Parallel solid-phase synthesis of diaryltriazoles

  1. Matthias Wrobel1,
  2. Jeffrey Aubé2 and
  3. Burkhard König1

1Fakultät für Chemie und Pharmazie, Universität Regensburg, D-93040 Regensburg, Germany, Fax: +49 9419431717
2Department of Medicinal Chemistry and the Chemical Methodology and Library Development Center, University of Kansas, Delbert M. Shankel Structural Biology Center, 2121 Simons Drive, West Campus, Lawrence, KS 66047, USA

  1. Corresponding author email

This article is part of the Thematic Series "Recent developments in chemical diversity".

Guest Editor: J. A. Porco Jr.
Beilstein J. Org. Chem. 2012, 8, 1027–1036. doi:10.3762/bjoc.8.115
Received 13 Apr 2012, Accepted 13 Jun 2012, Published 06 Jul 2012

Abstract

A series of substituted diaryltriazoles was prepared by a solid-phase-synthesis protocol using a modified Wang resin. The copper(I)- or ruthenium(II)-catalyzed 1,3-cycloaddition on the polymer bead allowed a rapid synthesis of the target compounds in a parallel fashion with in many cases good to excellent yields. Substituted diaryltriazoles resemble a molecular structure similar to established terphenyl-alpha-helix peptide mimics and have therefore the potential to act as selective inhibitors for protein–protein interactions.

Keywords: chemical diversity; Huisgen cycloaddition; library synthesis; peptidomimetics; solid phase synthesis; triazole

Introduction

The α-helix was the first-described secondary structure of peptides discovered by Linus Pauling in 1951 [1]. With about 30% of the amino acids in proteins being part of α-helices [2], it is the most common secondary structure found in proteins [3]. Protein–protein as well as protein–DNA and protein–RNA interactions often involve α-helices as recognition motifs on protein surfaces [4]. These helices are important targets for new drugs, but stabilization of the helix folding for small structures with less than 15 residues still remains a challenge [5,6]. Thus, new attempts have been made to design low-molecular-weight ligands that disrupt protein–protein interactions [7]. For example, fast proteolytic degradation observed with small peptide-based compounds [8], can be overcome by compounds stabilized by non-natural amino acids [9] or cross-linked between side chains and the backbone [10]. Replacement of the complete backbone by a nonpeptidic scaffold, which positions side chains in the typical i, i+3 and i+7 arrangement of an α-helix is another successful strategy [11]. Horwell pioneered this type of peptidomimetics and showed that 1,6-disubstituted indanes can imitate the helix residues i and i+1 [12,13]. Hamilton reported a 3,2′,2″-substituted terphenyl scaffold with a spatial orientation that mimics the i, i+3 and i+7 moieties on the surface of an α-helical peptide [14]. Inspired by the terphenyl-based α-helix mimetics 1, several related compounds containing three or more adjacent aryl rings (Scheme 1), such as 2, were reported [15]. However, the synthesis of substituted triaryl compounds can be tedious, and the predictability of their potency and selectivity as inhibitors is still limited. We have recently reported the synthesis of triazole-based α-helix mimetics 3 and 4 [16], which are efficiently available through azide–alkyne cycloadditions [17]. We now report the use of this chemistry to prepare libraries of potential inhibitors of protein–protein interactions.

[1860-5397-8-115-i1]

Scheme 1: Terphenyl scaffold 1 [13,14]; oxazole-pyridazine-piperazine 2 [14,15] and aryl-triazoles 3 and 4 [15,16] as α-helix mimetics.

Results and Discussion

Synthesis of azide-functionalized Wang resins

Two azide-functionalized resins were prepared for the solid-phase synthesis of diaryl-triazoles. The commercially available 4-(bromomethyl)benzoic acid (5) was converted into azide 6 in anhydrous DMF with sodium azide under heating. Coupling to Wang resin in dichloromethane, by using DIC and DMAP as coupling reagents, gave resin 7 in quantitative yield (Scheme 2) [18]. Commercially available 4-azidobenzoic acid (8) gave resin 9 in an analogous esterification of a Wang resin.

[1860-5397-8-115-i2]

Scheme 2: Synthesis of azido-functionalized resins 7 and 9.

