Peptides and Glycine Derivatives Via Direct C–H Bond Functionalization

Site-specific C-functionalization of free-(NH) peptides and glycine derivatives via direct C–H bond functionalization Liang Zhao, Oliver Basle´ , and Chao-Jun Li1 Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec H3A 2K6, Canada. Edited by Jack Halpern, University of Chicago, Chicago, IL, and approved January 14, 2009 (received for review September 11, 2008) A copper-catalyzed ␣-functionalization of glycine derivatives and short peptides with nucleophiles is described. The present method provides ways to introduce functionalities such as aryl, vinyl, alkynyl, or indolyl specifically to the terminal glycine moieties of small peptides, which are normally difficult in peptide modifications. Further- more, on functionalization, the configurations of other stereocenters in the peptides could be maintained. Because the functionalized Fig. 1. C-functionalization of N terminus of peptides. peptides could easily be deprotected and carried onto the next coupling process, our approach provides a useful tool for the peptide- based biological research. globally in the year 2000 (29). Although the Strecker synthesis (30–32), the Ugi reaction (33–36) and the Petasis reaction (37–39) amino acid ͉ C–C bond formation ͉ peptide modification are important tools to construct arylglycine derivatives; direct arylation of glycine derivatives or glycine moieties in peptides would ecent advances in proteomics demands innovative methods to be more powerful in cases where the glycine moiety is already Rrapidly generate and modify peptides and amino acids. Direct present. Herein, we wish to report the detailed study of a general and site-specific modification of amino acids and peptides takes method for site-specific C arylation, vinylation, alkynylation, and CHEMISTRY advantage of the existing structure and provides a convenient way indolylation of ␣-C–H bonds of glycines and short peptides at the to generate large arrays of diverse amino acids and peptides for N terminus (Fig. 1). biomedical applications. For amino acid C modifications, known methods include: alkylation of ␣-carbanions (preformed by depro- Results and Discussions tonation with a strong base) (1–4), via radicals [␣-bromination by Alkynylation of Glycine Derivatives. To find a general method to N-bromosuccinimide (5, 6) or UV photolysis in the presence of modify natural amino acids rapidly, we need a reaction system that di-tert-butyl peroxide (7)], the Claisen rearrangements (8, 9), and can directly activate the ␣-C–H bonds of an amino amide with high the recently reported palladium-catalyzed arylation of an amide chemo- and regioselectivity. The design of our methodology is to (10–14). In the field of peptide synthesis, stepwise mounting of catalytically generate, in situ, an electrophilic glycine inter- amino acids via solution and solid phase techniques has been mediate, which can be intercepted by a nucleophile to form a prevalent ever since they were first developed (15, 16). In another ␣-functionalized glycine derivative. scenario, direct site-specific C-functionalization of peptides pro- By using N-PMP (p-methoxyphenyl) glycine amide derivatives as vides an ideal approach that takes advantage of the preexisting the amine substrate, phenylacetylene as the nucleophile, in the peptides and provides rapid access to diverse peptide libraries for presence of CuBr as catalyst, TBHP as oxidant, the coupling biological studies. Recently, by using enolate chemistry, O’Donnell reaction proceeded very well at room temperature. The best solvent (17–19) and Maruoka (3, 4, 20–22) reported an elegant method to was found to be dichloromethane; other nonchlorinated solvents introduce alkyl groups into activated N-terminal glycine unit of a such as THF, 1,6-dioxane, and toluene afforded low yields of the short-chain peptide. However, a general method for site-specifically coupling product (Table 1). Under the optimized conditions, var- introducing various functional groups, leading to more elaborated ious glycine derivatives were coupled with aromatic alkynes (Table functionalized peptides such as aryl peptides, vinyl peptides, or 2). Secondary (Table 2, entries 1, 2, 3, and 4) and tertiary (Table 2, alkynyl peptides, still does not exist. This is largely because of the entry 5) amides all reacted well. For the aromatic alkyne counter- insurmountable difficulty in distinguishing the ␣-C–H bonds of part, 4-ethynylbiphenyl (Table 2, entry 6), 1-bromo-4-ethynylben- each amino acid unit in peptides by using existing methods. zene (Table 2, entry 7), and 4-ethynyltoluene (Table 2, entry 8) all Recently, we discovered that the ␣-C–H bond of tertiary amines or afforded the corresponding products in good yields. However, glycine derivatives can be alkylated by using