Organic Process Research & Development 2009, 13, 1145–1155
Development of a Scalable Synthesis of Dipeptidyl Peptidase-4 Inhibitor ABT-279 Todd S. McDermott,*,† Lakshmi Bhagavatula,‡ Thomas B. Borchardt,§ Kenneth M. Engstrom,‡ Jorge Gandarilla,‡ Brian J. Kotecki,‡ Albert W. Kruger,‡ Michael J. Rozema,‡ Ahmad Y. Sheikh,§ Seble H. Wagaw,‡ and Steven J. Wittenberger‡ Abbott Laboratories, Global Pharmaceuticals R&D, Formulation Sciences - Solids, 200 Abbott Park Road, D-R4P7, Building AP31, Abbott Park, Illinois 60064, Abbott Laboratories, Global Pharmaceuticals R&D, Process R&D, 1401 Sheridan Road, D-R450, Building R8, North Chicago, Illinois 60064, and Abbott Laboratories, Global Pharmaceuticals R&D, Solid State Chemistry, 1401 Sheridan Road, D-R452, Building R13, North Chicago, Illinois 60064
Abstract: A convergent, scalable synthesis of dipeptidyl peptidase-4 inhibitor, ABT-279, has been developed and demonstrated on multikilogram scale. The cis-2,5-disubstituted pyrrolidine is generated by cyclization of a Boc-amine onto an alkynyl ketone followed by stereospecific reduction of the resulting acyliminium intermediate. The amine coupling partner was prepared by a novel Hofmann rearrangement promoted by 1,3-dibromo-5,5-dimethylhydantoin. The final product was isolated as the L-malic acid salt. The scaleup campaign consisted of 15 steps and delivered 42 kg of ABT279 in 14% overall yield. A second-generation synthesis that addresses some of the issues encountered during scale-up was developed and demonstrated on kilogram scale. Introduction Diabetes is a disease that affects 170 million people worldwide. The prevalence of diabetes is expected to more than double by 2030.1 Inhibition of dipeptidyl peptidase-4 (DPP-4) has been shown to be effective at lowering levels in the blood of glucose and hemoglobin A1c (HbA1c) and has the potential to be disease altering by promoting regeneration of insulinproducing cells in the pancreas.2 As part of an active program in the area of DPP-4 inhibition, multikilogram quantities of DPP-4 inhibitor ABT-279 (1), Figure 1, were necessary to support regulatory toxicology studies and clinical evaluation.3 The retrosynthetic analysis of 1 leads to aminopyridine 2 and chloroacetamide 3 (Scheme 1). Aminopyridine 2 is ultimately derived from commercially available ethyl isonipecotate 4 and tert-butyl chloroisonicotinate 5, while chloroacetamide 3 comes from methyl N-Boc-pyroglutamate 6. Discussion Synthesis of Amine Coupling Partner. Commercially available ethyl isonipecotate, 4, was converted to salt 9 · TsOH * Author to whom correspondence may be sent. E-mail: todd.mcdermott@ abbott.com. † Formulation Sciences - Solids. ‡ Process R&D. § Solid State Chemistry.
(1) Wild, S.; Roglic, G.; Green, A.; Sicree, R.; King, H. Diabetes Care 2004, 27, 1047. (2) Zerilli, T.; Pyon, E. Y. Clin. Ther. 2007, 29, 2614. (3) Madar, D. J.; Kopecka, H.; Pireh, D.; Yong, H.; Pei, Z.; Li, X.; Wiedeman, P. E.; Djuric, S. W.; Von Geldern, T. W.; Fickes, M. G.; Bhagavatula, L.; McDermott, T.; Wittenberger, S.; Richards, S. J.; Longenecker, K. L.; Stewart, K. D.; Lubben, T. H.; Ballaron, S. J.; Stashko, M. A.; Long, M. A.; Wells, H.; Zinker, B. A.; Mika, A. K.; Beno, D. W. A.; Kempf-Grote, A. J.; Polakowski, J.; Segreti, J.; Reinhart, G. A.; Fryer, R. M.; Sham, H. L.; Trevillyan, J. M. J. Med. Chem. 2006, 49, 6416. 10.1021/op900197r CCC: $40.75 2009 American Chemical Society Published on Web 10/15/2009
Figure 1. DPP-4 Inhibitor ABT-279 (1). Scheme 1. Retrosynthesis of ABT-279 (1)
Scheme 2. Synthesis of amide 9a
a Reaction conditions: (a) Boc O, tol, RT. (b) LDA; MeI, THF/tol, -10 °C. 2 (c) NaNH2, THF/tol, 50 °C. (d) p-TsOH, IPA, 80 °C; 69% overall yield.