Solid-phase synthesis of diaryltriazoles

The conditions for the solid-phase synthesis of diaryltriazoles on functionalized Wang resin 7 were optimized by using five different alkynes 10ae, containing acyclic or cyclic aliphatic moieties, simple arenes and 1-(but-3-yn-2-yl)-3-(4-chlorophenyl)-1-methylurea (10c) as an example of a more complex alkyne. The azide–alkyne [3 + 2] cycloaddition was catalyzed with copper(II) sulfate pentahydrate and L-ascorbic acid in DMF overnight at room temperature. A solution of EDTA was added to remove the remaining copper cations from the resin. Resin cleavage under acidic conditions with TFA in DCM gave compounds 11ae in moderate to excellent overall yields of 57 to 90% (Table 1). Due to the solid phase synthesis protocol the crude material purity was typically high, ranging from 70 to 90%. Alkyne 10d and 10e bearing hydroxy groups were converted quantitatively, but elimination of water occurred in the presence of TFA. The dehydrated products were obtained in 57 and 71%. The remaining material was the corresponding hydroxylated product.

Table 1: Copper-catalyzed [2 + 3] cycloadditions of resin-bound azide 7 with five terminal alkynes.

[Graphic 1]
terminal alkyne 10 product 11 yield [%]
[Graphic 2]
10a
[Graphic 3]
11a
63
[Graphic 4]
10b
[Graphic 5]
11b
90
[Graphic 6]
10c
[Graphic 7]
11c
81
[Graphic 8]
10d
[Graphic 9]
11d
57
[Graphic 10]
10e
[Graphic 11]
11e
71

Parallel synthesis of a compound library

A larger compound library was prepared by using resins 7 and 9, 15 different terminal alkynes 10f–t and either copper or ruthenium-catalyzed [2 + 3] cycloadditions. The three reactions and the obtained products 11 (reaction 1), 12 (reaction 2) and 13 (reaction 3) are summarized in Table 2.

Table 2: Copper-catalyzed [2 + 3] cycloadditions of resin bound azide 7 with five terminal alkynes. Compounds 13, with the exception of 13f, were only characterized during compound library synthesis, by HPLC–MS analysis.

[Graphic 12]
alkyne 10 product after cleavage from resin (yield)
  reaction 1
resin 7, catalyst CuSO4
reaction 2
resin 9,
catalyst CuSO4
reaction 3
resin 7, catalyst Cp·RuCl(PPh3)2
[Graphic 13]
10f
[Graphic 14]
11f (95%)
[Graphic 15]
12a (49%)
[Graphic 16]
13a not obtained
[Graphic 17]
10g
[Graphic 18]
11g (99%)
[Graphic 19]
12b (44%)
[Graphic 20]
13b (43%)
[Graphic 21]
10h
[Graphic 22]
11h (97%)
[Graphic 23]
12c (45%)
[Graphic 24]
13c (89%)
[Graphic 25]
10i
[Graphic 26]
11i (98%)
[Graphic 27]
12d (44%)
[Graphic 28]
13d (89%)
[Graphic 29]
10j
[Graphic 30]
11j (98%)
[Graphic 31]
12e (41%)
[Graphic 32]
13e (90%)
[Graphic 33]
10k
[Graphic 34]
11k (97%)
[Graphic 35]
12f (21%)
[Graphic 36]
13f (48%)
[Graphic 37]
10l
[Graphic 38]
11l (98%)
[Graphic 39]
12g (52%)
[Graphic 40]
13g (94%)
[Graphic 41]
10m
[Graphic 42]
11m (88%)
[Graphic 43]
12h (38%)
[Graphic 44]
13h (96%)
[Graphic 45]
10n
[Graphic 46]
11n (98%)
[Graphic 47]
12i (41%)
[Graphic 48]
13i (80%)
[Graphic 49]
10o
[Graphic 50]
11o (97%)
[Graphic 51]
12j (63%)
[Graphic 52]
13j (82%)
[Graphic 53]
10p
[Graphic 54]
11p (99%)
[Graphic 55]
12k (47%)
[Graphic 56]
13k (89%)
[Graphic 57]
10q
[Graphic 58]
11q (98%)
[Graphic 59]
12l (40%)
[Graphic 60]
13l (89%)
[Graphic 61]
10r
[Graphic 62]
11r (98%)
[Graphic 63]
12m (61%)
[Graphic 64]
13m (92%)
[Graphic 65]
10s
[Graphic 66]
11s (94%)
[Graphic 67]
12n (26%)
[Graphic 68]
13n (95%)
[Graphic 69]
10t
[Graphic 70]
11t (96%)
[Graphic 71]
12o not obtained
[Graphic 72]
13o (74%)

