Helical Folding of Meta-Connected Aromatic Oligoureas - Organic

Letter pubs.acs.org/OrgLett

Helical Folding of Meta-Connected Aromatic Oligoureas Ting Hu,†,⊥ Alan L. Connor,‡,⊥ Daniel P. Miller,‡ Xiao Wang,† Qiang Pei,† Rui Liu,† Lan He,†,¶ Chong Zheng,§ Eva Zurek,‡ Zhong-lin Lu,*,† and Bing Gong*,†,‡ †

College of Chemistry, Beijing Normal University, 100875 Beijing, China Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, New York 14260, United States ¶ National Institute for Food and Drug Control, Institute of Chemical Drug Control, TianTanXiLi 2, Beijing, 100050, China § Department of Chemistry, Northern Illinois University, DeKalb, Illinois 60115, United States ‡

S Supporting Information *

ABSTRACT: Aromatic oligoureas composed of meta-linked residues bearing phenolic ether side chains are synthesized. The basic N,N′-diarylurea units adopt a trans,trans intramolecularly H-bonded conformation that is further strengthened by additional intermolecular H-bonding. Such basic units, in combination with meta-linked benzene residues, result in stably folded helical oligoureas in the highly polar DMF with up to four turns and with a small cylindrical inner pore that would be difficult to acquire.

F

oldamers have attracted wide interest since 1996.1 Various foldamers2 can be grouped into those having peptidomimetic1,3−7 or abiotic2c−e,8−13 backbones. An effective strategy for creating stable foldamers involves restricting the conformational freedom of backbones.2d,e,14 We have been interested in creating pore-containing foldamers.14 By incorporating three-center Hbonds,15 aromatic oligoamides are forced to adopt persistent shapes11e that contain nondeformable pores of 8 to >30 Å across. 2d,11b,14 Along with other void-containing foldamers2c,9,11f,g,12,13 and macrocycles having analogous backbones,16 our foldamers offer well-defined cavities and large aromatic surfaces that result in interesting self-assembly,17 molecular recognition,18 and other19 properties. An inherent limit of our oligoamide foldamers is that no pore smaller than ∼8.5 Å across, a size defined by oligomers composed of meta-linked benzene residues, can be created. Given the unusual mass-transport properties of subnanometer pores,19a,26 creating foldamers containing cylindrical pores of further reduced sizes could provide the structural basis for uncovering hitherto unknown properties and functions. Similar to the amide group, the urea linkage offers rigidity, planarity, and H-bonding capacity and has been adopted in the design of foldamers20 such as the ladder-type or pseudohelical N, N′-dimethyl-N, N′-diaryl urea oligomers,21 the peptidomimetic N, N′-linked aliphatic oligoureas,22 and N,N-linked oligourea templates.23,24 In addition, helical aromatic polyureas consisting of para-linked ureidophthalimide units were also reported.10a,b To create foldamers containing cavities with reduced sizes, we attempted to synthesize aromatic oligoureas shown by general structure A.10c Oligomers A with less than four benzene residues, prepared under rather harsh conditions, were found to adopt © 2017 American Chemical Society

curved conformations. Simple modeling showed that longer oligomers A may fold into helical conformations. However, repeated efforts to prepare longer A were impeded by the low reactivity of the electron-poor monomeric and oligomeric amines. As a result, helical aromatic oligoureas composed of meta-linked benzene residues have remained elusive.

We report herein oligoureas B, which can be regarded as being derived from A by replacing the ester side chains with phenolic ether groups. Oligomers B were designed to explore (1) the preparation of long oligomers capable of folding into multiturn helices, a feasible task given that the monomeric building blocks involve the highly activated dialkoxybenzenediamines; (2) the folding behavior of oligoureas B; and (3) the creation of small subnm pores with tunable lengths. Our studies indicate that oligoureas B with up to 15 residues can be obtained with excellent to good yields and adopt helical conformations with an inner pore of ∼5.5 Å across. For oligomers B to fold, each of the urea linkages should adopt the trans,trans conformation and be H-bonded with adjacent phenolic O atoms. However, it was not clear whether the S(5)Received: April 3, 2017 Published: May 5, 2017 2666

