Karolina Pulka-Ziach

Stéphanie Antunes

Camille Perdriau

Brice Kauffmann

Morgane Pasco

Céline Douat

Gilles Guichard

A post-elongation strategy for the introduction of guanidinium units in the main chain of helical oligourea foldamers

établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

Aliphatic N,N'-linked oligoureas 1 are a class of peptidomimetic oligomers which adopt a regular helical conformation stabilized by a network of H-bonds closing 12 and 14-membered pseudorings. 2 These non-peptide α-helicomimetic foldamers 3 possess specific advantages for possible (bio)applications, such as high folding fidelity and stability to enzymatic proteolysis. 4 Oligoureas do share high sequence modularity and synthetic accessibility with synthetic peptides. 5 Variation of substitution patterns and the introduction of isosteric backbone modifications are two complementary approaches to generate different spacing and projection of side chains at the surface and fine tune the properties of the helix. We have previously reported heterogeneous urea-based backbones containing various proportion of urea bond surrogates including amide (A), carbamate (C), 6,7 and thiourea (T) 8 units and have shown that the folding propensity can be controlled by the ratio between urea (U) and isosteric units and by their relative distribution in the sequence (Figure 1a).

To further modulate the molecular recognition and biological properties of oligourea helices, we have now considered the replacement of the neutral urea linkage by a guanidinium (G) unit at selected positions in the sequence (Figure 1b). The guanidinium moiety is protonated over a wide range of pH and displays many useful features for molecular recognition in competitive solvents, serving as H-bond donor, charge pairing agent and being capable of cation-π-stacking interactions.

It is a major functional group in proteins and natural products, 9 a useful pharmacophore in medicinal chemistry and its physicochemical properties have been largely exploited in supramolecular chemistry for the design of anion receptors and sensors, [10][START_REF] Houk | Anion Sensing[END_REF][START_REF] Blondeau | [END_REF][13] and in organocatalysis. 14 Guanidinium-rich molecules ranging from peptides to dendrimers and artificial oligomers efficiently enter living cells and have been widely explored as molecular transporters. [15][16][17][18][19][20] Oligomeric guanidines, i.e. backbones with N,N'-bridged guanidine units have also attracted interest as ligands of protein surfaces 21 and as abiotic folded systems. 22 For example, aromatic oligomeric N,N'-dimethyl-N,N'-diphenyl guanidines have been reported to fold into well-defined layered structures akin to that of corresponding aromatic oligoureas. Aliphatic oligomers with guanidinium backbones have also been developed as oligonucleotide [23][24][25][26] and peptide 27,28 mimics.

Several solid-phase synthesis approaches to access aliphatic oligomeric guanidines have been reported and the most effective methods are based on the use of arylsulfonyl (e.g. Pbf: 2,2,4,6,7-Pentamethyldihydrobenzofuran-5-sulfonyl), and alkoxycarbonyl (e.g. Fmoc: 9fluorenylmethyloxycarbonyl, Cbz: carbobenzoxyl and Troc: trichloroethoxycarbonyl) activated thiourea monomers. 27,[29][30][31] In these studies, the guanidylation reaction generally involves the transformation of the thiourea moiety into a carbodiimide by treatment with EDC, Mukaiyama reagent or HgCl 2 in the presence of TEA and subsequent reaction with the terminal amine of the growing chain. 32 This general approach and the use of N-((2-azidoalkyl)-Pbf-thioureas 27 as activated monomers are compatible with the solid-phase synthesis of aliphatic N,N'-linked oligourea foldamers on TFA-sensitive resins 5 and well-suited for the preparation of cognate urea/guanidine hybrid oligomers. Yet, this approach requests the preparation of specific activatedthiourea building blocks and the preparation of highly labile arylsulfonyl isothiocyanate and we were more interested by the possibility to introduce the guanidinium moiety at a later stage in the synthesis by using urea/thiourea hybrid foldamers 8 as advanced intermediates (Figure 1b).

Following exploration of reaction conditions and optimization in solution using model oligomers,

we have extended the approach to solid-phase synthesis and applied it to a longer oligomer carrying multiple functionalized side chains. In this work, we also report 1 H-NMR and CD (circular dichroism) data about the conformational preferences of the resulting urea/guanidinium (U/G) hybrids in direct comparison with the cognate urea/thiourea (U/T) and urea oligomers.

RESULTS AND DISCUSSION

Guanidinylation reaction of urea/thiourea hybrid oligomers in solution.

Short-chain N-tertbutyloxycarbonyl protected dimer 1a and tetramer 2a were initially selected as model urea/thiourea hybrid oligomers (TU and UUTU sequences, respectively) to test the best conditions to transform the thiourea linkage into the corresponding guanidinium. The oligomers were synthesized in solution using activated building blocks BB1 and BB2 as described previously (Figure 2 and see general procedure for details). We performed the guanidinylation reaction by a two-step strategy (Rathke synthesis 34 ) involving the S-methylation of the thiourea linkage by reaction with iodomethane (CH 3 I) to form the Smethylthiouronium iodide salt followed by a direct nucleophilic substitution in presence of the desired amine (Scheme 1). [START_REF] Touati-Jallabe | For an application to peptide macrocylization[END_REF] Initial efforts were dedicated to the formation of N,N'-disubstituted guanidinium derivatives 5 (Scheme 1a). Dimer 1a was thus first S-methylated with CH 3 I to form intermediate 6 and subsequent substitution reaction was carried out in 7.0 N ammonia in CH 3 OH as reaction medium with heating at 40-45 °C during the whole reaction process. The progress of the reaction was monitored by reversed phase HPLC (RP-HPLC) and showed complete disappearance of dimer 1a. However, RP-HPLC analysis of the crude product after reaction workup revealed the presence of two main products in a ratio of 33:67 that were subsequently isolated by silica gel chromatography. After careful structural characterization, it appeared that under such conditions the major product was the cyclic N,N',N"-guanidine derivative 7 (Scheme 1a, Table 1 and see supporting information for detailed characterization). A similar ratio of both the expected guanidine derivative 8 and the corresponding cyclic guanidine 9 as the main product was obtained when the reaction conditions were applied to tetramer 2a (Scheme 1b, Table 1 and see supporting information). At this stage, we assumed that the heterocyclic guanidine products formed because the N' nitrogen of the neighbouring urea moiety was sufficiently nucleophilic in basic conditions to quickly react with the newly formed S-methylthiouronium intermediate. This hypothesis was further confirmed by reacting S-methylthiouronium 10 with DIPEA as base in pure MeOH (Scheme 1b).

