Article pubs.acs.org/OPRD
Synthesis of BACE Inhibitor LY2886721. Part II. Isoxazolidines as Precursors to Chiral Aminothiazines, Selective Peptide Coupling, and a Controlled Reactive Crystallization Marvin M. Hansen,*,§ Daniel J. Jarmer,*,§ Enver Arslantas,‡ Amy C. DeBaillie,§ Andrea L. Frederick,§ Molly Harding,§ David W. Hoard,§ Adrienne Hollister,§ Dominique Huber,‡ Stanley P. Kolis,§ Jennifer E. Kuehne-Willmore,§ Thomas Kull,‡ Michael E. Laurila,§ Ryan J. Linder,§ Thomas J. Martin,‡ Joseph R. Martinelli,§ Michael J. McCulley,§ Rachel N. Richey,§ Derek R. Starkey,§ Jeffrey A. Ward,§ Nikolay Zaborenko,§ and Theo Zweifel‡ Downloaded by TEXAS A&M INTL UNIV on August 30, 2015 | http://pubs.acs.org Publication Date (Web): August 28, 2015 | doi: 10.1021/op500327t
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Small Molecule Design and Development, Lilly Research Laboratories, Eli Lilly and Company, Lilly Corporate Center, Indianapolis, Indiana 46285, United States ‡ Dottikon Exclusive Synthesis AG, P.O. Box 5605, Dottikon, Switzerland ABSTRACT: An efficient synthesis of LY2886721 (1) in five steps and 46% overall yield from the chiral nitrone cycloadduct 2 is presented. Minimizing formation of a des-fluoro impurity during hydrogenolysis to cleave the isoxazolidine ring and remove the benzyl chiral auxiliary was a key challenge. Installation of the aminothiazine moiety required careful stoichiometry control of the reagents BzNCS and CDI, including in situ conversion monitoring, to minimize byproduct formation. A remarkably regioselective peptide coupling afforded 1 without competing acylation at the aminothiazine nitrogen or bis-acylation. Consideration of the combined chemistry and crystallization process identified an optimal solvent system for the peptide coupling and a reactive crystallization that afforded 1 in high purity and with physical property control. A slurry milling operation near the end of the crystallization, followed by “pH cycles” to digest fines formed during milling, significantly reduced the crystal aspect ratio and provided desirable API bulk density and powder flow properties.
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INTRODUCTION LY2886721 (1)1 is a potent and selective inhibitor of betaamyloid cleaving enzyme (BACE) and was in Phase 2 clinical trials as a potential treatment for Alzheimer’s disease. The synthetic route used by the Lilly discovery chemistry group was developed to provide the first-generation development route shown in Scheme 1. This route was suitable for providing the active pharmaceutical ingredient (API) to support toxicology studies and early clinical trials, but an improved route was needed to provide larger amounts of API for development. Key issues with the first-generation route include: • long synthesis and low overall yield (1%); • moderate yield of the ketone in Step 4, despite significant optimization; • use of stoichiometric zinc as a reducing agent and concerns associated with waste disposal; • a “late” resolution at Step 8; • the cost and waste issues associated with use of the Mitsunobu2 conditions to promote ring closure in Step 10; • poor and inconsistent yields in the Step 11 amination process (the low overall yield for Steps 11−13 was due to the amination step yield). In addition, there was little data to support the benefit of a “technical-grade” API isolation as the HCl salt, followed by a separate freebasing operation to afford the API. The establishment of the key cis relative stereochemistry using a nitrone cycloaddition in Step 6 was a key benefit of the route. Issues © XXXX American Chemical Society
related to the ketone synthesis and development of an asymmetric nitrone cycloaddition that avoids the late stage resolution were described in Part I, the prior publication in this issue.3 Described herein is full development of a new route that addresses the issues listed above starting from the asymmetric nitrone cycloaddition product and leading to an efficient synthesis of this complex API. In addition, the development of a reactive crystallization process that controlled the physical properties of the API is highlighted.4
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RESULTS AND DISCUSSION Chiral Auxiliary Removal and Isoxazolidine Opening. As described in the prior publication in this issue, an asymmetric nitrone cycloaddition was developed using a chiral auxiliary based approach and afforded the nitrone cycloadduct 2 as a single diastereomer (dr >99.9:0.1, see Scheme 2).3 As shown in Scheme 3, the first challenge was to convert cycloadduct 2 to an enantiomerically enriched amino alcohol analogous to the racemic amino alcohol used in the firstgeneration route (see Step 7 of Scheme 1). As in the firstgeneration route, the amino alcohol was chosen as a precursor for construction of the aminothiazine ring system. Catalytic hydrogenolysis conditions were found for cleavage of the N−O bond that avoided use of stoichiometric zinc; however, these conditions created a new challenge. Regioselective removal of Received: October 16, 2014
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DOI: 10.1021/op500327t Org. Process Res. Dev. XXXX, XXX, XXX−XXX
Article
Organic Process Research & Development
Downloaded by TEXAS A&M INTL UNIV on August 30, 2015 | http://pubs.acs.org Publication Date (Web): August 28, 2015 | doi: 10.1021/op500327t
Scheme 1. LY2886721 First-Generation Routea
a
Index of abbreviations used in Scheme 1: TBAHS = tetra-n-butyl ammonium hydrogen sulfate; NMM = N-methylmorpholine; LDA = lithium diisopropylamide; L-DPTTA = L-di-p-toluoyltartaric acid; BSTFA = N,O-bis(trimethylsilyl)trifluoroacetamide; BzNCS = benzoylisothiocyanate; DTAD = di-tert-butyl azodicarboxylate; EDCI·HCl = 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride; HOBt = hydroxybenzotriazole.
