = 9.7 Hz, 1H), 5.37 (t, J = 9.4 Hz, 1H), 7.24 (dd, J = 7.8, 1.5 Hz, 1H), 7.33 (d, J = 8.0 Hz, 1H), 7.46 (s, 1H). 13C NMR (101 MHz, DMSO-d6) δ 10.5, 19.4, 20.1, 20.3, 20.4, 68.8, 72.1, 73.1, 77.4, 81.3, 126.1, 127.5, 131.0, 133.1, 135.7, 136.5, 168.5, 169.1, 169.5. (2S,3S,4R,5S,6R)-2-(4-Methyl-3-(4,4,5,5-tetramethyl1,3,2-dioxaborolan-2-yl)phenyl)-6-(methylthio)tetrahydro-2H-pyran-3,4,5-triyl Triacetate (10). A mixture of aryl chloride 20 (2.00 kg, 4.64 mol, 1.0 equiv), bis(pinacolato)diboron (1.76 kg, 6.95 mol, 1.5 equiv), potassium acetate (1.37 kg, 14.0 mol, 3.0 equiv), and dioxane (16 L) was degassed via four vacuum/nitrogen fill cycles. SPhos (76.2 g, 0.186 mol, 4.0 mol %) and palladium acetate (20.44 g, 0.091 mol, 2.0 mol %) were added, and the mixture was degassed (two cycles) and then aged at 60 °C until reaction completion (28 h). The reaction mixture was cooled to 20−25 °C, diluted with isopropyl acetate (IPAc) (10 L), stirred for 15 min, and then filtered through a pad of silica gel (4 kg). The silica gel pad was washed with IPAc, and the combined filtrate was concentrated under reduced pressure to ∼2.4 L. The distillation residue was diluted to 20 L with IPAc and treated with Darco G-60 (100 g) for 3 h at 50 °C. The mixture was filtered through a pad of diatomite, and the filter cake was washed with IPAc (2.25 L). The combined filtrate

morpholinoamide 1 (1.25 kg, 4.56 mol assay-corrected) in anhydrous THF (4.5 L) at −15 to −3 °C, and the resulting solution was held at −15 °C. In a separate reactor, isopropylmagnesium chloride in THF (2.65 L, 2.11 M, 5.47 mol, 1.09 equiv vs 13) was added to a solution of aryl iodide 13 (1.29 kg, 5.01 mol, 1.10 equiv vs 1) in anhydrous THF (3.5 L) at −16 to −8 °C, and the mixture was stirred until transmetalation was complete (IPC by GC after 1−2 h of aging). The aryl Grignard solution was then added to the solution of deprotonated morpholinoamide 1 at or below −3 °C. The reaction mixture was maintained at −10 °C until the reaction was complete and then quenched into a cold (∼5 °C) solution of citric acid (1.08 kg) in water (6.75 L) at or below 13 °C. The resulting biphasic mixture was agitated for 15 min at 15−20 °C. The upper organic layer was separated and washed twice with 25% brine (2 × 2.5 L). The washed organic phase was concentrated to ∼4 L, and the resulting suspension was warmed to 50−55 °C. n-Heptane (6 L) was added, and the mixture was cooled slowly to 0 °C, aged for 1 h, and filtered. The filter cake was washed with an n-heptane/THF mixture (4:1, 2.5 L) followed by n-heptane (2 L). The wet cake was dried under reduced pressure at 40−45 °C with a gentle nitrogen sweep to afford 1.30 kg (91% yield) of ketone 14 with >99% purity (HPLC area %). mp: 153 °C (DSC peak temperature). LC−MS: calcd [M + H]+ 313, found m/z 313. 1 H NMR (400 MHz, DMSO-d6): δ 1.28 (s, 3H), 1.48 (s, 3H), 2.40 (s, 3H), 4.44 (t, J = 3.8 Hz, 1H), 4.48 (d, J = 3.8 Hz, 1H), 5.52 (d, J = 3.5 Hz, 1H), 5.59 (d, J = 5.0 Hz, 1H), 6.02 (d, J = 3.5 Hz, 1H), 7.51 (d, J = 8.3 Hz, 1H), 7.80 (dd, J = 7.9, 1.6 Hz, 1H), 7.89 (d, J = 1.8 Hz, 1H). 13C NMR (101 MHz, DMSOd6): δ 19.8, 26.2, 26.8, 76.1, 84.3, 85.1, 104.6, 111.3, 126.8, 128.1, 131.4, 133.6, 135.6, 140.9, 193.2. (2R,3S,4R,5S,6S)-6-(3-Chloro-4-methylphenyl)tetrahydro-2H-pyran-2,3,4,5-tetrayl Tetraacetate (17). Luche Reduction To Give Diol 15. A solution of sodium borohydride (93.95 g, 2.48 mol) in 1.0 N sodium hydroxide (582 mL) was added to a mixture of ketone 14 (2.00 kg, 6.40 mol) and cerium trichloride heptahydrate (1.19 kg, 3.20 mol, 0.50 equiv) in ethanol (14 L) at −20 °C over 1.5 h and aged (diastereomeric ratio of diol 15 = 96:4). The reaction mixture was warmed to 0 °C, quenched with water (8 L) at ≤11 °C, and concentrated to ∼11 L. Ethyl acetate (8 L) was added, and the pH of the mixture was adjusted to 2.5 with 6 N HCl (∼535 mL). The organic layer was separated, washed sequentially with 0.25 N sodium hydroxide (2 × 4.5 L) and 25% brine (4 L), distilled to a low volume, and flushed with acetonitrile (12 L) to give a solution of diol 15 (∼9.5 L). Deprotection To Give Tetraol 16. Water (3.5 L) and 6 N HCl (260 mL) were charged to the solution of diol 15, and the mixture was aged for 3.5 h at 65−70 °C. After complete conversion, the reaction mixture was cooled to 25 °C and extracted with 2-MeTHF (10 L). The organic phase was washed with 25% brine (2 × 4 L), concentrated to ∼5 L, and then codistilled with acetonitrile in portions (34 L total) until residual water was

Process Development for a Locally Acting SGLT1 ... - ACS Publications

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