After 24 h, there was no more trace of 10 in the reaction mixture and cyclic guanidine 9 was observed as the main product (see supporting information for details).

Scheme 1. Synthesis of N,N'-disubstituted and N,N',N"-trisubstituted guanidinium oligomers from oligo(thio)urea oligomers 1a , 2a and 3.

A similar outcome was observed when attempting to prepare the corresponding N"-substituted guanidine derivatives with n-propylamine as the nucleophile. The cyclic guanidine 9 was obtained as the major product when starting from tetramer 2a in MeOH (Table 1, entry 3). The influence of the nature of the solvent on this guanidinylation reaction was next evaluated and we found that the ratio between cyclic and linear products could be reversed in pure acetonitrile. Under these conditions, the desired linear N,N',N''-trisubstituted guanidinium compound 11 was formed as the main product (Scheme 1b, Table 1, entry 4). However, the reaction in acetonitrile was much slower than in methanol and required larger excess of the primary amine to convert the starting thiourea into the N,N',N''-trisubstituted guanidinium derivative. This guanidinylation reaction in acetonitrile was repeated with a 0.5M ammonia solution in 1,4-dioxane. The expected linear N,N'-disubtituted guanidinium compound 8 was obtained as the main compound (Table 1, entry 5), which confirms the strong influence of the solvent on the outcome of the reaction. The linear to cyclic product ratio was further improved by using THF as a co-solvent (Table 1, entry 6). These conditions were finally applied to known hexamer 3 8 (scheme 1c) and the corresponding linear guanidinium derivative 12 was obtained together with the cyclized product 13 with a 75:25 ratio (

Guanidinylation of γ-amino acid-containing oligo(thio)urea substrates.

In order to avoid possible intramolecular cyclization leading to the incorporation of a heterocyclic guanidine in the foldamer backbone, (Scheme 1), we thought to substitute the urea residue (U unit) preceding the thiourea moiety by the corresponding γ 4 -amino acid derivative (A unit). Indeed, U and A units are isostructural and it has been shown previously that oligo(urea/γ-amide) hybrids display similar helical folding propensities compared to homooligoureas. 6 The corresponding dimer 1b and tetramer 2b equipped with thiourea and amide linkages (TA and UUTA sequences, respectively)

were synthesized in solution according to previously reported procedures. 6,8 X-ray diffraction analyses of single crystals of 4-mer hybrid 2b provided detailed information on the structural consequences of the local U  A replacement in 2. As shown in Figure 3a, the structure of 4-mer 2b is helical and compares well with the canonical 2.5-helical structure reported for the cognate U 4 homooligourea foldamer. 7 Root-mean square deviations (RMSDs) calculated for the backbone asymmetric C atoms (CH(R)) of the two molecules highlights the correlation between the two structures (RMSD for residues 1-4 = 0.129 Å). It is noteworthy that the intramolecular Hbond network is maintained with the thiocarbonyl group pointing towards the negative pole of the helix macrodipole similar to what was observed in longer helical oligo(thio)urea hybrids. These results provide further evidence that the combination of a -amino acid residue and a thiourea unit (TA sequence) at the first two positions of the sequence of oligoureas is tolerated and has limited influence on the overall helix geometry in the crystal.

The chemical transformation of the thiourea moiety into a guanidininum was next evaluated on dimer 1b and tetramer 2b (Scheme 2). Direct conversion of the S-methylthiouronium intermediate 14 derived from 1b in 7N ammonia in CH 3 OH gave the expected guanidinium-containing compound 15. However, RP-HPLC analysis of the crude product indicated that the guanidinylation product 15 was contaminated by several by-products and after flash silica gel chromatography dimer 15 was recovered in only 20% yield (Table 2, entry 1).

Scheme 2. Two-step guanidinylation reaction of -residue-containing substrates 1-2b and 4.

Much better results were obtained when the guanidinylation reaction was conducted in a non protic solvent like CH 3 CN. The reaction of S-methyluronium derivative of 2b with either 0.5M ammonia in 1,4-dioxane or n-propylamine as nucleophile in CH 3 CN gave the expected guanidinium tetramers 16 and 17 in 55% or 46% yield respectively after purification by silica gel chromatography (Table 2, entries 3 and 4). The reaction was carried out at 40-45°C; b The amine was added in several portions depending on the progress of the reaction.

These results motivated us to test these optimized conditions on a longer and potentially more folded oligomer. The selected hexamer 4 equipped with a -Leu at first position (sequence UUUUTA, Figure 2) and composed of the two triads with Val, Ala, Leu side-chains was synthesized in solution (see general procedure for details). Single crystals suitable for X-ray diffraction studies were obtained and as shown in Figure 3b Guanidinylation reaction on solid support. We next applied this post-elongation guanidinylation method to the solid support with the aim to speed the access to more functionalized and biologically relevant sequences. We have recently reported an efficient solid phase methodology to prepare oligo(thio)ureas on TFA-labile resins. 36 This optimized SPS protocol was applied here to synthesize a resin-bound 8-mer oligo(thiourea/amide/urea) hybrid on Rink-amide resin (20 R ) as a direct precursor for the guanidinylation steps (Scheme 3). This hybrid sequence derives from that of an antibacterial amphiphilic cationic oligourea sequence 4 The resin-bound methylthiourea 21 R was initially treated with 0.5 N ammonia in 1,4-dioxane under the conditions optimized in solution to access to 22. However, it appeared that the ammonia solution was not sufficiently concentrated to ensure conversion without an extended heating.

Conversely, complete conversion to the resin-bound guanidinium 22 R was obtained by treatment of 21 R with a saturated solution of ammonia gas in THF for 48 hours at 50°C. The purity of the crude product after TFA cleavage was estimated to be ≈ 66% by HPLC analysis. The expected guanidinium oligomer 22 was finally isolated in 17% overall yield (20 steps) after semi-preparative RP-HPLC.