DOI: 10.1021/acs.orglett.7b01005 Org. Lett. 2017, 19, 2666−2669

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longer oligomers 5−10. All oligomers were obtained, after extensive purification, in unoptimized yields ranging from 85% to 34%. The efficient synthesis of oligomers 2−10 demonstrates the feasibility of introducing phenolic ether side chains that lead to highly active intermediate amines. The weak intramolecular Hbonding also enhances the conformational flexibility of the oligoureas, which exposes reactive termini for coupling. The crystal structure27 of C (Figure 2a) reveals a conformation consistent with that predicted by computation. Single crystals of

type H-bonds of B, which are much weaker than the S(6)-type Hbonds of A,30 would still allow the diarylurea units to adopt the desired conformation. The conformational preference of compound C, which corresponds to the basic unit of B, was examined computationally with hybrid density functional theory (DFT) (Figure 1, with

Figure 1. Relative energies of conformers C′ and C″ resulting from rotation around the OC−NH and NH−Ph bonds of C.

details in the Supporting Information). Rotation around the carbonyl−NH bond involves an energy barrier of 8.34 kcal/mol. The planar trans,cis conformer C′ sits at a local minimum that is 2.03 kcal/mol less stable than C (Figure S1a). Rotation around the NH−Ph single bond shows that (Figures 1 and S1b), starting from C, rotation for 0−40° results in a small increase (1.05 kcal/mol) in energy, indicating that the H-bonded ring can tolerate considerable distortion. A rapid increase (3.27 kcal/mol) of energy occurs between 40° and 80° due to the interruption of the H-bonded ring, followed by a smoother change (1.05 kcal/mol) between 90° and 120°. The steepest ascent of energy (5.04 kcal/mol) is from 140° to 180°, which is attributed to the rapidly growing repulsive interaction between the lone pairs of urea and methoxy O atoms. Planar C″, with its urea and methoxy O atoms being in the closest proximity, is the least stable conformer occupying a global energy maximum that is 12.06 kcal/mol less stable than C. Thus, the basic unit of oligoureas B favors the trans,trans H-bonded conformation.25 An oligourea composed of such basic units should adopt a globally folded conformation. The stepwise, bidirectional coupling route for preparing aromatic oligoureas 2 through 10 is shown in Schemes 1 and S1. Building blocks 1-2−1-4 were prepared from 1-1. Dimers 2 were prepared from 1-3 by treatment with triphosgene or isocyanate 1-4. Oligomers 3−10 were synthesized by starting from dimer 2a or monomer 1-2b, which, upon being reduced to the corresponding diamine, was treated with 1-4, leading to 4mer 4 or 3mers 3. Repeating the reduction and coupling steps led to

Figure 2. Crystal structures of (a) C, (b) 2c, and (c) 3b. Hydrogen atoms, except for those of the urea groups, are omitted for clarity.

2c and 3b were obtained by slow evaporation of solvent from DMSO/CH2Cl2 (3/7, v/v) and MeOH/CHCl3 (8/2, v/v), respectively, at room temperature. The crystal structures of 2c (Figure 2b) and 3b (Figure 2c) show that the same conformational preference of C is shared. The X-ray structures of 2c and 3b also provided bond lengths and angles that allow the helical conformation of a longer oligomer to be projected, which revealed a helix with four residues per turn and a hydrophilic cavity of ∼5.5 Å across (Figure S2) Interestingly, the crystal structure of 2c shows that the urea NH groups engage in additional H-bonding with the O atom of a DMSO molecule. An O atom of a water molecule is H-bonded to one of the urea moieties of 3b. In each case, the two otherwise weak S(5)-type H-bonds are converted into much more favorable15 three-center H-bonds. These energetically favorable three-center H-bonds seem to reinforce the rigidity of 2c and 3b, resulting in nearly planar conformations. The observed H-bonding between 2c or 3b with solvent molecules suggests that the folding of this class of oligomers may be promoted by solvents having strong H-bond donors, which is indeed the case. The CD spectra of pentamer 5 to 15mer 10 in DMF, a solvent with a strong H-bond acceptor, give very similar bisignate Cotton effects (Figure S3) that reflect exciton coupling interaction between chromophores placed nearby in a chiral environment,28 suggesting that these oligoureas fold into similar chiral conformations. The intensities of the CD signals of 5−10 increase with increasing oligomer length (Figure S3), indicating that the folding of these oligomers involves positive cooperativity. The CD spectra of 10 measured in DMF at different temperatures (Figure 3a) or concentrations (Figure S4) reveal Cotton effects of the same overall shape and with intensities that weaken nonlinearly with rising temperature (Figure 3b) and enhance linearly with increasing concentration (Figure S4). Surprisingly, in chloroform, these oligomers give very weak Cotton effects (Figure S5), implying they are largely unfolded. The CD spectra of 9mer 7 were then measured in different solvents (Figure S6). While there are no CD signals in methanol and THF, strong CD signals appear in the nonpolar CCl4 or toluene, in which variable-concentration and temperature CD