Reaction 1 follows the established protocol and, gave after removal of the copper salts with a solution of EDTA and TFA cleavage, the corresponding products in good to quantitative yields (88–99%). Resin 9 was used in reaction 2 under otherwise identical reaction conditions. The use of an aromatic azide leads to more rigid products containing three adjacent aromatic rings: The central triazole and the phenyl ring of the benzoic acid as constant structural elements and the third ring consisting either of substituted benzenes, heteroarenes or a polycyclic aromatic compound. Lower product yields were obtained in this series of compounds, ranging from 21 to 63%. The lower reactivity of the aromatic azide and the increased steric demand may explain the decrease in yield in comparison to that of the former series of compounds. The formation of the compound 12o, bearing a particularly bulky substituent, was not observed. Replacing the copper(I) catalyst by a ruthenium(II) complex allows the preparation of regioisomers in reaction 3. Instead of the 1,4-disubstituted triazoles obtained from copper(I) catalysis, the complex pentamethylcyclopentadienylbis(triphenylphosphine)ruthenium(II) chloride leads to 1,5-disubstituted triazole compounds [19]. 4-(Azidomethyl)benzoic acid functionalized Wang resin 7 and the alkynes 10ft were reacted in DMF at 70 °C overnight, and the products 13bo were obtained after TFA cleavage from the resin in moderate to good yields ranging from 43–96%. Only compound 13a could not be obtained. The proton NMR analysis allows us to clearly distinguish between 1,4- and 1,5-disubstituted triazoles due to a characteristic shift of the triazole proton resonance. The triazole proton of the 1,4-disubstituted ring in compound 11k shows a 1H NMR resonance at δ = 8.71 (400 MHz, DMSO-d6), while the resonance signal for the triazole proton of product 13f is observed at δ = 8.00 (400 MHz, DMSO-d6). Compounds 13, with the exception of compound 13f, were only characterized by mass spectrometry during the synthesis of the compound library.

The copper-mediated [2 + 3] cycloadditions are restricted to terminal alkynes. However, the ruthenium-catalysis allows the use of internal alkynes. In preliminary work, resin 7 was therefore reacted with internal alkynes 14ac and pentamethylcyclopentadienylbis(triphenylphosphine)ruthenium(II) chloride as catalyst in DMF at 70 °C overnight followed by TFA cleavage [20]. LC–MS analysis of the crude product revealed the formation of compounds 15ac in high yields of 78–98% (Table 3).

Table 3: Ruthenium-catalyzed [2 + 3] cycloadditions of resin-bound azide 7 with three disubstituted alkynes.

[Graphic 73]
alkyne product 15 (yield)a
[Graphic 74]
14a
[Graphic 75]
15a (78%)
[Graphic 76]
14b
[Graphic 77]
15b (84%)
[Graphic 78]
14c
[Graphic 79]
15c (98%)

aYields based on LC–MS; compound 12b was used as standard.

Conclusion

Diaryltriazoles were obtained in an efficient three-step solid-phase procedure. Immobilization of aromatic azides on commercial Wang resin followed by copper(I)- or ruthenium(II)-catalyzed 1,3-cycloaddition and subsequent cleavage of the product from the resin gave the target structures in good to excellent yields with the possibility to introduce a wide variety of different substituents. The alternative use of copper or ruthenium catalysis for the on-bead cycloaddition gives regioisomeric products, which extends the diversity of the compound collection. The method may find application in the combinatorial search for selective protein–protein inhibitors. To that end, most of the compounds prepared herein were submitted to the Molecular Libraries Small Molecular Repository for ongoing inclusion in high-throughput screening activities.

Experimental

General procedures

GP 1 – Coupling of benzoic acid derivatives 6 and 8 on Wang resin: Wang resin (1 equiv) was preswollen in dichloromethane (0.8 mL/100 mg resin) for 2 h at room temperature. Subsequently, both coupling reagents N,N′-diisopropylcarbodiimide (3.5 equiv) and dimethylaminopyridine (0.5 equiv) were added. After the addition of the benzoic acid derivative 6 or 8 (2.5 equiv) the reaction mixture was stirred for 20 h at room temperature. The resin was first washed with dimethylformamide, methanol and dichloromethane (each solvent 3 × 0.8 mL/100 mg resin), and then dried in high vacuum for 3 h.