Effects on the conformation of thiourea  guanidinium replacements

The consequences on the helical conformation of oligo(thio)urea hybrids of discrete thiourea  The chemical shift variation () of  CH 2 of urea units was measured as a mean to further evaluate the effect of the guanidinium replacement on folding. It is now well-established 2,33 that when placed in a helical environment, the  CH 2 protons of U units exhibit a high degree of anisochronicity ( splitting) with  reaching values generally above 1 ppm. The Δδ values for urea residues in 18 and 19 were found to be almost similar to those already reported for the cognate homo-urea 6-mer (U 6 ) 8 and 4 suggesting that the helical structure running from residues 3-6 is not particularly affected by the introduction of the G and A units at positions 2 and 1. [START_REF]methylene protons in G and T units strongly differ by their electronic environment and hence their anisochronicity cannot be directly compared[END_REF] (Figure 5, bottom). Further insight into the relative folding propensity of 4 and 18 at the residue level was gained by comparing hydrogen/deuterium (H/D) exchange data of backbone urea and amide protons (see Figures S96-99

and Tables S10 and S11 in supporting information). The first observation is that urea protons of central residues (NH and N'H of Val U3 , Leu U4 and N'H of Ala U5 ) exchange at slow rates in both molecules (k ex ≤ 10 × 10 -3 min -1 ) supporting the view that folding from residue 3-6 is not affected by the T  G replacement. As expected, terminal urea protons (N'H of Val U6 and NH of Ala U5 ) and Boc-NH proton which are exposed to the solvent exchange at much higher rate in both molecules.

In contrast urea NH of the second residue was found to exchange at faster rate in 18 compared to 4

(72 × 10 -3 min -1 vs 11 × 10 -3 min -1 ). The same holds true for the terminal amide proton which exchanges at a rate of 275 × 10 -3 min -1 in 4, but too fast for the rate to be determined in 18. This is further supported by the observation that the amide NHMe proton resonance which appears ≈8 ppm in 4 is upfield shifted upon guanidinium replacement. These results confirm that folding is largely maintained over the first four urea-based residues but that the folding propensity of residues neighbouring the G units is reduced or eliminated. NMR evaluation of compounds 12 and 22 revealed a similar trend whereby the guanidinium moiety affects the helical character of the preceding residues in the sequence more significantly than those that follow. For example, a fall of anisochronicity at Leu U1 is observed in 12 compared to 3, whereas the anisochronicity at Leu U4 , Ala U5 and Val U6 remains largely unchanged after the T  G replacement (Figure 5, top).

anisochronicities at U units in 22 were quite revealing, showing that residues in i±2 relationship with the G unit (namely Trp U3 and Lys U7 ) are comparatively more affected than other residues in the sequence (see supporting information). This drop of anisochronicity can be explained by a significantly reduced folding propensity of the guanidinium moiety and concomitant loss of canonical intramolecular H-bonds. In contrast, the marginally weaker anisochronicity values at residues 2, 6 and 8 suggests that the overall geometry of these residues is maintained on this face of the helix. Although ECD has been used extensively to characterize helix formation among homoligoureas, 33 its potential utility here, is complicated by the presence of multiple chromophores (ureas, thioureas and amides). 6,8 Nevertheless, ECD analysis can provide qualitative information about the ability of modified oligomers to retain the typical signature (maximum of positive molar ellipticity [θ] ≈ 203 nm and a trough of weaker intensity at ≈ 188 nm) attributable to 2.5-helix formation. The ECD spectra of compounds 4, 18 and 19 were measured at a concentration of 0.2 mM in 2,2,2trifluoroethanol (TFE) and compared to that of the corresponding homooligourea U 6 8 (Figure 6).

CONCLUSIONS

The present work further expands the range of urea surrogates that can be introduced in the backbone of aliphatic oligourea foldamers to modulate their properties. While some of our earlier work focused on amide (A), carbamate (C) and thiourea (T) isosteric units, we have now exploited the reactivity of thioureas to introduce guanidinium linkages (G) at a late stage in the synthesis and possibly convert oligo(thio)ureas into the corresponding guanidinium/urea oligomers in a two-step procedure. One limitation of this approach was the competition with an intramolecular backbone cyclization that resulted in the formation of oligomers with heterocyclic guanidinium. Although this side reaction could be minimized by using moderately polar and non protic solvents, we found that the insertion of an isostructural -amino acid residue before the thiourea linkage was a simple alternative to cleanly install the guanidinium group in the sequence. By capitalizing on an efficient solid-phase synthesis of oligo(thio)ureas, we have adapted the guanidinylation procedure to solid support enabling fast elaboration of urea-based oligomers containing backbone guanidiniums.

Conformational studies in solution by NMR and ECD associated with XRD analyses confirm that the combination of a thiourea unit and a -amino acid residue and (T-A pair) in oligomeric precursors is compatible with helical folding with little deviation from the canonical 2.5-helical structure of oligoureas. Although spectroscopic characteristics of partially folded structures remain in oligoureas containing a guanidinium moiety, folding is significantly disrupted by the guanidinium replacement. A future goal of our research is to characterize the structures of oligo(urea/guanidinium) foldamers at atomic resolution using X-ray diffraction. It could also be of interest to evaluate how the nature of the counterion of the guanidinium linkage may affect the propensity for the oligomer to fold or unfold. [39][40][41][42][43][44] Another development of this work facilitated by the compatibility with SPS techniques will be to evaluate the impact of such backbone modifications on the biological activities of foldamers in terms of recognition of biological surfaces and also cell penetration. Encouraging results have recently been obtained with derivatives of membrane-active antibacterial oligoureas containing various surrogate units including guanidinium. 37 EXPERIMENTAL SECTION General procedures. All solvents and reagents were obtained from commercial sources. Rink amide MBHA resin (loading 0.52 mmol/g) was purchased from Merck-Millipore. Fmoc--Lys(Boc)-OH and benzotriazole-1-yl-oxy-tris-(dimethylamino)-phosphonium hexa-fluorophosphate (BOP) reagent were purchased from PolyPeptide Laboratories France. Solid phase peptide synthesis grade organic solvents (DMF, DCM) were used for solid phase synthesis and were purchased from Carlo Erba. RP-HPLC-quality acetonitrile (CH 3 CN) and MilliQ water were used for RP-HPLC analyses and purification. Anhydrous solvents were dispensed from a solvent purification system (DCM, THF). Thin layer chromatography (TLC) was performed on silica gel 60 F254 (Merck) with detection by UV light and charring with 1% ninhydrin in ethanol followed by heating. Flash column chromatography was carried out on silica gel (40-63 μm, Merck). Chemical shifts are reported in parts per million (ppm, δ) relative to the 1 H or 13 C residual signal of the deuterated solvent used. 1 H NMR splitting patterns with observed first-order coupling are designated as singlet (s), broad singlet (bs), doublet (d), triplet (t), or quartet (q). Coupling constants (J) are reported in hertz.

Analytical RP-HPLC analyses were performed on a Dionex U3000SD using a Macherey-Nagel Nucleodur column (4.6 × 100 mm, 3 μm) at a flow rate of 1 mL.min -1 . The mobile phase was composed of 0.1% (v/v) TFA-H 2 O (Solvent A) and 0.1% TFA-CH 3 CN (Solvent B). The detection was performed at 200 nm and the column temperature in the oven was 25°C. Semi-preparative purifications of oligoureas were performed on a Dionex U3000SD system using a Macherey-Nagel Nucleodur column (100-6 C18ec, 10  250 mm, 5 μm) at a flow rate of 4 mL.min -1 . The mobile phase was similar as for the analytic system, unless otherwise notified.

