Letter Cite This: Org. Lett. 2018, 20, 2526−2529
pubs.acs.org/OrgLett
Submonomer Strategy toward Divergent Solid-Phase Synthesis of α‑ABpeptoids Ganesh A. Sable, Kang Ju Lee, Min-Kyung Shin, and Hyun-Suk Lim* Department of Chemistry and Division of Advanced Material Science, Pohang University of Science and Technology (POSTECH), Pohang 37673, South Korea S Supporting Information *
ABSTRACT: A novel submonomer solid-phase synthetic method for α-ABpeptoid oligomers is reported. Iterative submonomer coupling and Fukuyama−Mitsunobu alkylation enable facile, divergent synthesis of α-ABpeptoid oligomers substituted with chemically diverse side chains in excellent yields.
bulky α-chiral side chains are able to fold into characteristic three-dimensional structures such as polyproline-type helical structures.35−38 In addition, Olsen and co-workers demonstrated that β-peptoids possessing α-chiral side chains also exhibit well-defined helical conformations.39 Recently, we have developed oligomers of N-substituted β2homoalanines as a new class of peptoids termed α-ABpeptoids (alpha-alkyl beta-peptoids).40 α-ABpeptoids have chiral methyl groups at α-positions and thus have backbone chirality unlike general peptoids (Figure 1). We anticipated that the chiral methyl groups play a similar role to those in peptoids with chiral side chains, enabling α-ABpeptoid oligomers to adopt folding structures. Indeed, oligomers of α-ABpeptoids carrying (R)-methyl groups displayed characteristic CD spectra with strong minimum at 200−210 nm and a weak maxima at 220− 230 nm, suggesting the formation of folded structures. αABpeptoid oligomers harboring (S)-methyl groups showed mirror images of CD spectra to the (R)-forms of oligomers, indicating that (R)- and (S)-forms of α-ABpeptoid oligomers form identical folding conformations of opposing handedness and the formation of such ordered structures is conferred by the backbone chirality. Of note, it is possible to create a vast library of α-ABpeptoids by introducing many structurally diverse alkyl groups (R) as side chains. In contrast, it is difficult to construct a chemically diverse library of peptoids with α-chiral side chains because there are only a few α-chiral methyl-containing primary amines. Indeed, synthesis of a large combinatorial library of peptoids with chiral side chains has not yet been reported. Collectively, α-ABpeptoids could find a wide range of applications in biomedical and material sciences. For example, a combinatorial library of structurally diverse αABpeptoids could be a rich source of protein ligands. However, our previous synthetic method is laborious and inefficient (Scheme 1). In this approach, α-ABpeptoid oligomers are
P
eptidomimetic foldamers are synthetic oligomers that can fold into well-defined conformations, thereby mimicking three-dimensional folding structures of natural proteins.1−3 They are generally resistant to proteolytic degradation unlike native peptides. Moreover, it is possible to generate far more structurally diverse peptidomimetics by incorporating many different proteogenic and nonproteogenic side chains. Due to their excellent proteolytic stability and structural diversity, unnatural peptidomimetic foldamers could serve as functional alternatives to native folding peptides. Over the past decades, many different classes of peptide-like foldamers have been developed, such as β- and γ-peptides,4−8 oligopyrrolidines,9,10 γ-AApeptides,11 α-aminoxy acids comprising oligomers,12 oligoureas,13,14 oligocarbamates,15 azapeptides,16,17 oligotriazoles,18 and peptoids.19−22 Among them, peptoids possess several desirable features as peptidomimetics, including ease of synthesis23 and resistance to proteolysis.24,25 Notably, due to the lack of amide protons, peptoids are shown to have relatively good cell permeability in contrast to other peptidomimetics having amide protons in their backbones.26 Owing to these important advantages, peptoid frameworks are emerging as a versatile class of peptidomimetics showing various applications.19,20,27−34 However, peptoids do not generally form folding structures, as they are devoid of backbone chirality and amide protons in their backbone structures unlike native peptides (Figure 1). Notably, it has been proven that α-peptoids substituted with
Figure 1. General structures of α-peptides, α- and β-peptoids, peptoids with α-chiral side chains, and α-ABpeptoids. © 2018 American Chemical Society
Received: February 25, 2018 Published: April 16, 2018 2526
DOI: 10.1021/acs.orglett.8b00661 Org. Lett. 2018, 20, 2526−2529
Letter
Organic Letters
Next, we examined whether this monomer unit could be used for solid-phase submonomer synthesis of α-ABpeptoids (Scheme 2b). The compound (S)-2 was loaded on Rink amide MBHA resin by a peptide coupling condition using HATU/ HOAt/DIPEA in DMF to give 3. For N-alkylation on a nosyl (Ns)-protected secondary amine of 3 for the introduction of side chains, we employed the Fukuyama−Mitsunobu condition that allows for convenient, direct N-alkylation of amine using various alcohols (Table 1).41−47 To demonstrate the feasibility
Scheme 1. (a) Previous Method for the Synthesis of Monomer Building Blocks 1; (b) Previous Method for the Synthesis of α-ABpeptoid Oligomers
Table 1. Condition Optimization for the Fukuyama− Mitsunobu Alkylation
synthesized by assembling alkylated 1 as monomer building blocks (Scheme 1b). In order to generate a library, each monomer building block 1 possessing different side chains should be prepared individually by a relatively long, six-step solution-phase synthesis (Scheme 1a). This synthetic procedure is not efficient (15% overall yield), and furthermore, each reaction step requires a cumbersome purification process by chromatography, thus not being amenable to large library synthesis. Herein, we report a facile strategy for divergent synthesis of structurally diverse α-ABpeptoid libraries using a submonomer solid-phase method. To circumvent the impediments of the previous method,40 we developed a submonomer strategy for the convenient solidphase synthesis of α-ABpeptoids as depicted in Scheme 2.
