Abstract
An efficient 2•3-component reaction (2•3CR; a 2-component reaction followed, in one pot, by a 3-component reaction) is presented for the synthesis of isoxazolino-β-ketoamides. This 2•3CR proceeds by (i) a Meldrum's acid-generated acyl ketene, which is trapped by an amine to form a β-ketoamide intermediate in a 2CR followed, in one pot, by (ii) a Mannich reaction followed by elimination of dimethyl amine•HCl to generate an α,β-unsaturated β-ketoamide dipolarophile that reacts in a nitrile oxide 1,3-dipolar cycloaddition reaction. This one-pot 2•3CR process delivers the targeted isoxazolino-β-ketoamide product. A total of 72 compounds are presented, all of which have been submitted to the NIH Molecular Libraries Small Molecule Repository for high-throughput biological screening.
Introduction
Multicomponent reactions offer an expedient route to complex targets1 since the combination of three or more reactants within a single reaction typically leads to a shorter reaction sequence and fewer purification steps.1 Moreover, multicomponent products are comparatively far more elaborate than their starting material counterparts and, by altering a single reactive component, a wide range of diversity can be achieved. For these reasons, multicomponent reactions, such as the Biginelli,2 Passerni,3 Hantzche,4 and Ugi5 reactions, have been widely used for the construction of combinatorial libraries.6 The development of new multicomponent reactions has broad implications in synthetic methodology, library production, and biological screening.
Isoxazolines have little precedence as pharmacophores. However, we have discovered that a class of isoxazolines function as a cystic fibrosis transmembrane regulator protein activator7 and others have previously shown this substructure to be bioactive in various other systems.8 As an under exploited heterocycle, isoxazolines could potentially find various new applications and serve as future synthetic targets. To address this possibility, expedient library synthesis is needed to enable the requisite random screening.
Isoxazolines are synthons for β-amino acids9 and γ-amino alcohols,10 which are abundant in natural products and medicinal compounds.11 Consequently, the diversity oriented synthesis of isoxazolines can have considerable significance. To this end, we present here the synthesis of a seventy-two membered library of isoxazolino-β-ketoamides prepared by a modified multicomponent reaction with the ability for easy diversification.
Results and Discussion
We recently reported a 4-component reaction (4CR) for the synthesis of isoxazolino-β-ketoamides12 as outlined in Scheme 1. This initial investigation was conducted as a proof-of-concept study for the development of a multicomponent macrocylization reaction, which we have now evolved into a general protocol for the synthesis isoxazolino-β-ketoamides. This procedure incorporates an acyl ketene precursor (1) and an amine (2) to form the β-ketoamide moiety, which then undergoes a Mannich reaction with Eschenmoser's salt (3) followed by elimination of dimethyl ammonium chloride. The resulting doubly activated Michael acceptor then serves as a dipolarophile in a nitrile oxide cycloaddition (⇒ 4) to, in one-pot, deliver the isoxazolino-β-ketoamide target (5). Our previously reported yields for this 4CR reaction ranged from 39 to 50%. Herein, we demonstrate modification and expansion of this versatile transformation.
Scheme 1.
4CR route to isoxazolino-β-ketoamides.
The combination of four reactants can lead to undesirable side-product formation and, consequently, lower isoxazolino-β-ketoamide yields. Despite these inherent limitations, our 4CR produces modest yields (39–50%) of the targeted product because of the high reactivity of the acyl ketene with the amine nucleophile. This high acyl ketene/amine reactivity effectively suppresses side reactions that form isoxazol-5(2H)-ones (6) and N-hydroxyimidamides (7). That said, we report here a modified method that delivers isoxazolino-β-ketoamides in both higher average yields and cleaner reactions – advancements that enable efficient library synthesis.
As delineated in Scheme 2, performing the multicomponent reaction as a 2•3CR (a 2-component reaction followed by a 3-component reaction), instead of a 4CR, results in isoxazolino-β-ketoamide yields up to 88% as well as excellent product purity. This 2•3CR was accomplished as a one-pot procedure by heating a DMF solution of the acyl ketene precursor in the presence of only the amine nucleophile, thus pre-forming the β-ketoamide in a 2CR. While it is known that dioxolinones and Meldrum's acids fragment thermally to acyl ketenes, it must be said that an alternative mechanism where the amine attacks first, followed by loss of CO2 and acetone, is possible. Subsequent addition of Eschenmoser's salt and chlorooxime reagents, to the same flask, results in a 3CR (collectively, a 2•3CR) that delivers the isoxazolino-β-ketoamide product. This 2•3CR modification essentially avoids both the chlorooxime + acyl ketene cycloaddition side reaction (→ isoxazol-5(2H)-ones; 6; Scheme 1) and the amine + chlorooxime side reaction (→ N-hydroxyimidamides; 7; Scheme 1), while proficiently effecting the desired transformation (→ isoxazolino-β-ketoamide; 5; Scheme 2).