Activated (S)-succinimidyl-{2-{[(tert-butoxy)carbonyl]amino}-2-X-ethyl}carbamate monomers

BB1 and BB3 for the introduction of urea bonds and activated (S)-tert-butyl (1-(1Hbenzo [d][1,2,3]triazole-1-carbothioamido)propan-2-yl)carbamate BB2 used for the introduction of thiourea unit were prepared from corresponding N-Boc protected ethylene diamine derivatives using recently reported procedures. 5,8,33 Compound 3 was synthesized as described previously. 8 Solid-phase oligourea synthesis was performed manually under microwave irradiation (vide infra)

on the Discover® System from CEM (CEM MWaves S.A.S., Orsay, France) using open reaction vessels and internal fiber optic temperature control.

Urea Formation in solution: General Procedure I. Boc-protected oligourea (1.0 equiv.) was dissolved in TFA (3 mL/g) cooled down in an ice/water bath and the reaction mixture was stirred at room temperature for 50 min under N 2 atmosphere. The reaction mixture was concentrated under reduced pressure, and the resulting residue was co-evaporated three times with cyclohexane. The crude product was then dissolved in CH 3 CN (5 mL/1.5 mmol). DIPEA (3.0 equiv.) was added, and the mixture was cooled down to 0 °C prior to the dropwise addition of the following activated carbamate BB1 (1.1 equiv.), dissolved in CH 3 CN (5 mL). The pH of reaction must be basic, so if required additional portion of DIPEA was added. After completion of the reaction, the reaction mixture was evaporated under reduced pressure, dissolved in EtOAc, and washed with 1 M KHSO 4 solution, saturated solution of NaHCO 3 and brine. The organic layer was then dried over Na 2 SO 4 and evaporated under reduced pressure. Flash column chromatography (CH 2 Cl 2 -MeOH (v/v), 98:2 to 95:5) over silica gel or trituration with Et 2 O gave the expected compound.

Thiourea Formation: General Procedure II. Boc-protected oligourea (1 equiv.) was treated with TFA (3 mL/g) cooled down in an ice/water bath and the reaction mixture was stirred at room temperature for 50 min under N 2 atmosphere. The reaction mixture was then concentrated under reduced pressure, and the resulting residue was co-evaporated three times with cyclohexane. The crude product was dissolved in CH 3 CN (5 mL/1.5 mmol). DIPEA (3.0 equiv.) was then added, and the mixture was cooled down to 0 °C prior to the dropwise addition of the thiocarbamoylbenzotriazole derivative BB2 (1equiv.) dissolved in CH 3 CN-CH 2 Cl 2 (85:15, 7mL).

The pH of reaction should be around 8, so if required additional amount of DIPEA was added. The Boc-Ala T -Leu-NHMe (1b). 1b was prepared according to procedure II starting from 25 (1.44 g, 5.3 mmol) and activated monomer BB2 (1.73 g, 5.4 mmol) and was obtained as a white foam (1.5 g, 69%). Guanidinylation reaction with 7N NH 3 in CH 3 OH: General procedure IV. The thiouronium iodide oligomer (0.14 mmol in 2 mL) was dissolved in a 7N ammonia solution in methanol, the flask was tightly closed with a septum and the reaction mixture was stirred at 40-45°C. The monitoring of the reaction was performed by RP-HPLC analysis. After 24-48 hours the reaction media was concentrated by evaporation under reduced pressure and the crude mixture was directly separated by silica gel column chromatography or semi-prep HPLC. In all cases, cyclic and/or linear substituted guanidiniums were formed.

S-methylation reaction

Compounds 5 and 7. 5 and 7 were prepared from thiouronium iodide salt 6 according to the general procedure III and were isolated in a ratio of 33/67 after silica gel flash column chromatography (CH 2 Cl 2 -CH 3 OH (v/v), from 98:2 to 95:5).

Compound 5 was recovered as a yellowish film in 11% yield (4 mg).

Circular dichroism

Circular dichroism (CD) experiments were performed on a Jasco J-815 spectrometer. Data are expressed in terms of the total molar ellipticity [deg.cm 2 .dmol -1 ]. CD spectra of the oligomers (0.2 mM) were acquired in spectrograde trifluoroethanol between 180 and 260 nm by using a rectangular quartz cell with a path length of 1 mm. To reduce the signal-to-noise ratio all spectra were recorded on an average of four consecutive scans.

Crystallographic data

See the supporting information for details. CCDC-1563646 and CCDC-1563645 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from

The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

ASSOCIATED CONTENT

*Supporting Information

Additional experimental procedures and characterization data of all compounds, supplemental

Figure 1 .

 1 Figure 1. a) Urea-based unit in aliphatic N,N'-linked oligoureas and corresponding A, C, G, and T units as urea bond surrogates; b) Schematic representation of the polar helical conformation of oligo(thio)urea foldamers and illustration of the thiourea  guanidinium replacement.

Figure 2 .

 2 Figure 2. Sequences of target (thio)urea-based hybrid oligomers 1-4 for investigating guanidinylation reaction in solution. Formulae of activated building blocks BB1 and BB2 used to prepare oligomers 1, 2 and 4. Oligomer 3 was described previously.8

Figure 3 .

 3 Figure 3. Crystal structures of (a) -residue-containing oligo(thio)urea 2b, (b) hexamer 4, and (c) structural alignment of the crystal structures of the hexamer 4 (light grey) with a cognate urea 6-mer (U 6 ) 8 (slate blue), the side-chains have been omitted for clarity reasons, calculated RMSD = 0.150 Å. Only one of the two independent molecules present in the asymmetric unit of the crystal of 4 is represented.

  (see supporting information for details), oligomer 4 adopts a well-defined helical structure in the crystal. An overlay with the known structure of the corresponding 6-mer homooligourea U 6 8 by fitting the six pairs of main chain asymmetric C atoms indicates a very close match (RMSD = 0.150 Å) between the two helices (Figure 3c). Conversion of 6-mer 4 to the corresponding N,N'-di-or N,N',N"-trisubstituted guanidiniumcontaining oligomers 18 and 19 by treatment of its S-methylthiouronium salt with either 0.5 M ammonia solution in 1,4-dioxane or n-propylamine in CH 3 CN was straightforward (Scheme 2c and

Scheme 3 .