entrya
R−OH
1 2 3 4 5 6 7 8 9 10 11 12
benzyl 2-naphthylmethyl isobutyl 2-2-diphenylethyl benzyl 2-naphthylmethyl isobutyl 2-2-diphenylethyl benzyl 2-naphthylmethyl isobutyl 2-2-diphenylethyl
a b
time
temp
solvent
convb (%)
12 12 12 12 8 8 12 12 6 6 8 8
rt rt rt rt 37 37 37 37 55 55 55 55
THF THF THF THF THF THF THF THF toluene toluene toluene toluene
100 100 50 20 100 100 80 65 100 100 100 100
h h h h h h h h h h h h
°C °C °C °C °C °C °C °C
For all conditions, DIAD (10 equiv) and PPh3 (10 equiv) were used. Determined by analytical reversed-phase HPLC of crude products.
of this on-resin dehydrative coupling reaction, benzyl alcohol was first used. Treatment of 3 with benzyl alcohol in the presence of PPh3 and DIAD in THF at 37 °C, followed by Nsdeprotection, afforded the desired N-benzylated product 4 in quantitative yield (Figure S1). The coupling and alkylation reactions were repeated to synthesize oligomers of desired length (2-mer to 8-mer). Finally, the synthesized benzylated oligomers 5a−g were cleaved from resin by using trifluoroacetic acid (TFA). The released products were analyzed by HPLC and MS or MALDI-TOF MS for their identity and purity (SI, Table S2). HPLC analysis for the crude products demonstrated that our solid-phase submonomer synthesis was remarkably efficient, generating N-benzylated oligomers in high yields (65−92%), irrespective of the chain length without major byproducts (Figure 2 and Figure S3). The crude products were purified by preparative HPLC for further studies (SI, Figure S4). To further explore the efficiency of the solid-phase submonomer method, we synthesized a series of α-ABpeptoid oligomers using different alcohols such as 2-naphthylmethanol, 2,2-diphenylethanol, and isobutanol. When using 2-naphthylmethanol, oligomers substituted with naphthylmethyl side chains (2-mer to 6-mer) were successfully prepared under the same reaction conditions used for the synthesis of N-benzylated oligomers (Table 2 and Table S2). However, when using isobutanol or 2,2-diphenylethanol, the on-resin Fukuyama−Mitsunobu reaction was slow and incomplete. After testing several reaction conditions, we found that elevating the reaction temperature up to 55 °C in toluene resulted in complete alkylation to obtain N-alkylated products 4 (Table 1 and Figure S1). Using this condition, α-ABpeptoid oligomers with various side chains were efficiently synthesized, demonstrating the robustness of our submonomer solid-phase route (Figure 2, Table 2, Table S2, and Figures S3, S4).
Scheme 2. (a) Synthesis of Nosyl-Protected 3-Amino-2methylpropanoic Acid (S)-2 as a Submonomer Unit; (b) Submonomer Route for the Synthesis of α-ABpeptoid Oligomers Using On-Resin Fukuyama−Mitsunobu Alkylation
Initially, we synthesized (S)-2-methyl-3-((4-nitrophenyl)sulfonamido)propanoic acid (S)-2 as the submonomer unit by nosyl protection of (S)-3-amino-2-methylpropanoic acid (Scheme 2a). Acid (R)-2 was also prepared by the same procedure starting from (R)-3-amino-2-methylpropanoic acid (Scheme 2a). This single step reaction provided the nosylprotected monomer in a good yield (78%) without racemization (Supporting Information (SI), Table S1). 2527
DOI: 10.1021/acs.orglett.8b00661 Org. Lett. 2018, 20, 2526−2529
Letter
Organic Letters
Figure 3. CD data. (a) CD spectra of N-benzylated α-ABpeptoid oligomers 5a−g (2-mer to 8-mer). (b) CD spectra of N-naphthylated α-ABpeptoid oligomers 6a−e (2-mer to 7-mer). (c) CD spectra of Ndiphenylethylated α-ABpeptoid oligomers 7a−e (2-mer to 7-mer).