by standard transformations (Scheme 2). Boc protection of 4 in toluene was followed by deprotonation with LDA in toluene/ THF at -10 °C and then addition of methyl iodide to give ester 7. After workup, the ester was converted to the corresponding amide 8 by treatment with sodium amide in toluene/THF at 50 °C. Finally, the Boc group was removed, and the p-toluenesulfonic acid salt, 9 · TsOH, was isolated as a crystalline solid. The overall yield for the four-step sequence was 69%. The pyridine-coupling partner necessary to access aminopyridine 2 is tert-butyl-2-chloroisonicotinate, 5. Commercially available 2-chloroisonicotinic acid, 10, was protected as its tertbutyl ester by treatment with excess Boc2O and catalytic DMAP in N-methyl-2-pyrrolidone (NMP) solvent (Scheme 3). Attempts Vol. 13, No. 6, 2009 / Organic Process Research & Development
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Scheme 3. Synthesis of aminopyridine coupling partner 2
to effect the tert-butyl ester formation under more standard conditions4 resulted in low yields due to incomplete reactions. After extraction with methyl tert-butyl ether (MTBE) and concentration from toluene, compound 5 was obtained as an oil containing toluene and di-tert-butyl carbonate, which do not interfere with subsequent reactions.5 The strategy for coupling chloropyridine 5 and various substituted piperidines was initially focused on palladiumcatalyzed N-arylation reactions.6 Sensitivity of the tert-butyl ester of 5 to the highly basic conditions necessary for these coupling reactions led to low yields and complicated reaction mixtures. The major impurities were related to hydrolysis of the tertbutyl ester of starting material and product. An alternative thermal coupling strategy was developed. Treatment of chloropyridine 5 with mild base and 1.5 equiv of 9 · TsOH at 100 °C in dimethyl sulfoxide (DMSO) for 36 to 48 h led to amide 11 (Scheme 3). The major side reaction was cleavage of the tert-butyl ester of the starting chloropyridine. Upon completion, addition of water to the reaction mixture resulted in crystallization of amide 11. Water washes of the solid completely removed the chloroisonicotinic acid byproduct resulting in pure material (>98 LC A%). The final transformation to produce amine 2 was a Hofmann rearrangement of amide 11. This transformation could be carried out under typical Hofmann rearrangement conditions; however, low yields or hard-to-control reactions resulted.7 An alternative Hofmann reaction was developed, utilizing 2,5-dibromo-3,3dimethylhydantoin (DBDMH) as the bromine source (Scheme 3).8 Initially, the Hofmann reaction was hampered by formation of acid 12, by hydrolysis of the ester, and urea 13 due to buildup of product in the presence of the intermediate isocyanate (Figure (4) (a) Baldwin, J. E.; Bamford, S. J.; Fryer, A. M.; Rudolph, M. P. W.; Wood, M. E. Tetrahedron 1997, 53, 5233. (b) Wright, S. W.; Hageman, D. L.; Wright, A. S.; McClure, L. D. Tetrahedron Lett. 1997, 38, 7345. (c) Dhaon, M. K.; Olsen, R. K.; Ramasamy, K. J. Org. Chem. 1982, 47, 1962. (5) During the pilot-plant campaign, distillation of the MTBE/toluene product solution resulted in a higher than expected concentration, and compound 5 crystallized in the reactor. A crystallization procedure from MeOH/water was subsequently developed and demonstrated on 1-kg scale. (6) Wagaw, S.; Buchwald, S. L. J. Org. Chem. 1996, 61, 7240. (7) The use of NaOCl as oxidant led to incomplete reaction, whereas the use of 2,5-dichloro-3,3-dimethylhydantoin led to loss of the tert-butyl ester group due to long reaction times. The used of bromine as oxidant was possible, but addition rate and stoichiometry were difficult to control. (8) Engstrom, K.; Henry, R.; Hollis, S.; Kotecki, B.; Marsden, I.; Pu, Y.M.; Wagaw, S.; Wang, W. J. Org. Chem. 2006, 71, 5369–5372. 1146
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Figure 2. Major impurities formed during the synthesis of ABT-279.