Scheme 2.
Isoxazolino-β-ketoamides by a multicomponent (2•3CR) process.
In addition to the 2•3CR synthetic modification, we wanted to expand the scope of this reaction by utilizing an array of diverse reactants (Figure 1). A total of eight amines were used. Both aliphatic and aromatic amines worked well; however, yields dropped when aminothiazole, ethanolamine, or acetohydrazide where used. For the library reported here, three different acyl ketene precursors (p-methoxyphenyl,13 cyclopropyl,14 and methyl) as well as three different chlorooximes (o-pyridinyl,15o-trifluoromethylphenyl,16 and carboethoxy17) were used. We also demonstrate that these substituted acyl ketenes and chlorooximes tolerate the 2•3CR reaction conditions. Isolated yields for all variations are shown in Table 1.
Figure 1.
2•3CR diversity inputs.
Table 1.
Library of isoxazolino-β-ketoamides.
| Entry | Acyl Ketene | Chloro-Oxime | Amine | Product | Yield |
|---|---|---|---|---|---|
| 1 | {1} | {1} | {1} | 5{1,1,1) | 80% |
| 2 | {1} | {1} | {2} | 5{1,1,2) | 79% |
| 3 | {1} | {1} | {3} | 5{1,1,3} | 30% |
| 4 | {1} | {1} | {4} | 5{1,1,4} | 40% |
| 5 | {1} | {1} | {5} | 5{1,1,5} | 40% |
| 6 | {1} | {1} | {6} | 5{1,1,6} | 51% |
| 7 | {1} | {1} | {7} | 5{1,1,7} | 76% |
| 8 | {1} | {1} | {8} | 5{1,1,8} | 88% |
| 9 | {1} | {2} | {1} | 5{1,2,1) | 75% |
| 10 | {1} | {2} | {2} | 5{1,2,2) | 76% |
| 11 | {1} | {2} | {3} | 5{1,2,3} | 31% |
| 12 | {1} | {2} | {4} | 5{1,2,4} | 44% |
| 13 | {1} | {2} | {5} | 5{1,2,5} | 31% |
| 14 | {1} | {2} | {6} | 5{1,2,6} | 50% |
| 15 | {1} | {2} | {7} | 5{1,2,7} | 78% |
| 16 | {1} | {2} | {8} | 5{1,2,8} | 83% |
| 17 | {1} | {3} | {1} | 5{1,3,1) | 81% |
| 18 | {1} | {3} | {2} | 5{1,3,2) | 80% |
| 19 | {1} | {3} | {3} | 5{1,3,3} | 19% |
| 20 | {1} | {3} | {4} | 5{1,3,4} | 48% |
| 21 | {1} | {3} | {5} | 5{1,3,5} | 41% |
| 22 | {1} | {3} | {6} | 5{1,3,6} | 38% |
| 23 | {1} | {3} | {7} | 5{1,3,7} | 75% |
| 24 | {1} | {3} | {8} | 5{1,3,8} | 75% |
| 25 | {2} | {1} | {1} | 5{2,1,1) | 30% |
| 26 | {2} | {1} | {2} | 5{2,1,2) | 50% |
| 27 | {2} | {1} | {3} | 5{2,1,3} | 8% |
| 28 | {2} | {1} | {4} | 5{2,1,4} | 15% |
| 29 | {2} | {1} | {5} | 5{2,1,5} | 35% |
| 30 | {2} | {1} | {6} | 5{2,1,6} | 44% |
| 31 | {2} | {1} | {7} | 5{2,1,7} | 55% |
| 32 | {2} | {1} | {8} | 5{2,1,8} | 64% |
| 33 | {2} | {2} | {1} | 5{2,2,1) | 60% |
| 34 | {2} | {2} | {2} | 5{2,2,2) | 58% |
| 35 | {2} | {2} | {3} | 5{2,2,3} | 17% |
| 36 | {2} | {2} | {4} | 5{2,2,4} | 31% |
| 37 | {2} | {2} | {5} | 5{2,2,5} | 30% |
| 38 | {2} | {2} | {6} | 5{2,2,6} | 33% |
| 39 | {2} | {2} | {7} | 5{2,2,7} | 60% |
| 40 | {2} | {2} | {8} | 5{2,2,8} | 60% |
| 41 | {2} | {3} | {1} | 5{2,3,1) | 62% |
| 42 | {2} | {3} | {2} | 5{2,3,2) | 42% |
| 43 | {2} | {3} | {3} | 5{2,3,3} | 12% |
| 44 | {2} | {3} | {4} | 5{2,3,4} | 46% |
| 45 | {2} | {3} | {5} | 5{2,3,5} | 60% |
