Article pubs.acs.org/joc
Synthesis of the γ‑Secretase Modulator MK-8428 Steven P. Miller,* William J. Morris,* Robert K. Orr, Jeffrey Eckert, Jay Milan, Mathew Maust, Cameron Cowden, and Jian Cui Department of Process Research, MRL, Merck & Co., Inc., Rahway, New Jersey 07065, United States S Supporting Information *
ABSTRACT: The synthesis of the γ-secretase modulator MK-8428 (1) is described. The synthesis is highlighted by an enzyme-catalyzed reaction to access 3,4,5-trifluoro(S)-phenylglycine, a 1-pot activation/displacement/deprotection sequence to introduce the aminooxy functionality and a dehydrative intramolecular cyclization under mild conditions to form the oxadiazine heterocycle of 1. In situ reaction monitoring was employed to understand the deleterious role of water during the formation of a methanesulfonate ester in the 1-pot activation/displacement/deprotection sequence.
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INTRODUCTION
RESULTS AND DISCUSSION The retrosynthesis of 1 is presented in Figure 1. The key oxadiazine pharmacophore5 was generated via intramolecular dehydrative cyclization of an intermediate aminooxy nucleophile derived from carbinol 2. This carbinol was accessible after coupling of α,β-unsaturated acid 3 with amino-alcohol 4. Acid 3 was formed via Horner−Wadsworth−Emmons olefination of aldehyde 5, and the amino-alcohol 4 was prepared via an enzyme-mediated conversion of an α-keto carboxylic acid 6 to an optically pure α-amino acid followed by reduction of the carboxylic acid. The route employed to produce α,β-unsaturated acid 3 is shown in Scheme 1. 3-Hydroxy-4-nitro benzoic acid (7) underwent bis-alkylation with dimethylsulfate to generate 8. The nitro group was reduced in the presence of Raney-Ni to provide aniline 9. This aniline was formylated and subsequently alkylated with chloroacetone to yield 11, which upon treatment with ammonium acetate in acetic acid underwent dehydrative cyclization to afford 4-methyl-imidazole 12. The methyl ester was subsequently converted to aldehyde 56 using a two-step reduction/oxidation sequence. Aldehyde 5 was coupled to phosphonate 135a under Horner−Wadsworth−Emmons7 conditions, yielding a mixture of olefin isomers (E:Z 4.7:1). Finally, exposure to trifluoroacetic acid resulted in the cleavage of the tert-butyl ester to afford the trifluoroaceate salt of 3. The synthesis of the amino-alcohol coupling partner 4 is shown in Scheme 2. Readily available 5-bromo-1,2,3-trifluorobenzene (15) was acylated with diethyl oxalate, and the resulting ester was hydrolyzed to yield α-keto carboxylic acid 6. In a key transformation, exposure of 6 to an amino acid dehydrogenase (L-AADH-108)8 produced 3,4,5-trifluoro-(S)phenylglycine 16 in high yield and with excellent enantioselectivity (>99% ee). Glucose and glucose dehydrogenase were used to regenerate the NADH cofactor from NAD+, producing D-glucono-1,5-lactone (Figure 2) in the process. The carboxylic
Alzheimer’s disease (AD) is a neurodegenerative disorder that results in gradual loss of memory and impairment of vocal and motor control before ultimately resulting in death. By 2025, it is estimated that greater than 7 million Americans will be suffering from AD, a number that represents a 40% increase from the number of Americans living with the disease in 2015.1 The current standard of care involves treatment with a cholinesterase inhibitor to improve cognition, but there are currently no disease-modifying therapies available to treat AD. Patients suffering from AD possess two distinctive biomarkers: extracellular amyloid plaques and intracellular neurofibrillary tangles, both of which are believed to result in the loss of neurons in the cerebral cortex. The amyloid plaques are formed when amyloid precursor protein (APP) is cleaved sequentially by proteases BACE1 and γ-secretase, leading to the formation of Aβ42. Novel inhibitors or modulators of the γ-secretase protease can therefore be assessed as possible treatments for AD.2 During the course of their clinical evaluation, γ-secretase inhibitors were found to lead to higher incidence of adverse events, likely due to the promiscuity of the γ-secretase enzyme, which possesses a range of endogenous substrates and is involved in a variety of key cellular processes.3 To mitigate these concerns, a new class of molecules was developed that changes the enzyme’s interaction with APP without further cellular impact. These new molecules, γ-secretase modulators, alter the APP specificity and result in the elevated formation of the shorter, less toxic Aβ37 and Aβ38 fragments, resulting in lower levels of Aβ42.4 MK-8428 (1) was identified as a novel γsecretase modulator possessing favorable in vitro activity and an excellent pharmacokinetic profile and was subsequently targeted for further development. To support preclinical and early clinical development of this compound, a robust and efficient synthesis of 1 was required. © XXXX American Chemical Society
Received: December 13, 2016
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DOI: 10.1021/acs.joc.6b02979 J. Org. Chem. XXXX, XXX, XXX−XXX
Article
The Journal of Organic Chemistry
Figure 1. Retrosynthesis of 1.