 3 Scheme 3. Post elongation and on-resin approach to introduce guanidinium linkages in urea-based foldamers. (a-f) Synthesis of the resin bound 8-mer oligo(thiourea/amide/urea) hybrid 20 R ; (g) Smethylation of 20 R and (h) guanidinylation reaction of 21 R into 22 R . The RP-HPLC chromatograms ( = 200 nm) and ESI spectra correspond to the crude products isolated after each step (i.e. crude 20, 21 and 22) following micro-TFA-cleavage of few resin beads.

  guanidinium replacements were investigated by NMR and ECD. We concentrated on oligomers 12, 18, 19, and 22 which differ by the relative position of the backbone guanidinium in the sequence.The guanidinium unit is located close to the C-terminus in compounds 18 and 19, whereas it occupies a more central position in 12 and 22. NMR spectra of all compounds and their oligo(thio)urea precursors were recorded in the range 1-3 mM in CD 3 CN (or CD 3 OH) and proton resonances were assigned by using combination of homonuclear COSY, TOCSY, ROESY and heteronuclear 1 H-13 C HSQC 2D experiments (see supporting information). As shown in Figure4for compounds 4, 18 and 19, thiourea and guanidinium NHs generally appear as broad signals and at higher frequencies compared to the urea NHs. Of note, the urea NH resonances of the two guanidinium-containing 6-mers 18 and 19, are not significantly affected by the T  G replacement and display qualitative features typical of a well-folded conformation : dispersion over a large range of chemical shifts (5.0-6.8 ppm); large vicinal coupling constants with β CH(R) protons(6-10 Hz) and a strong differentiation between vicinal coupling constants with diastereotopic -methylene (  CH 2 ) protons of chiral diamine units.

Figure 4 .

 4 Figure 4. Part of 1 H NMR spectra (300 MHz) of oligomers 4, 18-19 in CD 3 CN (3mM) at 298 K, showing the NH region. The urea NHs signals are spread between 6.8 and 5.0 ppm. Thiourea NHs annotated with a star, guanidinium NHs annotated with a circle, amide NHMe annotated with a triangle.

Figure 5 .

 5 Figure 5. Anisochronicity at urea residues in oligo(thio/amide)urea 3 and 4 as well as related guanidinium hybrids 12, 18, and 19. Ratios to the values measured for the cognate oligourea U 6 8 are reported.

Figure 6 .

 6 Figure 6. ECD spectra of oligomers 4, 18 and 19 compared to that of the cognate homo-oligourea U 68 (all spectra were recorded in TFE at 0.2 mM).

4 [

 4 reaction mixture was left to come back to RT and stirred overnight. CH 3 CN was then evaporated under reduced pressure and the crude mixture was dissolved in EtOAc, washed twice with 1 M KHSO 4 solution, saturated solution of NaHCO 3 , water and brine, dried over MgSO 4 and evaporated TOF) m/z: [M + Na] + Calcd for C 30 H 61 N 9 O 5 NaS 682.4414; Found 682.4431. C 18 RP-HPLC (10-100% B in 10 min): t R = 9.29 min. Boc-γ-Leu-NHCH 3 (25): Boc-γ-Leu-OH (1.4 g, 5.4 mmol) was suspended in CH 3 CN (50 mL) and cooled in ice/water bath. After addition of DIPEA (3.76mL, 21.6 mmol) all the suspension was dissolved. HBTU (2.46 g, 6.48 mmol) was added and the reaction mixture was stirred for 10 min. at 0°C. Methylamine hydrochloride (0.73 g, 10.8 mmol) was added in one portion. The reaction mixture became green and after 10 min. yellow. The reaction was left to reach RT and magnetic stirring was maintained overnight. CH 3 CN was evaporated and the residue was dissolved in EtOAc (80 mL), washed with 1M KHSO 4 (5  35 mL), NaHCO 3 (3  35 mL, emulsion), brine (1  35 mL), dried over Na 2 SO 4 . The crude product was directly engaged to the next step without purification. Yellowish solid; (1.44g, 98% (crude, purity >95%)), 1 H NMR (CDCl 3 , 300 MHz) δ 6.46 (bs, 1H), 4.30 (d, J = 9.7, 1H), 3.71-3.59 (m, 1H), 2.85 (d, J = 4.8, 3H), 2.25 (t, J = 6.9, 2H), 1.92-1.81 (m, 1H), 1.75-1.54 (m, 1H), 1.48 (s, 9H), 1.38 -1.21 (m, 2H), 0.93 (d, J = 6.7, 3H), 0.92 (d, J = 6.5, 3H); t R = 6.93 min. (10-100% B in 10 min); ESI-MS (+): 273.2 [M+H] + , 545.2M+H] + .

:

  General procedure III. To a solution of oligo(thio)urea (0.35 mmol in 20 mL) in a mixture of CH 3 CN/THF (1:1, v/v) was added CH 3 I (10 equiv.). The flask was tightly closed with a septum and the mixture was heated to 40-45°C. The monitoring of reaction was performed by RP-HPLC analysis. The reaction was completed after 2-5 hours. The solvent was evaporated and the crude material was directly engaged to the following step without any further purification.