| 46 | {2} | {3} | {6} | 5{2,3,6} | 45% |
| 47 | {2} | {3} | {7} | 5{2,3,7} | 70% |
| 48 | {2} | {3} | {8} | 5{2,3,8} | 68% |
| 49 | {3} | {1} | {1} | 5{3,1,1) | 49% |
| 50 | {3} | {1} | {2} | 5{3,1,2) | 78% |
| 51 | {3} | {1} | {3} | 5{3,1,3} | 23 |
| 52 | {3} | {1} | {4} | 5{3,1,4} | 31% |
| 53 | {3} | {1} | {5} | 5{3,1,5} | 37% |
| 54 | {3} | {1} | {6} | 5{3,1,6} | 44% |
| 55 | {3} | {1} | {7} | 5{3,1,7} | 52% |
| 56 | {3} | {1} | {8} | 5{3,1,8} | 62% |
| 57 | {3} | {2} | {1} | 5{3,2,1) | 77% |
| 58 | {3} | {2} | {2} | 5{3,2,2) | 70% |
| 59 | {3} | {2} | {3} | 5{3,2,3} | 18% |
| 60 | {3} | {2} | {4} | 5{3,2,4} | 39% |
| 61 | {3} | {2} | {5} | 5{3,2,5} | 35% |
| 62 | {3} | {2} | {6} | 5{3,2,6} | 43% |
| 63 | {3} | {2} | {7} | 5{3,2,7} | 54% |
| 64 | {3} | {2} | {8} | 5{3,2,8} | 62% |
| 65 | {3} | {3} | {1} | 5{3,3,1) | 76% |
| 66 | {3} | {3} | {2} | 5{3,3,2) | 62% |
| 67 | {3} | {3} | {3} | 5{3,3,3} | 9% |
| 68 | {3} | {3} | {4} | 5{3,3,4} | 34% |
| 69 | {3} | {3} | {5} | 5{3,3,5} | 69% |
| 70 | {3} | {3} | {6} | 5{3,3,6} | 32% |
| 71 | {3} | {3} | {7} | 5{3,3,7} | 63% |
| 72 | {3} | {3} | {8} | 5{3,3,8} | 40% |
Considerable functional group diversity is represented in this library in which product molecular weights range from 272 to 496. Manipulation of the acyl ketene-, amine-, and chlorooxime-derived R-groups allow for dramatic alteration of the isoxazolino-β-ketoamide three-dimensional shape. Together, the ease of altering functional groups, molecular weights, and molecular geometry with this 2•3CR-based methodology allows for the rapid assembly of a library of biological relevant isoxazolino-β-ketoamides.
Conclusion
We have successfully synthesized a library of seventy-two isoxazolino-β-ketoamides with novel 2•3CR-based methodology. This library effectively expands the scope of our multicomponent methodology and demonstrates its application in diversity-oriented synthesis. Compared to its 4CR variant, this one-pot 2•3CR method delivers the targeted scaffold with much improved yields – as much as 38% in side-by-side comparative reactions. The compounds reported here have been deposited in the NIH Molecular Libraries Small Molecule Repository for high-throughput biological screening and assay results for these molecules and similar analogs will likely follow.
Supplementary Material
Acknowledgements
The authors thank the National Institutes of Health (GM0891583 and RR1973) and the National Science Foundation [CHE-0910870; and CHE-0443516, CHE-0449845, CHE-9808183, and DBIO 722538 for NMR spectrometers] for their generous support.
Footnotes
Supporting Information Available Detailed experimental procedures, characterization data, and 1H and13C NMR spectra of 20 representative compounds. This material is available free of charge via the Internet at http://pubs.acs.org.