Scheme 1. Preparation of α,β-Unsaturated Acid 3
Scheme 2. Preparation of Amino Alcohol 4
to previously disclosed routes to 4 with respect to overall yield and stereocontrol.5e Amino-alcohol 4 was coupled to unsaturated acid 3 under HOBT/EDC conditions, resulting in the formation of amide 17 (Scheme 3). Treatment of amide 17 with NaOMe promoted the intramolecular alkylation, yielding the δ-lactam 2. Assembly of the aminooxy moiety was carried out in a 2-step sequence beginning with the addition of N-hydroxyphthalimide after activation of 2 as the methanesulfonate ester10 to yield 18. It was determined that isolation of 18 was not required, as direct addition of aqueous hydroxylamine resulted in the cleavage of the phthalimide to provide 19. Hydroxylamine was chosen as the nucleophile for this transformation, as the byproduct of the reaction (N-hydroxyphthalimide) was easily separated from 19 by washing with aqueous base during the reaction workup. The overall yield from 2 to 19 was 86% and
Figure 2. Enzymatic mediated conversion of keto-acid 6 to amino acid 16.
acid of 16 was then reduced to provide 4, which was isolated as the mandelate salt.9 The targeted amino alcohol was prepared in 3 steps with an overall yield of 42% and provided advantages B
DOI: 10.1021/acs.joc.6b02979 J. Org. Chem. XXXX, XXX, XXX−XXX
Article
The Journal of Organic Chemistry Scheme 3. Installation of the Aminooxy Moiety
Table 1. Impact of Water on the Activation/Displacement Approach to 18
entry 1 2 3 4
H2O content (mol %)
age time (h)a
2 (%)b
18 (%)b
0.5 0.5 3 3
3.7 0.3 11.0 0.3
91.9 97.2 81.2 96.7
c
60 6d 60 5
a Time between the end of the MsCl addition and the N-hydroxyphtalimide charge. bContent was recorded after reaction was deemed complete by HPLC. Numbers shown are HPLC area percents. cKF values were recorded for individual components (2 and THF, respectively) for this analysis. d KF value was the solution of 2 in THF.
The impact of water on the conversion of 2 to 18 was further explored using in situ IR spectroscopy.11 In situ IR provided a tool to monitor the formation of the methanesulfonate ester 20, as this reactive intermediate was unstable under reverse phase HPLC conditions.12 An experiment was carried out where water removal by azeotropic distillation was omitted (Table 1, entry 3). Following the addition of methanesulfonyl chloride, the batch was aged for 3 h. The IR spectroscopy data showed initial formation of the of the methanesulfonate ester (Figure 3, orange band), which was followed by clear decomposition of this reactive intermediate during the course of the 3 h holding period (Figure 3, red band). N-Hydroxyphthalimide was then added to the reaction, and the final content of 2 was observed to be 11% by HPLC. A subsequent experiment was conducted in which water was removed by azeotropic distillation (Table 1, entry 4). Following completion of the methanesulfonyl chloride addition, the reaction was again aged for 3 h (Figure 4, pink band). In stark
yielded the precursor for the pivotal oxadiazine formation (vide infra). During the course of our optimization work on installing the N-hydroxyphthalimide group of 18, it was observed that small amounts of 2 persisted at the end of the reaction (Table 1, entry 1). Given the stoichiometry of the methanesulfonyl chloride employed (1.5 equiv), we considered it unlikely that the carbinol was not fully converted to the methanesulfonate ester. A more plausible scenario was that adventitious water reacted with the newly formed methanesulfonate ester 20 to return carbinol 2. To evaluate this hypothesis, water introduced from 2 and THF was azeotropically removed prior to addition of the methanesulfonyl chloride and triethylamine. This operation reduced the measured water content to 6 mol % (Table 1, entry 2). Following formation of the methanesulfonate ester and addition of the N-hydroxyphthalimide, the amount of 2 observed was lowered to 0.3% by HPLC with a concomitant increase in the purity of 18 by HPLC. C
DOI: 10.1021/acs.joc.6b02979 J. Org. Chem. XXXX, XXX, XXX−XXX
Article
The Journal of Organic Chemistry
Figure 3. IR spectrum from mesylation of 2 without distillation prior to reaction (3 h hold). Formation of the intermediate methanesulfonate ester 20 was established by monitoring the appearance of the S−O stretch in the IR spectrum. Unlabeled stretches represent time points prior to the completion of the MsCl addition.