GP 2 – Huisgen 1,3-dipolar cycloaddition of solid-phase-immobilized azides with terminal alkynes by copper(I) catalysis: An azide-functionalized Wang resin 7 or 9 (1 equiv) was preswollen in dimethylformamide (1.5 mL/100 mg resin) for 2 h at room temperature. The copper(I) catalyst was prepared in situ by using L-ascorbic acid (0.5 equiv) as reducing agent and copper(II) sulfate pentahydrate (10 mol %). After the terminal alkyne (4 equiv) was added, the reaction mixture was stirred for 22 h at room temperature. The resin was washed with dimethylformamide, methanol and dichloromethane (each solvent 2 mL/100 mg resin). The remaining copper cations were complexed and removed by using a solution of ethylenediaminetetraacetic acid disodium salt. For this purpose, a 1:1 mixture of dimethylformamide and disodium EDTA (aq., sat.) was added to the resin and stirred for 10 min at room temperature. Again washing steps with water, dimethylformamide, methanol and dichloromethane (each solvent 3 × 2 mL/100 mg resin) were carried out.

GP 3 – Huisgen 1,3-dipolar cycloaddition of solid-phase-immobilized azides with terminal or internal alkynes by ruthenium(II) catalysis: The azide functionalized Wang resin (1 equiv) was preswollen in dimethylformamide (2 mL/100 mg resin) for 2 h at room temperature. Subsequently, the catalyst complex pentamethylcyclopentadienylbis(triphenylphosphine)ruthenium(II) chloride, Cp·RuCl(PPh3)2, (5 mol %) and either a terminal or an internal alkyne (4 equiv) were added. After the reaction mixture was stirred for 22 h at 70 °C, the resin was washed with dimethylformamide, methanol and dichloromethane (each solvent 3 × 2 mL/100 mg resin).

GP 4 – Cleavage of solid-phase resin-bound molecules with TFA: The swollen resin was treated with a 1:4 mixture of trifluoroacetic acid and dichloromethane (1 mL/100 mg resin). After being stirred for 10 min at room temperature, the cleaved product was rinsed out of the resin using dichloromethane (1.5 mL/100 mg resin). The resin was treated once more with the 20% trifluoroacetic acid solution (1 mL/100 mg resin), stirred for 10 min at room temperature and washed with dichloromethane (3 × 1 mL/100 mg resin). The solvent was evaporated and the product was dried in high vacuum for 4 h.

4-(Azidomethyl)benzoic acid (6) [21]: The synthetic procedure leading to this literature-known compound was improved. 4-(Bromomethyl)benzoic acid (5, 1.2 g, 5.58 mmol, 1.0 equiv) and sodium azide (907 mg, 13.95 mmol, 2.5 equiv) were suspended in 25 mL of anhydrous dimethylformamide under a nitrogen atmosphere. After the reaction mixture was stirred for 15 h at 50 °C the solvent was evaporated. The colorless residue was dissolved in 90 mL of water and the solution was treated with 17 mL of hydrochloric acid (c 1 mol/L). The precipitate was separated with a Büchner funnel, dissolved in dichloromethane and dried over potassium sulfate. After filtration, and evaporation of the solvent, 4-(azidomethyl)benzoic acid (6, 880 mg, 4.97 mmol, 89%) was yielded as a colorless solid and dried in high vacuum overnight; mp 135.6–136.6 °C; 1H NMR (300 MHz, CDCl3) δ 4.46 (s, 2 H, H-6), 7.44 (d, 3JHH = 8.4 Hz, 2H, H-4), 8.14 (d, 3JHH = 8.3 Hz, 2H, H-3); 13C NMR (75 MHz, CDCl3) δ 54.3 (−, 1C, C-6), 128.0 (+, 2C, C-4), 129.1 (Cq, 1C, C-2), 130.8 (+, 2C, C-3), 141.5 (Cq, 1 C, C-5), 171.4 (Cq, 1C, C-1); IR (cm−1) [Graphic 80]: 2933 (w), 2880 (w), 2817 (w), 2656 (w), 2110 (m), 2086 (m), 1950 (w), 1682 (s), 1293 (s), 1239 (s), 707 (s), 545 (s); EIMS m/z: 177.0 (40) [M]+, 148.0 (80) [M − N2]+, 135.0 (100) [M − N3]+; Anal. calcd for C8H7N3O2: C, 54.24; H, 3.98; N, 23.72; found: C, 54.28; H, 4.25; N, 23.75.

Supporting Information

Supporting Information File 1: Experimental details and spectra.
Format: PDF Size: 1.5 MB Download

Acknowledgments

We thank Ben Neuenswander for carrying out the purification of library members. Financial support from the National Institute of General Medical Sciences (KU CMLD center, P50 GM69663).

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