a copper-catalyzed 2-methoxyphenylacetylene (Table 2, entry 9) is less reactive than cross-coupling reaction (23–25). We also made the preliminary the other substrates, indicating that the steric hindrance on the observation that glycine amides could be alkynylated in the pres- alkyne retarded its reactivity. In the meantime, R1 being a substi- ence of glycine ester to alkynylated glycine amide (23). An inter- tuted amine moiety is also very important for the success of this esting and important nonproteinogenic class of amino acids is the transformation. When R1 was switched to an OEt group (Table 2, arylglycines. It has attracted more and more attention because the entry 10), the coupling reaction did not occur at all at room frequency of isolating arylglycines has increased rapidly in the past few decades. For example, vancomycin (26–28), which was the first glycopeptide antibiotic discovered, contains a heptapeptide in Author contributions: C.-J.L. designed research; L.Z. and O.B. performed research; L.Z. and which three of the amino acid residues are arylglycines. Besides that, C.-J.L. analyzed data; and C.-J.L. wrote the paper. arylglycines are important intermediates in the commercial pro- The authors declare no conflict of interest. duction of ␤-lactam antibiotics. Phenylglycine (ampicillin, cefa- This article is a PNAS Direct Submission. chlor) and p-hydroxyphenylglycine (amoxicillin, cefadroxil) are the Freely available online through the PNAS open access option. predominant representatives in this family. According to World 1To whom correspondence should be addressed. E-mail: [email protected]. Health Organization (WHO) data, ampicillin and amoxicillin to- This article contains supporting information online at www.pnas.org/cgi/content/full/ tally accounted for almost half of the ␤-lactam antibiotics produced 0809052106/DCSupplemental. www.pnas.org͞cgi͞doi͞10.1073͞pnas.0809052106 PNAS Early Edition ͉ 1of6 Downloaded by guest on October 2, 2021 *Reaction conditions: glycine derivative (0.10 mmol), alkyne (0.30 mmol), TBHP (18 ␮L, 5–6 M in decane), CuBr (0.01 mmol), CH2Cl2 (0.2 mL). †NMR yields using an internal standard. DCE, dichloroethane; DME, dimethoxyethane; THF, tetrahydrofuran; NP, no product. temperature; whereas switching R1 to a phenyl group (Table 2, peptides are more prevalent in nature and more important synthons entry 11) afforded a mixture of unidentified compounds. This in organic syntheses, we decided to focus on the arylation of indicates that R1 being a substituted amine moiety could probably peptides. Simple dipeptides (Table 5, entries 1–8) and tripeptides reduce the oxidation potential of the substrate and stabilize the (Table 5, entries 12–19) all reacted with arylboronic acids, affording imine intermediate being generated. the coupling products in good yields in most cases. The scope of arylboronic acids is very similar to the one examined for N-PMP Arylation of Glycine Derivatives. With the success of alkynylation, we glycine amide. A dipeptide (Table 5, entry 2) and tripeptides (Table then examined the C-functionalization with other nucleophiles. 5, entry 18 and 19) with an amino acid moiety other than glycine Among all of the examined nucleophiles, such as tributylphenyltin, also afforded the cross-coupling products. Interestingly, similar trimethylphenylsilane, and phenylboronic acid, only phenylboronic diastereoselectivities were observed when the preexisting chiral acid afforded the desired arylation product. With 10 mol% CuBr center is either one (Table 5, entry 2) or two (Table 5, entry18 and and 1.0 equiv TBHP in 1,2-dichloroethane (DCE), the arylation 19) amino acid units away from the N-terminal glycine moiety. reaction proceeded efficiently at 100 °C, affording the coupling To examine the scope of this method for peptide modifications, product in 75% isolated yield, using a slight excess of N-PMP glycine other nucleophiles such as phenylacetylene (Table 5, entry 9 and 20) amide (1.5 equiv, Table 3, entry 3). Other nonchlorinated solvents, and indole (Table 5, entry 10 and 21) were also tested. The coupling such as THF, 1,6-dioxane, or toluene, afforded low yields of the reactions went very well at conditions even milder than with coupling product (Table 3, entries 4–8). With this result in hand, we arylboronic acids. It should also be noted that all of the function- then briefly investigated the scope of this arylation reaction (Table alizations occurred exclusively at the N terminus of the peptides 4). Aryl boronic acids bearing electron-donating groups (Table 4, without any scrambling on other amino acid moieties. entries 2 and 5), a weak electron-withdrawing group (Table 4, entry 4), or a steric hindered functional group (Table 4, entry 3) all Importance of

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