Scheme 1. Synthesis of Oligoureas 2−10

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DOI: 10.1021/acs.orglett.7b01005 Org. Lett. 2017, 19, 2666−2669

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residues, an oligomer needs eight or more residues to fold into a helix of two or more turns. The NOESY spectrum of 9mer 7 in DMF-d7 reveals numerous NOEs between the NH protons and those of their side chains (Figures 5 and S12), indicating a crescent backbone. NOEs

Figure 3. (a) CD spectra of 15mer 10 (10 μM) in DMF measured at different temperatures (path length = 1 cm). (b) Plot of θ322 vs temperature from −5 to +65 °C.

spectra of 9mer 7 (Figure S7) and 15mer 10 (Figure S8) reveal intense Cotton effects with the same shape as those in DMF, suggesting the presence of the same chiral structure in these distinctly different solvents. Plotting CD signals vs ratios of CHCl3 in CCl4 reveal sigmoidal curves, which reveals the cooperative nature for the folding of 7 and 10.29 In DMF, oligomers 5−10 do not aggregate, as shown by their well-dispersed 1H NMR peaks (Figure S9) at 0.5 mM. The line width and position of the 1H NMR resonances of oligomer 10 also remain unchanged from 0.5 to 10 mM in DMF-d7 (Figure S10). In contrast, the signals of 10 in CCl4/CDCl3 broaden with increasing proportion of CCl4 (Figure S11), indicating the occurrence of aggregation. The chemical shifts of protons located near (≤7 Å) an aromatic ring undergo an upfield shift,31 which indicates parallel stacking of aromatic rings.32 Oligomers 2a−10 were compared by examining protons H2, H4, and NH-1 in DMF-d7 (Figure 4). From dimer 2a

Figure 5. Diagnostic NOEs (red arrows) revealed by NOESY spectrum indicating the helical folding of 7. Except for those between protons g6 and the side chains (shown as R), other side chain NH NOEs are not indicated for clarity.

between protons e and b, and between e and c1, are also visible in the NOESY spectrum, which confirms the adoption of helical conformation in which these protons are close. The efficient synthesis of oligoureas 2−10 has overcome a long-standing obstacle and should be equally applicable for preparing longer oligomers. The H-bonded, trans,trans conformation of the basic units, along with meta-linked benzene rings, results in helical conformation for oligomers with five or more residues. Helical folding is demonstrated by CD spectra and confirmed with evidence including shifts of 1H resonances with oligomer length and NOEs between remotely located protons. The observed cooperative folding in both polar and nonpolar solvents is very intriguing and warrants further studies. These helically folded oligoureas contain hydrophilic pores with a subnanometer diameter. These helical aromatic foldamers, with their ready synthetic availability and tunability, provide subnanometer pores of tunable lengths and should lead to fundamental and practical impacts.

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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.7b01005. Additional figures and UV, CD, and NMR spectra; synthetic procedures; analytical data; computational details (PDF) X-ray data for compound 2c (CIF) X-ray data for compound 3b (CIF)

Figure 4. Chemical shifts of (a) NH-1 and (b) H2 and H4 of 2a, 3a, 4, 5, 6, 7, 8, 9, and 10 (0.5 mM/each) vs oligomer length.

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to pentamer 5, the resonances of protons H2 and NH-1 move upfield; from 5 to longer oligomers, protons H2 and NH-1 no longer exhibit a noticeable shift. In contrast, the shifts of H4 do not plateau until the oligomer length reaches eight residues. The observed trends can be attributed to the helical folding of these oligomers: with four residues per turn, it takes five or more residues for the terminal residues of an oligomer to stack. For proton(s) H4 on the central residue(s) to “feel” nonadjacent

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]ffalo.edu. ORCID

Daniel P. Miller: 0000-0003-1507-2667 Eva Zurek: 0000-0003-0738-867X 2668

DOI: 10.1021/acs.orglett.7b01005 Org. Lett. 2017, 19, 2666−2669

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Zhong-lin Lu: 0000-0001-5473-2889 Bing Gong: 0000-0002-4155-9965 Author Contributions ⊥

T.H. and A.L.C. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (91227109 and 21372032) and the US National Science Foundation (CHE-1306326 and CBET1512164). D.M. was funded by the US NSF (HRD-1345163). We acknowledge support from the Center of Computational Research at SUNY Buffalo.

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