2). In order to increase the rate of isocyanate hydrolysis, the reaction was carried out in a biphasic system of THF/4 M NaOH in the presence of a phase transfer catalyst. The use of 1 equiv of tetrabutylammonium bromide (TBAB) limited the formation of urea while not greatly affecting the ester hydrolysis. The amount of TBAB could be reduced to about 0.4 equiv without an increase in urea formation, but the slower reaction time resulted in additional ester hydrolysis. During the preparation for scale up, it was noted that the Hofmann reaction is very sensitive to temperature. It was critical to keep the reaction temperature below 5 °C during the addition of DBDMH, which is very exothermic. On small scale, increasing the reaction temperature from 5 to 10 °C resulted in a 4-fold increase in the amount of urea 13 formed. During the pilot plant campaign, the reaction temperature was controlled by limiting the rate of addition of DBDMH and the reaction resulted in 87% yield of the desired amine (Scheme 3). Synthesis of Chloroacetamide Coupling Partner. The synthesis of the other coupling partner, chloroacetamide 3, began with chemistry reported by Martin et al. in 2004.9 They reported opening of carbamate-protected pyroglutamates followed by cyclization and reduction by treatment with BF3 · OEt2 in the presence of triphenylsilane to give 2,5-cis-pyrrolidines. In the case of the isopropyl carbamate, the yield and selectivity were excellent. In the case of Boc-protected pyroglutamates, the protecting group was unstable to the Lewis acidic conditions, and alternative cyclization/reduction conditions were employed. The Martin chemistry provided a good starting point for the synthesis of chloroacetamide 3. For our purposes, the Boc protecting group was preferable to the isopropyl carbamate because of our previous experience with Boc-protected intermediates. The addition of the magnesium chloride salt of trimethylsilylacetylene to pyroglutamate 6 gave clean conversion to propargylic ketone 14 (Scheme 4). Although the ester was added to an excess of Grignard reagent, very little bis-addition product was detected, suggesting that the initially formed tetrahedral intermediate is stabilized by the presence of the adjacent Boc group. The major byproduct encountered was ketone 15, the result of base-promoted self-condensation of 6 (Figure 2). This product could be avoided by slow addition of a solution of the ester to a cold THF solution of the Grignard (9) Brenneman, J. B.; Machauer, R.; Martin, S. F. Tetrahedron 2004, 60, 7301.
