Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Topologically Diverse Shapes Accessible by Modular Design of Arylopeptoid Macrocycles Thomas Hjelmgaard,§,† Lionel Nauton,‡ Francesco De Riccardis,∥ Laurent Jouffret,‡ and Sophie Faure*,‡ §
Department of Chemistry, Section for Chemical Biology and Nanobioscience, Faculty of Science, University of Copenhagen, Thorvaldsensvej 40, 1871 Frederiksberg C, Denmark ∥ Department of Chemistry and Biology, University of Salerno, Via Giovanni Paolo II n. 132, I-84084 Fisciano, SA, Italy ‡ Université Clermont Auvergne, CNRS, SIGMA Clermont, Institut de Chimie de Clermont-Ferrand, F-63000 Clermont-Ferrand, France S Supporting Information *
ABSTRACT: N-Substituted aromatic cyclooligoamides composed of different combinations of ortho-, meta-, and/or paraarylopeptoid residues carrying methoxyethyl side chains have been efficiently synthesized by macrocyclization of the corresponding linear oligomers. The study of the architectures of these macrocycles in solution and solid state has revealed that tetracyclic arylopeptoids adopt sequence-dependent shapes with different backbone amide conformations and side-chain orientations. Remarkably, despite the absence of intramolecular Hbonding ability, some of these arylopeptoid macrocycles show well-defined architectures in solution.
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acrocycles have, over the past decades, become a key molecular motif in drug discovery1 and nanotechnology.2 An extensive interest in cyclic peptides as potential therapeutic agents has emerged due to the undeniable advantages resulting from cyclization: higher metabolic stability, increased cell permeability, and conformational rigidity enhancing binding affinity and selectivity toward target biomolecules.3 A macrocycle could be regarded as a scaffold displaying functions or ligands for molecular interaction, or as a receptor to host various guests. For both of these uses, the shape, size, orientation, and flexibility of the macrocycle are crucial criteria to be taken into account. Cyclodextrins4 and calixarenes5 have for decades been the macrocycles of choice to address these issues. However, in recent years, other classes of interesting macrocyclic architectures have emerged.6 Among them, the aromatic oligoamide macrocycles take advantage of backbone rigidification induced by aromatic cores and intra- or intermolecular hydrogen bonds.7 Indeed, these types of macrocycles exhibit persistent ring-shapes usually leading to discrete or extended tubular assemblies.8 By contrast, examples of aromatic cyclooligoamides that do not capitalize on intramolecular H-bonding in their structuration remain rare. Two main types of N-substituted aromatic cyclooligoamides have been studied: the cyclic N-alkylated benzanilides9 and cyclophanamides (Figure 1).10 Notably, the cyclophanamides have shown promise as selective hosts or artificial enzymes, but their development may have been impeded by the low tunability of these scaffolds.10 Highly tailorable N-substituted aromatic © XXXX American Chemical Society
Figure 1. Structures of (a) cyclic N-alkyl benzanilides, (b) N-alkyl paracyclophanamides, (c) cyclic N-alkyl arylopeptoids, and (d) cyclic Nalkyl benzylopeptoids.
cyclooligoamides, macrocyclic arylopeptoids11 and benzylopeptoids,12 have been recently reported. Despite the additional backbone methylene compared to the benzanilide structure and the absence of NH−O type H-bonding ability, certain cycloarylopeptoids showed the propensity to adopt well-defined architectures in solution.11 This type of cyclooligoamides is ideally suited to access a large panel of topologically varied scaffolds for ligand display or molecular recognition. Herein, we present the synthesis of macrocycles with diverse sizes and shapes by combining N-substituted ortho-, meta-, and para-aminobenzamide subunits, and the study of their conformational behavior in solution and in the solid state. Received: November 24, 2017
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DOI: 10.1021/acs.orglett.7b03660 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
Information for details, Table S1). The optimization study prompted us to select the uronium salt COMU as coupling reagent. All the tetramers and hexamers were subjected to the optimized cyclization conditions after Boc removal with TFA, and the macrocycles formed were purified by flash chromatography (Table 1). The cyclization in the para-series was found to be problematic for short oligomers as previously observed for para-triarylopeptoids.11 Cyclization of pppp-1 gave rise to a mixture of cyclotetramer 13 and cyclooctamer 14 in ∼70/30 proportion under high dilution conditions (2.5 mM). No cyclodimerization was encountered in other series. For the meta-series, the cyclization of tetramer 3 gave the cyclotetramer 16 with lower yield (52%) than reported for the metatriarylopeptoids cyclization (>80%).11 It is important to note that the efficiency of the cyclization may be dependent on the sequence pattern of linear precursors. This issue was studied with the peptoids 5, 7, 6, and 8 of the mixed meta,para-series. The yield of cyclotetramer 18 was 10% higher when synthesized from mpmp-5 than from pmpm-7. Different yields were