References
- 1).(a) Trost BM. Atom economy – a challenge for organic synthesis: homogeneous catalysis leads the way. Angew. Chem., Int. Ed. 1995;34:259–281. [Google Scholar]; (b) Noyori R. synthesizing our future. Nat. Chem. 2009;1:5–6. doi: 10.1038/nchem.143. [DOI] [PubMed] [Google Scholar]; (c) Bienayme H, Hulme C, Oddon G, Schmitt P. Maximizing synthetic efficiency: multi-component rransformations lead the way. Chem. Eur. J. 2000;6:3321–3329. doi: 10.1002/1521-3765(20000915)6:18<3321::aid-chem3321>3.0.co;2-a. [DOI] [PubMed] [Google Scholar]; (d) Posner GH. Multicomponent one-pot annulations forming 3 to 6 bonds. Chem. Rev. 1986;86:831–844. [Google Scholar]; (e) Boger DL, Desharnais J, Capps K. Solution-phase combinatorial libraries: modulating cellular signaling by targeting protein-protein or protein-DNA Interactions. Angew. Chem., Int. Ed. 2003;42:4138–4176. doi: 10.1002/anie.200300574. [DOI] [PubMed] [Google Scholar]
- 2).(a) Biginelli P. The urea-aldehyde derivatives of acetoacetic esters. Ber. 1891;24:2962–2967. [Google Scholar]; (b) Biginelli P. The urea-aldehyde derivatives of acetoacetic esters. Gazz. Chim. Ital. 1893;23:360–416. [Google Scholar]
- 3).Passerni M. Isonitriles. II. Compounds with aldehydes or with ketones and monobasic organic acids. Gazz. Chim. Ital. 1921;51:181–189. [Google Scholar]
- 4).Hantzch A. Synthesis of pyridine derivatives from acetoacetic esters and aldehydeammonia. Liebigs Ann. Chem. 1882;215:1–82. [Google Scholar]
- 5).(a) Ugi I, Meyr R, Fetzer U, Steinbruckner C. Studies on isonitriles. Angew. Chem. 1959;71:386. [Google Scholar]; (b) Ugi I, Steinbruckner C. Concerning a new condensation principle. Angew. Chem. 1960;72:267–268. [Google Scholar]; (c) Ugi I. The α-addition of immonium ions and anions to isonitriles coupled with secondary reactions. Angew. Chem. 1962;74:9–22. [Google Scholar]
- 6).(a) Ugi I, Domling A, Werner B. Since 1995 the new chemistry of multicomponent reactions and their libraries, including their heterocyclic chemistry. J. Hetero. Chem. 2000;37:647–658. [Google Scholar]; (b) Ugi I. The improved search of new products by multicomponent reactions and their libraries. Lat. Kim. Z. 1999;1:47–58. [Google Scholar]; (c) Ugi I. Fast and permanent changes in preparative and pharmaceutical chemistry through multicomponent reactions and their `libraries.'. Eesti. Tead. Akad., Keemia. 1995;44:237–273. [Google Scholar]; (d) Bienayme H, Hulme C, Oddon G, Schmitt P. Maximizing synthetic efficiency: multi-component transformations lead the way. Chem. Eur. J. 2000;6:3321–3329. doi: 10.1002/1521-3765(20000915)6:18<3321::aid-chem3321>3.0.co;2-a. [DOI] [PubMed] [Google Scholar]
- 7).Sammelson RE, Ma T, Galietta LJV, Verkman AS, Kurth MJ. 3-(2-Benzylocyphenyl)isoxazoles and isoxazolines: synthesis and evaluation as CFTR activators. Bioorg. Med. Chem. Lett. 2003;13:2509–2512. doi: 10.1016/s0960-894x(03)00482-7. [DOI] [PubMed] [Google Scholar]
- 8).(a) Del Corral JMM, Gordaliza M, Castro MA, Lopez-Vazquez ML, Garcia-Gravalos MD, Broughton HB, San Feliciano A. Bioactive isoxazoline and oxime derivatives from 7-Ketolignans. Tetrahedron. 1997;53:6555–6564. [Google Scholar]; (b) Cheng C, Yu P, Li Z, Shu J, Wang X. Synthesis of 2-methyl-3-(4-methoxyphenyl)-5-(3-methoxyphenyl)isoxazoline and its bioactivity. Youji Huazue. 2010;30:833–836. [Google Scholar]