Scheme 4. Initial cyclization/reduction sequence
Table 1. Cyclization/reduction of 18
entry
solvent
reductant
cis/trans
1 2 3 4 5 6 7
toluene DCM acetonitrile DCM toluene acetonitrile EtOAc
Et3SiH Et3SiH Et3SiH NaBH(OAc)3 NaBH(OAc)3 NaBH(OAc)3 NaBH(OAc)3
1:8 1:7 1:4 1.5:1 2:1 5.1:1 11:1
Scheme 6. Synthesis of Boc-acid 19a Scheme 5. Synthesis of enyne 18
a Reaction conditions: (a) NaBH(OAc) ; TFA, IPAc, -10 °C. (b) LiOH · H O, 3 2 THF/EtOH, 0 °C, crystallization from IPAc/heptane, (65%).
reagent. This reaction gave 85% yield during the pilot plant campaign and the product, which was about 95 LC A% pure (excluding IPAc), was used without purification after extractive workup. Initial attempts to effect cyclization/reduction of 14 were unsuccessful. Using the conditions reported by Martin, the compound rapidly decomposed. Changing the solvent to THF led to cyclization to the corresponding enyne (16), which slowly reduced to pyrrolidine 17. However, during the long reaction time degradation of enyne 16 and product was seen (Scheme 4). In order to work with a more stable system, the triisopropylsilyl (TIPS)-protected enyne (18) was produced by addition of tri-isopropylalkynyl lithium to 6 and treatment with BF3 · OEt2 followed by silica gel chromatography (Scheme 5). Enyne 18 is a stable material isolated in 45% yield and allowed for the controlled investigation of the desired reduction. The desired reduction could be achieved by treatment of 18 with trifluoroacetic acid (TFA) in the presence of reducing agents. Indeed, 18 could be selectively reduced to either the trans- or cis-pyrrolidine, depending on the conditions. Use of Et3SiH as the reducing agent led preferentially to the trans product (Table 1, entries 1-3). Reduction with a bulkier reagent, sodium triacetoxyborohydride (NaBH(OAc)3), led preferentially to the cis-pyrrolidine product (Table 1, entries 4-7). For both reducing agents, the use of polar solvents (IPA or THF) led to removal of the Boc group and decomposition. The highest cisselectivity was observed when EtOAc was used as the solvent.10 Although the above reduction produced the desired cispyrrolidine product, it was preferable to use the TMS-protected acetylene, rather than the harder to remove TIPS protecting group. Unfortunately, isolation of enyne 16 was not possible (10) An investigation of the scope and limitations of the cyclization/ reduction reaction is underway and will be reported separately.
due to the lability of the TMS group. However, it was not necessary to isolate the enyne intermediate. Ketone 14, when exposed to TFA in the presence of NaBH(OAc)3, gave the desired pyrrolidine (17) in good yield and selectivity (Scheme 6).11 In practice, a mixture of 14 and 1.5 equiv of NaBH(OAc)3 in IPAc was treated with 3 equiv of TFA at -10 °C. Upon warming to room temperature the reaction was complete.12 The crude product was solvent switched to EtOH and treated with LiOH to hydrolyze the methyl ester and the TMS group. On pilot-plant scale the cyclization, reduction, and hydrolysis sequence gave a 12:1 cis/trans ratio. Crystallization of acid 19 from IPAc/heptane resulted in material that contained less than 0.5% of the trans isomer, but substantial losses to the mother liquors occurred (about 19%). The overall yield of the process was 65% on 50-kg scale (Scheme 6), and the material produced was greater than 97% pure by HPLC analysis. Carboxylic acid 19 was converted to the corresponding amide and then dehydrated to nitrile 20 under Vilsmeier conditions (Scheme 7). The Boc group was removed with p-TsOH. One of the side products of this reaction was the pyrrolidine tert-butyl amide 21 (Figure 2), resulting from attack of the nitrile on the tert-butyl cation produced during the Boc deprotection. The use of CH3CN as solvent provided an excess of an alternative nitrile and resulted in formation of significantly lower amounts of 21 (2-4 LC A%) and the production of 1 equiv of tert-butylacetamide as a byproduct, which does not interfere with the next reaction. After removal of the Boc protecting group, the chloroacetamide (3) was formed directly (11) For the use of TFA to convert a Boc-protected amino-ketone to a Boc-enamine see: Clark, R. D.; Muchowski, J. M.; Fisher, L. E.; Flippin, L. A.; Repke, D. B.; Souchet, M. Synthesis 1991, 871. (12) Other protic acids (HCl, H2SO4, MeSO3H, H3PO4) and TMSCl failed to effect the cyclization/reduction. The selectivity of the reaction is independent of temperature (12:1 at 0 or 25 °C), although slightly higher yields are obtained at 0 °C. IPAc and EtOAc are comparable solvents for the reduction reaction. IPAc was used to avoid issues with EtOAc hydrolysis in the subsequent step. Vol. 13, No. 6, 2009 / Organic Process Research & Development
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Scheme 7. Synthesis of chloroacetamide 3a
Scheme 9. Alternative Hofmann and amine coupling sequencea
a Reaction conditions: (a) i-BuOCOCl; NH OH, IPAc, 0 °C. (b) SOCl /DMF, 4 2 THF, rt, (87%). (c) TsOH, CH3CN RT, (d) ClCH2COCl, Hunig’s base, CH3CN, 0 °C; crystallization from IPA/water, (78%).