6. 37 (

 37 d, J = 7.0 Hz, 1H), 6.26 (dd, J = 9.7 Hz, 1.8, 1H), 5.92 (d, J = 10.2 Hz, 1H), 5.73 (d, J = 10.3 Hz, 1H), 5.53 (dd, J = 6.9 Hz, 4.8 Hz, 1H), 5.44 (d, J = 10.3, 1H), 5.22 (d, J = 10.0 Hz, 1H), 3.99-3.92 (m, 1H), 3.89-3.82 (m, 1H), 3.75-3.47 (m , 8H), 3.32 (dt, J = 10.3 Hz, 4.9, 1H), 3.20 (dd, J = 12.4 Hz, 6.8 Hz, 2H), 3.08 -3.01 (m, 1H), 2.71 (d, J = 4.7 Hz, 3H), 2.62 -2.50 (m, 1H), 2.45-2.25 (m, 5H), 1.87-1.80 (m, 1H), 1.73-1.65 (m, 7H), 1.63-1.56 (m, 1H), 1.49 (s, 9H), 1.42-1.31 (m, 1H), Compound 20. 20 was obtained from of resin-bound 20 R(16 µmol) by performing cleavage of the resin in TFA/TIS/H 2 O solvent mixture (95:2.5:2.5, v/v/v, 2.5 mL). After cold Et 2 O precipitation, the crude material was purified by semi-preparative RP-HPLC (10-45% of B in 20 min at 4 mL•min -1 ) to furnish 3.9 mg of pure 20 in 17 % yield.1 H NMR (CD 3 OH , 400 MHz, see TableS8); HRMS (ESI-TOF) m/z: [M + H] + Calcd for C 64 H 113 N 21 O 8 S 1336.8880; Found 1336.8904. RP-HPLC (10-100% B in 10 min): t R = 6.04 min. Solid phase synthesis of resin-bound 21 R . Resin bound oligo(thio)urea 20 R (26 µmol) wasengaged in methylation activation with CH 3 I (10.0 equiv. relative to resin loading) in DMF (0.5 mL) under microwave irradiation (40°C, 20 W, 30 min). The resin was successively washed with DMF (3  3 mL) and DCM (3  3 mL). This alkylation step was repeated three times with the same washings between each activation. Solid phase synthesis of resin-bound 22 R . The S-methylated resin-bound oligomer 21 R was washed with anhydrous THF (3  3 mL), then transferred in a sealed reactor. Anhydrous THF (5 mL) was added and ammonia gas was bubbled into the reactor for 30 min. The reactor was then sealed and stirred for 48 hours at 50 °C. Compound 22. 22 was obtained from resin-bound 22 R by performing cleavage of the resin in TFA/TIS/H 2 O solvent mixture (95:2.5:2.5, v/v/v, 2.5 mL). After cold Et 2 O precipitation, the crude material was purified by semi-preparative RP-HPLC (15-41% of B in 20 min at 4 mL.min -1 ) to furnish 6 mg of pure 22 in 17% yield. 1 H NMR (CD 3 OH , 400 MHz, see Table S9); HRMS (ESI-TOF) m/z: [M] + Calcd for C 64 H 115 N 22 O 8 1319.9268; Found 1319.9262. RP-HPLC (10-100% B in 10 min): t R = 5.35 min.

Figures

  Figures S1-S79 and Tables S1-S8 (PDF)

  8,33

Table 1

 1

	, entry

Table 1 . Outcome of the guanidinylation reaction on oligo(thio)ureas 1a, 2a and 3 entry substrate nucleophile equiv. of amine (co)solvent a time (h) ratio linear:cyclic product b

 1

	1	1a	7N NH 3	~ 150	CH 3 OH	48	33:67 (5:7)
	2	2a	7N NH 3	~ 300	CH 3 OH	24	26:74 (8:9)
	3	2a	n-PrNH 2	20	CH 3 OH	24	15:85 (11:9)
	4	2a	n-PrNH 2	45 c	CH 3 CN	120	73:27 (11:9)
			0.5M NH 3
	5	2a	in 1,4-dioxane	45 c	CH 3 CN	120	85:15 (8:9)
	6	2a	0.5M NH 3 in 1,4-dioxane	45 c	THF	38	97:3 (8:9)
	7	3	0.5M NH 3 in 1,4-dioxane	60 c	THF	150	75:25 (12:13)

Table 2 .

 2 Outcome of the guanidinylation reaction of model γ-amino acid-containing oligo(thio)urea substrates 1b, 2b and 4.

	entry substrate	nucleophile	equiv. of amine	solvent a	time (h)	cmpd	yield (%)
	1	1b	7N NH 3	~ 150	MeOH	48	15	20
	2	2b	0.5M NH 3 /1,4-dioxane	45 b	CH 3 CN	90	16	55
	3	2b	0.5M NH 3 /1,4-dioxane	40 b	THF	38	16	74
	4	2b	nPrNH 2	45 b	CH 3 CN	90	17	46
	5	4	0.5M NH 3 /1,4-dioxane	20	CH 3 CN	24	18	92
	6	4	nPrNH 2	20	CH 3 CN	60	19	60
	a

Table 2 entries

 2

4 & 5)

. The conversion to 19 required 60 hours to reach completion whereas conversion to 18 was nearly complete after only 24h as monitored by RP-HPLC analysis. Both oligo(guanidinium/amide/urea) hybrids 18 and 19 were recovered in 92% and 60% yield respectively after silica gel chromatography purification.

  Calcd for C 18 H 36 N 4 O 3 NaS 411.2406; Found 411.2396. C 18 RP-

	(m, 4H), 2.11-2.02 (m, 1H), 1.73-1.54 (m, 3H), 1.49 (s, 9H), 1.38-1.20 (m, 6H), 1.09 (d, J = 6.9,
	3H), 1.03 (d, J = 6.9, 3H), 0.98-0.91 (m, 18H); t R = 9.98 min. (10-100% B in 10 min then 100% B
	for 3 min); ESI-MS (+): 759.5 [M+H] + , 1518.1 [2M+H] + .
	Boc-Val U -Ala U -Leu U -Val U -Ala T -Leu-NHMe (4): 4 was prepared as described in the general
	procedure I and was isolated as a white solid (1.1g, 72% yield). 1 H NMR (CD 3 CN, 300 MHz, see
	also Table S1) δ 7.94 (s, 1H), 7.68 (d, J = 7.5 Hz, 1H), 7.24 (d, J = 9.3 Hz, 1H), 6.55 (dd, J = 9.7
	Hz, 2.6, 1H), 6.39 (dd, J = 9.9 Hz, 2.6 Hz, 1H), 6.18 (d, J = 10.7 Hz, 1H), 5.81 (d, J = 10.2 Hz,
	1H), 5.78 (d, J = 9.7 Hz, 1H), 5.73 (dd, J = 9.9 Hz, 3.3 Hz, 1H), 5.46-5.41 (m, 2H), 5.04 (d, J =
	10.0 Hz, 1H), 4.65-4.53 (m, 1H), 4.49-4.32 (m, 1H), 4.17-4.02 (m, 1H), 3.98-3.79 (m, 2H), 3.72-
	3.46 (m, 6H), 2.68 (d, J = 4.7 Hz, 3H), 2.61-2.49 (m, 2H), 2.43-2.24 (m, 5H), 2.1-2.02 (m, 1H),
	1.77-1.55 (m, 4H), 1.50 (s, 9H), 1.38-1.28 (m, 3H), 1.24-1.17 (m, 2H), 1.08-0.81 (m, 30H); 13 C
	NMR (CD 3 OD, 75 MHz) δ 175.1, 160.5, 160.5, 160.3, 160.2, 160.2, 159.5, 158.3, 79.0, 56.4, 54.6,
	51.2, 50.1, 46.1, 46.0, 45.5, 44.9, 43.3, 42.60, 41.5, 32.2, 30.7, 30.5, 27.5, 25.1, 25.0, 24.8, 22.3,
	22.2, 21.9, 21.4, 18.9, 18.8, 17.9, 17.7, 16.9, 16.7. mp 208-209 °C. HRMS (ESI-TOF) m/z: [M +
	Na] + Calcd for C 41 H 82 N 12 O 7 NaS 909.6048; Found 909.6066. C 18 RP-HPLC (10-100% B in 10 min
	then 100% of B for 3 min): t R = 11.56 min.
	1 H NMR (CD 3 CN, 300 MHz) δ 6.55+6.27 (s+bs, 2H), 5.58 (bs, 1H), 4.48 (bs, 1H), 3.75
	(bs, 2H), 3.46 (bs, 2H), 2.70 (d, J = 4.6 Hz, 3H), 2.29-2.16 (m, 2H), 1.77-1.60 (m, 2H), 1.44 (s+m,
	10H), 1.35-1.26 (m, 2H), 1.13 (d, J = 6.7 Hz, 3H), 0.95-0.92 (m, 6H); 13 C NMR (CD 3 OD, 125
	MHz) δ 176.5, 158.4, 80.2, 53.4, 50.0, 47.8, 45.7, 33.7, 33.1, 28.8, 26.4, 26.1, 23.6, 23.0, 18.8.
	HPLC (10-100% B in 10 min): t R = 7.25 min.