likewise obtained when accessing macrocycle 19. In this case, it was the pmpmpm linear sequence that provided the highest yield. The cyclotetramers or cyclohexamers derived from 5 and 7 or 6 and 8, respectively, provided superimposable NMR spectra indicating that the cyclization step had no influence on the conformational preference of the final macrocycle. The best cyclization yields were obtained in the meta,ortho-series. The cyclic tetramer 20 and the cyclic hexamer 21 were synthesized with 79% and 71% yields, respectively. Overall, ten macrocycles 13−23 were prepared with yields ranging from 47 to 79%. The first examination of 1H NMR spectra of the macrocycles in CDCl3 revealed coalescent signals for most of them, indicating a relative flexibility of these macrocycles in solution. However, spectra obtained for the cyclotetramers pppp-13, momo-20, and popo-22 were significantly different. Cyclotetramer pppp-13 displayed simplified 1H and 13C spectra indicative of a very fast conformational exchange in solution or a well-defined conformation with a high level of symmetry. The cyclodimeric species obtained during the pppp-1 macrocyclization are likely due to conformational restriction preventing the physical proximity of the extremities. Accordingly, it was not expected that the cyclotetramer pppp-13 would be highly flexible in solution. The absence of aggregation was checked by performing NMR at various concentrations (0.2, 1, and 5 mM, Figure S1). Finally, a variable temperature study revealed that a predominant well-defined conformation with a four-fold rotational symmetry exists in CDCl3 at room temperature (Figure S2). The four backbone amides should therefore be all cis or all trans. By contrast, 1H NMR spectra of pppp-13 in aprotic or protic polar solvents (CD3CN and CD3OD) was drastically different from the one obtained in CDCl3 (Figure S3 and S4). Two sets of signals now appeared to be present: one identical to those observed in CDCl3 and a second one. The pppp-13 easily crystallized from CH3CN to give crystals suitable for X-ray diffraction enabling structure determination (Figure 2a). The 28-membered ring pppp-13 belongs to the P21/c space group and adopts a centrosymmetric collapsed oval structure (approximately 13 × 4 Å) with a cis-trans-cis-trans conformation of the backbone amides. Accordingly, the solid-state structure was different from the one observed in CDCl3 for which all amides should adopt the same conformation. 1H NMR spectra of the cyclotetramer momo-20 revealed the coexistence of two conformations sufficiently stable to be observed by NMR at room temperature. However, coalescence was rapidly observed upon
The linear precursors were synthesized on solid-phase using a submonomer protocol, which does not need fastidious preparation of protected monomeric precursors and atomconsuming coupling reagents. 13 In brief, the aromatic oligoamides are constructed by an iterative acylation/substitution sequence using acid chlorides and primary amines as building-blocks (Scheme 1). Scheme 1. Solid-Phase Synthesis of Linear Precursors 1−12 and Macrocyclization
The initial attachment onto the 2-chlorotrityl chloride copoly(styrene−1% DVB) resin was performed using 3- or 4(chloromethyl)benzoic acid and diisopropylethylamine in CH2Cl2. The substitution step was carried out by reaction with excess of 2-methoxymethyl amine for 1 h at 50 °C. Microwave activation may be employed at this stage to decrease the reaction time.14 The acylation step was performed using 2-, 3-, or 4(chloromethyl)benzoyl chloride to access ortho-, meta-, or paraarylopeptoid units, respectively. The terminal amine was protected by a Boc group followed by cleavage from the resin using hexafluoroisopropanol. The crude product was purified by simple acidic washing to provide the linear arylopeptoid in sufficient purity for use in the ensuing steps without further purification. Twelve tetramers and hexamers 1−12 with different sequence patterns were obtained with good yields (56−75%) and HPLC purities ≥95% (Table 1). The optimization of the macrocyclization process was carried out using four selected linear arylopeptoids and the following coupling reagents: HATU, COMU, and PyBOP (see Supporting Table 1. Yields and Purities of Linear Arylopeptoids 1−12 and Macrocycles 13−23 linear precursor
macrocycle
sequencea
#
yield %b (purity %)c
#
yield % (purity %)c
pppp pppppp mmmm mmmmmm mpmp mpmpmp pmpm pmpmpm momo momomo popo popopo
1 2 3 4 5 6 7 8 9 10 11 12
59 (95) 56 (96) 67 (95) 65 (95) 72 (95) 60 (95) 65 (95) 56 (95) 75 (96) 73 (95) 67 (95) 65 (95)
13/14d 15 16 17 18 19 18 19 20 21 22 23
47/19e(>99) 62 (>97) 52 (>99) 60 (87) 73 (>99) 50 (98) 64 (>99) 61 (98) 79 (>98) 71 (>97) 56 (>99) 69 (98)
a p for para, m for meta, and o for ortho. bIsolated yield after aqueous workup. cMeasured by HPLC. d13, cyclotetramer; 14, cyclooctamer. e Reaction performed at 2.5 mM.
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DOI: 10.1021/acs.orglett.7b03660 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
(Figure S10), the 24-membered macrocycle mmmm-16 provided single crystals from CH3CN/Et2O. Crystal structure revealed a “chair-like” shape of the ring where the aromatic rings are pairwise parallel in an alternative manner (Figure 4).