Scheme 8. Endgame strategya
a Reaction conditions: (a) DBDMH, aq KOH/CH CN, 0 °C. (b) p-TsOH, 3 MeOH, 60 °C, (92%). (c) Boc2O, DMAP, NMP; Et3N, RT (88%). (d) K3PO4, NMP 80 °C (97%). (e) H2, Pd/Al2O3, K3PO4, NMP, 40 °C (96%).
a Reaction conditions: (a) K PO /KI, NMP, 35 °C. (b) D-tartaric acid, IPA, 3 4 (88%). (c) H3PO4/H2O, 60 °C. (d) L-malic acid, EtOH/H2O, 45 °C (93%).
by treatment with chloroacetyl chloride in the presence of Hunig’s base. Chloroacetamide 3 was crystallized from IPA/ water to give 78% yield of material that was 99 LC A% pure and >200:1 cis/trans. Endgame Strategy. With the coupling partners in place, all that remained was the final coupling and deprotection. The coupling of hindered amine 2 required slightly forcing conditions. An NMP solution of chloroacetamide 3 was added to a mixture of 2, milled K3PO4, and KI in NMP solvent at 35 °C (Scheme 8). The key concern with this reaction was epimerization of the nitrile stereocenter. At temperatures above 40 °C, the amount of epimerization was beyond the level that could be removed during isolation. At 45 °C, the cis/trans ratio was 87:13, and salt formation and crystallization resulted in a final ratio of 89:11. A reaction temperature of 35 °C gave a good balance between reaction rate (completion in ∼4 h) and epimerization (less than 1%). The product, 22, was isolated by extraction with MTBE followed by crystallization as its D-tartaric acid salt. The isolation of the penultimate product allowed very clean material (>99 LC A%) to be taken into the final deprotection step. Early attempts at the tert-butyl ester deprotection suffered from formation of Ritter reaction byproducts, as had been seen with the Boc deprotection of 20. The strategy of using CH3CN as a solvent, which had been successful previously, did not completely alleviate the problem. Carrying out the final deprotection with phosphoric acid in water avoided formation of the tert-butyl amide byproduct. After the deprotection step, ABT279 (1) was isolated as its crystalline L-malic acid salt from EtOH/H2O. The final deprotection/isolation sequence proceeded in 93% yield and gave 42 kg of product. The material was 99.8 LC A% pure and was 99.4% ee by HPLC analysis. The cis/ trans diastereomeric ratio of the final solid was >99.9:0.1. Alternative Synthetic Sequence. The synthesis carried out in the pilot plant produced material to support the project through phase I and into phase II development. However, there were several issues with the scale-up synthesis that needed to be addressed prior to further scale-up work. As described earlier, 1148
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the DBDMH Hofmann reaction is very sensitive to reaction temperature (see Scheme 3). The primary reason for this sensitivity is that amide 11 has low solubility in most solvents, making hydrolysis of the intermediate isocyanate slow. As an alternative, the Hofmann reaction was carried out on amide 8, a compound with more favorable solubility characteristics (Scheme 9). Because of the improved solubility of 8, a phase transfer catalyst was not necessary.13 The resulting Boc amine was treated with p-TsOH in MeOH at 60 °C to effect cleavage of the Boc group. The product was isolated as its bis-p-TsOH salt directly from the reaction mixture. This sequence gave 92% isolated yield of salt 23 for two steps, which is a substantial improvement of the previous Hofmann protocol. An additional improvement to the scale-up campaign was the use of the more activated