HRMS (ESI-TOF) m/z: [M + Na] +

reaction in the presence of DIPEA

  ). S-methylated tetramer 10 (0.017 mmol) was dissolved in MeOH (0.5 mL) and DIPEA was added(5 equiv., 15µL). The flask was closed with septum and the mixture was heated to 40-45°C. Cyclic product 9 was formed after 24 hours of 42.7, 40.9, 40.4, 30.0, 27.7, 25.5, 24.4, 22.1, 21.4, 20.6, 18.7, 17.4, 17.2, 10.4. HRMS (ESI-TOF) m/z: [M] + Calcd for C 33 H 69 N 10 O 5 685.5452; Found 685.5467. C 18 RP-HPLC (10-100% B in 10 min): t R = 7.81 min. (conversion 96% for the guanidylation step, 20 equiv. of n-propylamine) as a yellowish foam in 60% yield after silica gel flash column chromatography (CH 2 Cl 2 -CH 3 OH (v/v), from 98:2 to 92:8). H NMR (CD 3 CN , 400 MHz, see Table S7) δ 8.02 (bs, 1H), 6.88 (bs, 1H), 6.43 (d, J = 8.4 Hz 1H),

	Compound 9 (stirring. The product was purified by semi-preparative RP-HPLC (30-100% B in 20 min) and ESI-Compound 17. 17 was obtained from 2b after 90 hours according to the general procedures III and
	MS analysis confirmed the formation of 9. V (conversion 82% for the guanidylation step, 45 equiv. n-propylamine, CH 3 CN) as a yellowish
	Compound 12. Compound 12 was obtained from 3 (scale 0,035 mmol) according to the general foam in 46% yield after silica gel flash column chromatography (CH 2 Cl 2 -CH 3 OH (v/v), from 98:2
	procedures III and VI (THF as co-solvent) with a conversion of 56% for the guanidylation step to 92:8). 1 H NMR (CD 3 CN , 300 MHz, see also Table S5) δ 8.02 (bs, 1H), 6.96 (bs, 1H), 6.00 (d, J =
	1 H NMR (CDCl 3 , 300 MHz) 6.0 Hz, 1H), 5.74 (dd, J = 8.0 Hz, 3.4, 1H), 5.49 (pseudo-t, J = 6.0 Hz, 1H), 5.42 (d, J = 9.7 Hz, 1H), 5.17 (d, J = 9.6 Hz, 1H), 3.88 -3.72 (m, 1H), 3.70-3.51 (m, 4H), 3.35-3.27 (m, 2H), 3.21 (q, J = 9.6 Hz, 2H), 3.10 -2.98 (m, 1H), 2.71 (d, J = 4.7 Hz, 3H), 2.68-2.58 (m, 2H), 2.31 (t, J = 6.2 Hz, using 60 equiv. of 0.5 M NH 3.94 2H), 1.76-1.62 (m, 6H), 1.46 (s, 9H), 1.38-1.22 (m, 3H), 1.19 (d, J = 6.7 Hz, 3H), 1.03-0.87 (m,
	δ 7.64 (bs, 1H), 7.42 (bs 2H), 6.13 (bs, 1H), 5.30 (bs, 1H), 4.82 (bs, 1H), 3.67 (bs, 1H), 3.56-3.41 (m,1H), 3.87 (m,1H), 3.77-3.72 (m,2H), 3.69 (m,1H), 3.65-3.58 (m,2H), 3.55-3.48 (m, 2H), 3.46 21H); 13 C NMR (CD 3 CN, 75MHz) δ 54.7, 50.0, 48.6, 46.1, 45.0, 42.9, 42.4, 41.0, 30.9, 30.1, 27.9,
	(bm, 2H), 3.32-3.24 (m, 1H), 3.03 (dd, J = 14.0, 6.4 Hz, 1H), 2.78 (d, J = 4.8 Hz, 3H), 1.46 (s, 9H), (m,1H), 3.27 (m,1H), 3.16 (m,1H), 3.08-3.03 (m,2H), 2.87 (m,1H), 2.69 (d,3H), 2.56 (m,1H), 2.38-25.0, 24.7, 21.8, 21.3, 20.6, 18.5 17.4, 10.4. HRMS (ESI-TOF) m/z: [M] + Calcd for C 34 H 70 N 9 O 5
	1.31 (d, J = 6.7 Hz, 3H), 0.97 (d, J = 6.6 Hz, 3H), 0.89 (d, J = 6.5 Hz, 2H). 13 C (CDCl 3 , 125MHz, 2.30 (m,2H), 1.75 (m,1H), 1.72-1.65 (m,2H),1.63 (m,1H),1.47 (s,9H),1.30 (m,2H),1.23-1.13 684.5500; Found 684.5491. C 18 RP-HPLC (10-100% B in 10 min): t R = 6.04 min.
	cryoprobe) δ 160.5, 156.8, 156.7, 81.1, 51.3, 47.3, 47.0, 46.2, 29.7, 28.4, 28.3, 27.0, 24.9, 23.2, (m,5H), 1.03 (d,3H), 0.98-0.94 (dd,6H), 0.93-0.88 (m,18H). 13 C NMR (CD 3 OH, 700 MHz) δ
	21.5, 18.4. HRMS (ESI-TOF) m/z: [M] + Calcd for C 17 H 37 N 6 O 3 373.2927; Found 373.2930. C 18 RP-56.5,53.5, 48.9, 48.2,47.8, 47.2, 45.9, 45.1, 44.9, 44.4, 42.7, 41.6,44.4, 35.1, 30.6, 30.4,29.3, 27.4
	HPLC (10-100% B in 10 min): t R = 5.71 min. (3C), 26.7, 25.9, 24.8, 22.3, 20.9 (2C), 18.7 (2C), 17.6, 16.9, 16.1,13.03. HRMS (ESI-TOF) m/z:
	Compound 7 was recovered as a yellowish film in 26% yield (12 mg); 1 H NMR (CDCl 3, 300 MHz) [M] + Calcd for C 40 H 83 N 14 O 7 871.6564; Found 871.6545. RP-HPLC (10-100% B in 10 min then
	δ 9.64 (s, 1H), 9.48 (b sign, 0.5H), 7.47 (bs, 1H), 4.94 (d, J = 4.6 Hz, 1H), 4.68 (t, J = 8.8 Hz, 1H), 100% B for 2 min): t R = 8.22 min.
	4.30-4.20 (m, 1H), 4.14-4.09 (m, 1H), 3.69-3.57 (m, 2H), 3.39 (m, 1H), 2.88 (d, J = 4.5 Hz, 3H), Compound 15. 15 was prepared from 1b according to general procedure III and IV in CH 3 OH and
	2.02-1.89 (m, 1H), 1.85-1.79 (m, 1H), 1.58-1.54 (m, 1H), 1.47 (s, 9H), 1.34 (d, J = 6.6 Hz, 3H), was recovered in 20% yield (10mg). 1 H NMR (CD 3 CN , 300 MHz) δ 7.53 (bs, 1H), 7.15 (bs, 2H),
	1.00 (t, J = 6.6 Hz, 6H); 13 C NMR (HSQC, CDCl 3 , 75MHz) 52.8, 52.2, 49.0, 46.5, 43.8, 29.5, 28.0,