- 9).(a) Fuller AA, Chen B, Minter Aaron, R., Mapp AK. Succinct synthesis of β-amino acids via chiral isoxazolines. J. Am. Chem. Soc. 2005;127:5376–5383. doi: 10.1021/ja0431713. [DOI] [PubMed] [Google Scholar]; (b) Sewald N. Synthetic Routes toward enantiomerically pure β-amino acids. Angew. Chem. Int. Ed. 2003;42:5794–5795. doi: 10.1002/anie.200301692. [DOI] [PubMed] [Google Scholar]; (c) Minter AR, Fuller AA, Mapp AK. A Concise approach to structurally diverse β-amino acids. J. Am. Chem. Soc. 2003;125:6846–6847. doi: 10.1021/ja0298747. [DOI] [PubMed] [Google Scholar]
- 10).(a) Tang S, He J, Sun Y, He L, She X. Efficient and regioselective synthesis of 5-hydroxy-2-isoxazolines: versatile synthons for isoxazoles, β-lactams, and γ-amino alcohols. J. Org. Chem. 2010;75:1961–1966. doi: 10.1021/jo1000065. [DOI] [PubMed] [Google Scholar]; (b) Jaeger V, Buss V, Schwab W. Syntheses via isoxazolines. III. Diastereoselective synthesis of γ-amino Alcohols with 2 and 3 chiral centers. Tetrahedron Let. 1978;34:3133–3136. [Google Scholar]
- 11.(a) Tirel PJ, Vaultier M, Carrie R. Ligand assisted hydride delivery: an expeditious stereoselective total synthesis of (±)-Norsedamine and its pyrrolodino analog. Tetrahedron Lett. 1989;30:1947–1950. [Google Scholar]; (b) Dondoni A, De Lathauwer G, Perrone D. A convergent synthesis of the renin inhibitor SPP-100 using a nitrone intermediate. Tetrahedron Lett. 2001;42:4819–4823. [Google Scholar]; (c) Feske BD, Stewart JD. Chemoenzymatic formal total synthesis of (−)-Bestatin. Tetrahedron Asym. 2005;16:3124–3127. [Google Scholar]; (d) Aggarwal VK, Roseblade S, Alexander R. The use of enantiomerically pure ketene dithioacetal bis(sulfoxides) in highly diastereoselective intramolecular nitrone cycloadditions. Application in the total synthesis of the p-amino acid (−)-Cispentacin and the First asymmetric synthesis of the cis-(3R,4R)-4-aminopyrrolidine-3-carboxylic acid. Org. Biomol. Chem. 2003;1:684–691. [PubMed] [Google Scholar]; (e) Jiang J, Schumacher K, Joullie AA, Davis FA, Reddy RE. Approaches toward the total syntheses of isatins A, B, and C. Tetrahedron Lett. 1994;35:2121–2124. [Google Scholar]
- 12).Knapp JM, Fettinger JC, Kurth MJ. Multicomponent macrocyclization reactions (MCMRs) employing highly reactive acyl ketene and nitrile oxide intermediates. Org. Lett. 2011;13:4732–4735. doi: 10.1021/ol202024a. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 13).Yamamoto Y, Watanabe Y, Ohnishi S. 1,3-Oxazines and related compounds. XIII. Reactions of acyl Meldrum's acids with Schiff bases giving 2,3-disubstituted 5-acyl-3,4,5,6-tetrahydro-2H-1,3-oxazine-4,6-diones and 2,3,6-trisubstituted 2,3-dihydro-1,3-oxazin-4-ones. Chem. Pharm. Bul. 1987;35:1860–1870. [Google Scholar]
- 14).Beyer WRC, Woithe K, Luke B, Schindler M, Antonicek H, Scherkenbeck J. Asymmetric total synthesis of the indole alkaloid cyclopiazonic acid and first structure-activity data. Tetrahedron. 2011;67:3062–3070. [Google Scholar]
- 15).Grob JE, Nunez J, Dechantsreiter MA, Hamann LG. Regioselective synthesis and slow-release Suzuki-Miyaura cross-coupling of MIDA boronate-functionalized isoxazoles and triazoles. J. Org. Chem. 2011;76:10241–10248. doi: 10.1021/jo201973t. [DOI] [PubMed] [Google Scholar]
- 16).Guggenheim KG, Butler JD, Painter PP, Lorsbach BA, Tantillo DJ, Kurth MJ. Synthesis of spiro-fused pyrazolidoylisoxazolines. J. Org. Chem. 2011;76:5803–5812. doi: 10.1021/jo200924y. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 17).Kozikowski AP, Tapadar S, Luchini DN, Kim KH, Billadeau DD. Use of the nitrile oxide cycloaddition (NOC) reaction for molecular probe generation: a wew class of enzyme selective Histone Deacetylase inhibitors (HDACIs) showing picomolar activity at HDAC6. J. Med. Chem. 2008;51:4370–4373. doi: 10.1021/jm8002894. [DOI] [PMC free article] [PubMed] [Google Scholar]
Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.