tert-butyl 2,6-dichloroisonicotinate (25) as the pyridine coupling partner. Protection of the commercially available 2,6-dichloroisonicotinic acid (24) was carried out using conditions similar to those reported earlier for t-butyl ester formation.14 In this case, the ester (25) was isolated as a crystalline solid in 88% yield. The N-arylation reaction using compound 25 is considerably more facile. Treatment of 25 with a slight excess of 23 in NMP in the presence of base gave a nearly quantitative yield of chloropyridine 26 in a few hours at 80 °C. Dechlorination of 26 was carried out by treatment with Pd/Al2O3 in the presence of H2 and base in NMP to give 96% yield of coupling partner 2 (Scheme 9). After workup, the product was used in the final coupling reaction without further purification. During the first pilot-plant campaign the sequence from amide 8 to amine 2 proceeded in four steps and 66% overall yield. The alternative sequence proceeded in five steps and 76% overall yield and gave material that was indistinguishable from material produced during the pilot-plant campaign. This sequence was carried out on 1-kg scale in order to demonstrate its scalability. In this case, each of the reactions was worked up as usual (see Experimental Section). However, the tert-butyl ester formation, the N-arylation, the dechlorination, and the final coupling (Scheme 9) are each carried out in the NMP/base system, allowing the possibility of telescoping these (13) Mixtures of 8, CH3CN, and 2 M KOH became homogeneous upon addition of DBDMH. Running the reaction under dilute conditions, where layer separation occurred, resulted in formation of the corresponding urea byproduct. (14) One equivalent of Et3N was added to the reaction to increase the nucleophilicity of the acid. In the absence of base the reaction exhibits a long induction period and a hard-to-control exotherm.
reactions. In a small-scale run, these reactions were carried out without workup of any of the intermediates. Optimization of this sequence should allow telescoping of the final four steps of the process. Conclusion We have described a convergent and practical synthesis of ABT-279, a DPP-4 inhibitor. The synthesis was carried out in the pilot plant to produce 42 kg of ABT-279. The key transformations include cyclization of a Boc-protected aminoketone, stereospecific reduction of the resulting acyliminium ion with sodium triacetoxyborohydride, and a novel DBDMHpromoted Hofmann rearrangement to give the amine coupling partner. Substantial improvements were made to the initial synthesis and were demonstrated on 1-kg scale. Experimental Section All reactions were carried out under a nitrogen atmosphere in either glass vessels or glass-lined stainless steel vessels with overhead mechanical stirring unless otherwise noted. HPLC data were collected using an Agilent 1100 series HPLC. Gas chromatography data were collected using a Hewlett-Packard 6890 series gas chromatograph. Chromatographic conditions are reported as part of the experimental descriptions below. Retention times are uncorrected. Elemental analysis samples were run by Quantitative Technologies Inc., Whitehouse, NJ (U.S.A.). Reagents were purchased from commercial suppliers and used as received. Preparation of 1-tert-Butyl 4-ethyl 4-methylpiperidine1,4-dicarboxylate (7). Boc protection. A solution of Boc2O (97 kg, 444 mol, 1.03 equiv) and toluene (140 kg, KF 0.16 wt % water) at 15 °C was treated with ethyl isonipecotate (4, 68 kg, 433 mol, 1 equiv) at