Scheme 2. Establishment of Stereochemistry Using an Asymmetric Nitrone Cycloaddition
Amination5 or amidation (see below) of aryl bromide 2 afforded the amine/amide substrates 3−5 (Scheme 3). These three derivatives and bromide 2 were evaluated under a variety of hydrogenolysis conditions using Pd/C and H2.6 As expected, the C−Br bond of bromide 2 was rapidly cleaved, followed by N−O and C−N bond hydrogenolysis to provide a 5:1 mixture of the desired amino alcohol A and the deamination byproduct B. Product stability studies for all substrates showed that deamination product B does not result from overhydrogenol-
the chiral auxiliary by preferential cleavage of the exocyclic benzylic C−N bond (product A) in the presence of an endocyclic benzylic C−N bond (product B) was required (Scheme 3). Choice of X in Scheme 3 was also important, as the aryl bromide would not survive the catalytic hydrogenation conditions that were considered. Eventually the X group will be comprised of a C−N bond in the API, and the most convergent route would incorporate the entire pyridine amide side chain as the X group. B
DOI: 10.1021/op500327t Org. Process Res. Dev. XXXX, XXX, XXX−XXX
Article
Organic Process Research & Development Scheme 3. Removal of Chiral Auxiliary and N−O Bond Cleavage
Downloaded by TEXAS A&M INTL UNIV on August 30, 2015 | http://pubs.acs.org Publication Date (Web): August 28, 2015 | doi: 10.1021/op500327t
Scheme 4. Acetamide Amidation of Aryl Bromide
Scheme 5. Hydrogenolysis To Remove Chiral Auxiliary and Cleave N−O Bond
also increased with higher CuI loading; however, it is unclear how copper can function as a reducing agent under these conditions. N−O bond reduction did not occur in a control experiment without acetamide. Product isolation was designed to remove copper, iodide, and phosphate, as all of these, and particularly iodide, were shown to be catalyst poisons in the next step. Partitioning between 10% aqueous NH4Cl and isopropyl acetate was useful for extracting the copper residue into the aqueous phase. Two additional 10% aqueous NH4Cl washes were needed to ensure that the residual levels of inorganics in the isolated product were below the acceptance limits (≤100 ppm iodide, ≤250 ppm of Cu, ≤250 ppm phosphate). Removal of the isopropyl acetate by a solvent exchange into xylenes afforded the desired amide product 4 in 87% yield over four batches on 190 kg scale. Hydrogenolysis for Chiral Auxiliary Removal and N− O Bond Cleavage. Conversion of the amide isoxazolidine 4 to the amino alcohol 8 with the removal of the chiral auxiliary was required at this stage. As previously discussed, N−O bond cleavage and selective cleavage of the exocyclic benzylic C−N bond in the presence of the endocyclic C−N bond was desired. Initial hydrogenolysis catalyst screening showed that the desired benzylic C−N bond could be cleaved with high selectivity (>50:1) using the optimized conditions shown in Scheme 5. Initial cleavage of the C−N bond to afford intermediate 7, followed by N−O bond reduction was the predominant pathway, although the intermediate where the N− O bond was reduced first could also be observed in situ. Two byproducts became the focus of more detailed development studies: defluorination of the aromatic ring (des-F, 9) and loss of the acetyl group under the acidic reaction conditions (desAc, 10). Of these, the des-F impurity was more important, because it was not well rejected during crystallization of the
ysis of the product but rather forms via intermediate C. With the unprotected aniline 3, the hydrogenolysis proceeded with improved regiocontrol (A:B = 49/1), but the diamine−alcohol product was difficult to isolate. The envisioned convergent route using the full pyridine amide side chain in 5 was thwarted by concomitant reduction of the pyridine ring in competition with N−O and C−N bond cleavage.7 Reduction of the acetyl protected aniline derivative 4 gave the most promising results (>50:1 ratio of A:B, facile product isolation) in this initial evaluation, and this substrate was chosen for detailed study as described below. Amidations using acetamide are relatively rare in the literature,8 and significant effort was required to develop an amidation process.9 Preliminary screening using a model system identified dimethyl ethylene diamine (6) as an excellent ligand for this transformation as shown in Scheme 4.10 The utility of this ligand was previously demonstrated by Buchwald and co-workers for other amides.11 As reported for related reactions, KI improved the reaction rate, and the ArI intermediate was observed in situ.12 Evaluation of the stoichiometry showed no further rate enhancement with more than 0.7 equiv of KI. The acetamide stoichiometry was evaluated over a range of 2−5 equiv, and 4 equiv was chosen to minimize waste while suppressing diarylation (2.5% diarylation with 2 equiv, and 1.2% with 4 equiv). A reaction temperature above 100 °C was required for reaction completion in