Figure 2. HPLC chromatograms of crude α-ABpeptoid oligomers 5g, 6e, and 7e.
Table 2. Synthesized α-ABpeptoid Oligomers: Sequence, Purity, and Mass Confirmation
compd
R
n
% puritya
calcd mass [M + H]+
obsd massb [M + H]+
5b 5c 5d 5e 5f 5g 6d 6e 7d 7e
Bn Bn Bn Bn Bn Bn Naph Naph Dpe Dpe
3 4 5 6 7 8 5 6 5 6
92 82 81 74 66 65 80 80 62 60
542.33 717.43 892.53 1067.62 1242.72 1417.82 1142.60 1367.72 1342.76 1607.91
543.3 718.4 893.5 1068.6 1244.6 1419.7 1143.5 1390.8c 1343.7 1630.9c
In addition, we also investigated the CD of N-naphthylated oligomers (6a−e). Interestingly, the CD spectra for this series of oligomers exhibited completely different features from those of benzylated oligomers (5a−g). The trimer or longer oligomers of N-naphthylated α-ABpeptoids displayed a strong positive band at 220 nm, a negative band at near 230 nm, and the weak maxima at near 236 nm (Figure 3b). The CD spectra closely resemble those of N-naphthylated β-peptoids, which exhibit intense minima at 224−228 nm, maxima at 215−218 nm, and a slightly positive peak at 232 nm,39 suggesting that Nnaphthylated α-ABpeptoid oligomers might adopt ordered folding structures. The CD spectrum N-diphenylethylated oligomers (7a−e) had different patterns compared to other series. Oligomers with more than three residues (7b−e) showed two maxima at 196−200 and ∼ 210 (Figure 3c). The CD spectra of this family of α-ABpeptoids are similar to those of peptoids with α-chiral side chains.49 Overall, α-ABpeptoid oligomers with different side chains showed distinctive CD spectra, suggesting the presence of ordered folding conformations. Further studies, including X-ray and NMR studies, are underway to obtain the precise structures of α-ABpeptoids. In conclusion, we have developed a facile submonomer solidphase strategy for the synthesis of α-ABpeptoid oligomers. This method uses nosyl-protected 3-amino-2-methylpropanoic acid as a readily accessible submonomer and highly efficient on-resin Fukuyama−Mitsunobu alkylation for introducing structurally diverse side chains. Therefore, this approach greatly facilitates the divergent synthesis of α-ABpeptoid oligomers without the need for labor-intensive synthesis and purification of individual monomer building blocks. Moreover, given the high efficiency and robustness, this strategy would enable the generation of vast combinatorial libraries of α-ABpeptoids aiming at the identification of novel modulators of a given target protein.
a
Determined by analytical reversed-phase HPLC of crude products. Mass spectrometry data were acquired using ESI techniques. cMass data acquired using MALDI [M + Na]+. b
Recently, Morimoto and co-workers have reported a submonomeric synthetic method using reductive amination for the synthesis of oligomers of N-substituted β3-homoalanines.48 Although this elegant method is convenient, the synthetic efficiency is not sufficient (20−30% yields for pentamers) for constructing large combinatorial libraries, particularly for lengthy oligomers. In contrast, our approach enables the preparation of α-ABpeptoid oligomers up to 8-mer in excellent yields (60−80%), highlighting that our submonomer solid-phase synthetic method is suitable not only for preparation of relatively long oligomers but also for vast combinatorial library construction. We then obtained circular dichroism (CD) spectra of the synthesized oligomers. As expected, α-ABpeptoid oligomers substituted with the benzyl group (5a−g) or/and isobutyl group (8a−f, 9a−c) showed characteristic CD signals with a strong positive ellipticity at near 208 nm, which were identical in shape and intensity to those of the same molecules prepared by the previous method (Figure 3a, Figures S5 and S6).
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b00661. 2528
DOI: 10.1021/acs.orglett.8b00661 Org. Lett. 2018, 20, 2526−2529
Letter
Organic Letters
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Experimental details, synthesis and characterization of all new compounds, NMR spectra, CD spectra, and HPLC data (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hyun-Suk Lim: 0000-0003-4083-2998 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Samsung Research Funding Center of Samsung Electronics (SRTF-BA1402-13).
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DOI: 10.1021/acs.orglett.8b00661 Org. Lett. 2018, 20, 2526−2529