Figure 4. IR spectrum from mesylation of 2 with distillation prior to reaction (3 h hold). Formation of the intermediate methanesulfonate ester 20 was established by monitoring the appearance of the S−O stretch in the IR spectrum. Unlabeled stretches represent time points prior to the completion of the MsCl addition.
polyphosphate byproducts13 which complicated the isolation of 1. Acids were screened to determine their effectiveness in promoting the conversion of 19 to 1 (Table 2). While the conversion in the presence of most acids was high, the assay yields of the desired oxadiazine product were low due to the competing side reactions. Gratifyingly, we discovered that when treating a solution of 19 with hexamethyldisilazane (HMDS) and catalytic amounts of trimethylsilyl trifluormethanesulfonate (TMSOTf), the desired intramolecular cyclization took place in high yield.14 This silyl-mediated dehydration provided a milder alternative to Brønsted acids and a new approach to access this interesting heterocycle (1). Finally, isolation as the hemifumarate salt provided access to our targeted γ-secretase modulator 1 (Scheme 4).
contrast to the experimental data shown in Figure 3, no discernible mesylate decomposition was observed by IR when the water was removed prior to the addition of methanesulfonyl chloride and triethylamine (Figure 4, light blue band). Following completion of the reaction, the final content of 2 was determined to be 0.3% by HPLC. Taken together, the water titration and in situ IR data provided strong support that water dramatically impacts the conversion of 2 to 18 presumably by reacting with the intermediate mesylate to regenerate carbinol 2. With an understanding of the conversion of 2 to 19 in place, we were poised to introduce the oxadiazine heterocycle using an intramolecular dehydrative cyclization. Previously reported conditions using P2O5 in EtOH5a led to the formation of D
DOI: 10.1021/acs.joc.6b02979 J. Org. Chem. XXXX, XXX, XXX−XXX
Article
The Journal of Organic Chemistry Table 2. Acid Screen for Intramolecular Dehydration
entrya
acid
conversion (%)b
assay yield of 1 (%)
1 1 2 3 4 5 6 7 8c
none HCl MsOH AcOH TFA HBF4 H3PO4 Tf2NH HMDS/TMSOTf
0 60 98 98 99 99.5 99.5 99.8 99.9
0 42 65 54 52 70 68 75 92
a
Entries 1−7 were run using EtOH as a solvent under reflux conditions for 14 h. bBased on HPLC area percent of 19. cReaction solvent was 2-MeTHF, and reaction temperature was 30 °C.
Scheme 4. End Game Approach to 1
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CONCLUSION
preclinical development of this potentially important diseasemodifying therapy for AD.
A convergent synthesis of γ-secretase modulator 1 was described. Highlights of this synthesis included an enzymemediated conversion of an α-keto carboxylic acid to access an enantiopure 3,4,5-trifluoro-(S)-phenylglycine (16). In situ reaction monitoring was employed to understand the deleterious role of water on the 1-pot mesylation/displacement sequence that was employed to introduce the aminooxy functionality. Finally, mild dehydration conditions were identified to form the key oxadiazine heterocycle in high yield. The route described enabled production of 1 to support
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EXPERIMENTAL SECTION
General Information. All reactions were carried out under a nitrogen atmosphere in dried glassware with either magnetic stirring or overhead agitation. 1H NMR spectra were recorded on either a 500 or 400 MHz spectrometer and are reported in ppm using deuterated solvent as an internal standard. Data are reported as ap = apparent, s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet, b = broad; coupling constant(s) in Hz; integration. Proton-decoupled 13C NMR spectra were recorded on a 125 or 100 MHz spectrometer and are E
DOI: 10.1021/acs.joc.6b02979 J. Org. Chem. XXXX, XXX, XXX−XXX
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The Journal of Organic Chemistry reported in ppm using deuterated solvent as an internal standard. Electrospray mass spectra (ESI-MS) were obtained using a triple quadrupole mass spectrometer. L-AADH-108 and GDH-105 were purchased from Codexis. (E)-1-(4-(2-Carboxy-5-chloropent-1-en-1-yl)-2-methoxyphenyl)4-methyl-1H-imidazol-3-ium 2,2,2-trifluoroacetate (3). To a roundbottom flask at 5 °C were added potassium tert-butoxide (3.5 g, 31.1 mmol) and THF (100 mL). A solution of tert-butyl diethylphosphonoacetate (7.5 g, 29.7 mmol) in THF (19 mL) was added to the potassium tert-butoxide solution at a rate to maintain the internal temperature