Figure 2. Structures showing amide conformations and X-ray crystal structures of cyclotetramers (a) pppp-13 and (b) momo-20.
Figure 4. (a) Structure of mmmm-16 showing amide conformation (cis amide in blue and trans amide in red), (b) X-ray crystal structure, and (c) crystal packing.
heating (Figure S5). Crystallization of momo-20 by slow evaporation of CH3CN enabled the determination of the solidstate structure. This 22-membered ring belongs to the P21/c space group and adopts an extended zigzag arrangement of the backbone (rectangular shape of approximately 10 × 4.5 Å) with an all-trans conformation of the amides (Figure 2b). The four carbonyl groups and the four side-chains are roughly perpendicular to the mean plane of the ring with two consecutive groups pointing toward one face and the two others toward the opposite face. Remarkably, popo-22 adopted one privileged conformation in solution at room temperature according to 1H NMR spectra in CDCl3 and CD3CN (Figures 3a, S6, and S7).
A cis-trans-cis-trans conformation of the amides was observed and induced a close proximity of vicinal side-chains by the pair. Indeed, the distance observed between the two terminal methyl groups is approximately 4 Å instead of 8 or 12 Å in the crystal structure of pppp-13 and momo-20, respectively. In the crystal lattice, the molecules of mmmm-16 stack on each other along the a axis to form tubular assemblies with a rectangular cavity of approximately 4.5 × 8 Å (Figures 4c and S11). These tubular assemblies interact with each other to form a packing with a checkerboard pattern depending on the orientation of the cycles. Several weak interactions such as CH−O-type H-bonds and van der Waals contacts are observed between vicinal tubes and may stabilize the whole structure. This highly organized packing probably explains the crystallization ability of this macrocycle even though no particular conformational preferences were observed in solution. Unfortunately no single crystal could be obtained from the last cyclotetramer pmpm-18. However, on the basis of the existent crystallographic structures, we propose by molecular modeling a model for the pmpm system (see SI for details, Figure S12). A preliminary binding study performed by NMR using sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaTFPB)12,15 revealed different complexing properties of these cyclic tetraarylopeptoids (see Supporting Information for details, Figures S14−S17). While the cycles with larger cavities such as pppp-13 or pmpm-18 gave scarce or no complexation, the NMR of cycle momo-20 showed the formation of a major symmetric complex (Figure S17). However, the titration experiments revealed the formation of multiple sodiated species hampering the evaluation of the association constant of the major complex formed with momo-20 and the extraction of the complex stoichiometry. The conformationally rigid cycle popo-22 retained its conformation in the presence of increasing amounts of NaTFPB. The present study shows that topologically diverse shapes are easily accessible by modular design of arylopeptoid macrocycles. Conformational analysis in solution and solid state revealed that cyclic tetramers adopt sequence-dependent shapes and sidechain orientations. Remarkably, despite the absence of intramolecular H-bonding, some of these macrocycles showed a discrete conformation in solution. Although preliminary qualitative binding studies with sodium cation revealed weak complexing abilities, the synthesis of topologically defined
Figure 3. (a) 1H NMR spectra of popo-22 (5 mM) in CDCl3 (bottom curve) and CD3CN (upper curve) at 298 K, (b) structure of popo-22 showing backbone amide conformation determined by NMR, and (c) X-ray crystal structure of popo-22.
A combination of NMR experiments (COSY 1H/1H, HSQC, and HMBC 1H/13C) enabled the attribution of each signal of popo-22, thus allowing for the determination of the amide conformation by NOESY experiments (Figures S8 and S9). The macrocycle popo-22 appears to adopt a cis-trans-cis-trans conformation of the backbone amides (Figure 3b). Single crystals of compound popo-22 were obtained by slow evaporation of CDCl3. Consistent with solution NMR analysis, the macrocycle popo-22 adopts a cis-trans-cis-trans conformation of the backbone amides in the solid-state (Figure 3c). Unlike other cyclic tetraarylopeptoids, popo-22 crystallizes in a noncentrosymmetric orthorhombic system (space group P212121). Despite conformational heterogeneity observed in solution C
DOI: 10.1021/acs.orglett.7b03660 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
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artificial scaffolds based on cyclic arylopeptoids represents a notable leap toward the construction of prototypes with specific molecular domains and chemically addressable architectures.
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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.7b03660. Experimental procedures and characterization (PDF) Accession Codes
CCDC 1575313, 1575334, 1575349, and 1575351 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing
[email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Francesco De Riccardis: 0000-0002-8121-9463 Sophie Faure: 0000-0001-5033-9481 Present Address † Rockwool International A/S, Hovedgaden 584, Entrance C, 2640 Hedehusene, Denmark.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully thank the Carlsberg Foundation (Grant 2011_01_0432 to T.H.), A. Job (ICCF UMR CNRS 6296) for HPLC, DRX facilities of UCA-partner, and the Mésocentre Clermont Auvergne.
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DOI: 10.1021/acs.orglett.7b03660 Org. Lett. XXXX, XXX, XXX−XXX