3 in 1,4-dioxane (150h). The ratio of linear product 12 versus cyclic product 13 was 75/25. Compound 12 was isolated after semi-preparative RP-HPLC purification (40-60% B in 20 min). White solid. 1 H NMR (CD 3 OH, 700 MHz, see also Table S4) δ 7.30 (m, NH), 6.65 (d, NH), 6.58 (m, NH), 6.40-6.37 (d, NH), 6.05-6.00 (m, NH), 5.95-5.87 (m, NH), 6.83 (s, 1H), 5.72 (s, 1H), 3.61 (bs, 2H), 3.34-3.26 (bm, 1H), 3.14-3.04 (bm, 1H), 2.72 (d, J = 4.8 Hz, 3H), 2.35 -2.27 (m, 2H), 1.88-1.60 (m, 4H), 1.46 (s, 9H), 1.42-1.30 (m, 1H), 1.21 (d, J = 6.7 Compound 19. 19 was obtained from 4 after 60 hours according to the general procedures III and V 1

ACKNOWLEDGMENTS

This work was supported by European Union (Marie-Curie FP7-PEOPLE-2010-IEF-273224 postdoctoral fellowship to K.P.-Z.) and by ANR and DGA (Project ANR-12-ASTR-0024). Predoctoral fellowships from DGA and Conseil Régional d'Aquitaine (to S.A.) and from ANR (to C. P., ANR-15-CE07-0010) are gratefully acknowledged. We thank Estelle Morvan for her assistance with the NMR measurements. This work has benefited from the facilities and expertise of IECB Biophysical and Structural Chemistry platform (BPCS), CNRS UMS3033, Inserm US001, Univ. Bordeaux.

Hz, 3H), 0.95 (d, J = 6.6 Hz, 3H), 0.88 (d, J = 6.6 Hz, 3H); 13 C NMR (HSQC, CD 3 CN, 75MHz) δ 50. 2, 46.6, 46.2, 42.3, 31.2, 30.1, 27.1, 25.3, 24.6, 22.3, 20.8, 16.5. HRMS (ESI-TOF) m/z: [M] + Calcd for C 18 H 38 N 5 O 3 372.2969; Found 372.2984. C 18 RP-HPLC (10-100% B in 10 min): t R = 5.88 min.

Guanidinylation reaction with n-propylamine: General procedure V. S-methylated oligomer intermediate was dissolved in CH 3 CN or CH 3 OH (0.034 mmol/mL) and n-propylamine (10 equiv.) was added. The flask was closed tightly with septum and the reaction mixture was heated to 40-45°C. The monitoring of the reaction was performed by RP-HPLC analysis. Additional amounts of n-propylamine were added regularly. Reaction was stopped when no further progress was observed by RP-HPLC analysis.

Compounds 9 and 11: 9 and 11 were prepared on small scale (0.034 mmol) in CH 3 CN from 2a according to the general procedures III and V in the presence of n-propylamine (45 equiv.) for 120 hours. The ratio of linear tetramer 11 versus cyclic product 9 was 73/27 and both compounds were isolated after semi-preparative RP-HPLC purification (30-100% B in 20 min). Yields after purification for compound 9 was 16% and for compound 11 was 26%.

When the reaction was performed in CH 3 OH in presence of n-propylamine (20 equiv.) after 24 hours compound 11 and 9 were obtained in a ratio of 15/85 and were isolated after semi-RP-HPLC purification (30-100% B in 20 min).

Compound 11. 1 H NMR (CD 3 CN , 300 MHz, see also Table S3) δ 6.02 (bs, 1H), 5.39 (d, J = 6.9, building blocks according to the recently reported solid phase oligo(thio)urea synthesis procedure by locally using N-Fmoc isothiocyanate building block BB4 and azide Trp-type activated monomer BB5 under microwave irradiation with the azide strategy on a 50 µmol scale, starting from NovaPeg Rink Amide resin (111 mg, 0.45 mmol/g). 37 Coupling of N-Fmoc--Lys-OH (1.5 equiv. relative to the resin loading, 1.5 mmol, 45 mg) was performed in DMF (3 mL) in presence of DIPEA (3.0 equiv.) and BOP (3.0 equiv.) under microwave irradiation (50 W, 50 °C, ramp time 5 min, hold time 30 min). This coupling step was repeated once. Fmoc removal was performed with a solution of piperidine in DMF (2:8, v/v, 1 mL), three times 20 min.

AUTHOR INFORMATION

* Corresponding author, e-mail: g.guichard@iecb.u-bordeaux.fr ORCID Karolina Pulka-Ziach: 0000-0002-2861-1466 Brice Kauffmann: 0000-0002-2932-3255 Morgane Pasco: 0000-0002-1556-2802