HETEROCYCLES

Paper | Special issue | Vol. 84, No. 1, 2012, pp. 515-526
Received, 18th April, 2011, Accepted, 23rd May, 2011, Published online, 2nd June, 2011.
DOI: 10.3987/COM-11-S(P)13

■ Efficient Microwave-Assisted Synthesis of 1,2,4-Triazole-Based Peptidomimetics Using Benzotriazole Methodology

Finn K. Hansen, Lucas K. Beagle, Ekaterina Todadze, and Alan R. Katritzky*

Center for Heterocyclic Compounds, Department of Chemistry, University of Florida, Gainesville, FL 32611-7200, U.S.A.

Abstract

A microwave-assisted three-step protocol allowed the rapid and convenient construction of a series of 1,2,4-triazole substituted amino acids and dipeptides as potential building blocks for peptidomimetics.

INTRODUCTION
The coupling of microwave-assisted reactions with traditional organic synthesis allows a large range of compounds to be synthesized in an efficient manner. By utilizing the strengths of both methods organic chemists can quickly create large libraries of molecules, which can lead to the expeditious discovery of novel drugs.1-4
1,2,4-Triazole derivatives have attracted considerable interest among medicinal chemists because of their versatile biological properties. For instance, 1,2,4-triazoles have been found to exhibit a wide range of antifungal and antibacterial activities.5,6 The 1,2,4-triazole moiety was also found in potent CRF1 receptor antagonists,7 H2 receptor antagonists8 and muscarinic receptor ligands.9,10 Ring acylated 1,2,4-triazole derivatives have shown substantial inhibition of Janus associated kinases (TYK2 and JAKs1-3)11 and cyclin-dependent kinases (CDKs, Figure 1).12
Various efficient methods for the preparation of 1,2,4-triazole-3,5-diamine derivatives have been published which involve the use of (i) N-cyanoguanidines,13,14 (ii) S,S-dimethyl-N-cyanodithioimidocarbonate15 and (iii) diphenyl cyanocarbonimidate.16-19 However, due to limited scope method (i) does not show great versatility and diphenyl cyanocarbonimidate (method iii) is a relatively expensive starting material.

Recently discovered bioactive peptides play diverse roles, including functioning as hormones, enzyme inhibitors and neurotransmitters.20,21 However, their clinical application has been limited due to their rapid hydrolysis by peptidase enzymes. One approach to overcome the drawbacks of natural peptides is the use of peptidomimetics. These are small protein-like molecules designed to mimic natural peptides or proteins.22 Bioisosteric replacement of the amide bond is an important aspect in the design of peptidomimetics.23 In particular 1,2,4-triazole derivatives were successfully utilized as amide bond mimetics with increased hydrolytic stability.23,24
N-Acylbenzotriazoles are stable solids, easy to handle and advantageous for N-, O-, C- and S-acylation.25 We herein demonstrate a novel microwave-assisted approach for the synthesis of 1,2,4-triazole-based peptidomimetics using benzotriazole methodology and starting from inexpensive and versatile starting materials.

RESULTS AND DISCUSSION
Isothiourea derivatives 2a-c were prepared by reacting commercially available S,S-dimethyl-N-cyanodithioimidocarbonate (1) with primary or secondary amines according to literature procedures (63-89% yield, Scheme 1, Table 1).15,26,27 However, the preparation of 2a-c under conventional conditions required relatively long reaction times (4-5 h, 63-89% yield). Therefore, we repeated the synthesis of 2a-c using microwave irradiation. Under microwave heating, significant reduction in reaction times were observed, which provided the isothiourea derivatives 2a-c within 5-30 minutes in comparable or greater yields (61-86%, Table 1).

Treatment of isothioureas 2a-c with hydrazine hydrate (2 equiv.) in refluxing ethanol furnished the 1,2,4-triazole-3,5-diamine derivatives 3a-c in 75-90% yield within 4-5 hours (Scheme 1, Table 2). Furthermore, the reaction conditions were optimized using microwave irradiation to shorten reaction times and to improve yields. The microwave-assisted preparation (80 °C, 100 W) afforded 3a-c within 5-10 minutes in significantly higher yields (89-95%, Scheme 1, Table 2).

We then investigated the preparation of the desired 1,2,4-triazole substituted amino acids and dipeptides. In initial experiments the reaction of 3a with Cbz-L-Ala-Bt (Bt = benzotriazol-1-yl) was incomplete after heating the reaction mixture under reflux for 12 hours. Subsequently, we studied the microwave-assisted N-acylation of 3a,c. Interestingly, the reaction of 3a,c with N-(protected α-aminoacyl)benzotriazoles under microwave irradiation (70 °C, 100 W) was complete after 30 minutes and afforded the ring acylated products 4a-e (65-95% yield, Table 3).

Acylation of the ring nitrogen was confirmed by observation of an amino signal (2H) in the 1H NMR spectrum. The downfield shift of the amino group signal (> 7ppm) indicates clearly that the ring acylation has taken place at the N1 position proximal to the amino group consistent with the findings by Reiter et al.,15 who showed that the exocyclic amino group proximal to the methyl substituted ring nitrogen in 1-methyl-1H-1,2,4-triazole-3,5-diamine induced a downfield shift whereas the distal exocyclic amino group was shifted upfield by ca. 2 ppm.15 D’Andrea et al. found a similar trend with an analog of compound JNJ-7706621, from which the chemical shift of the amino group upon acylation at N2 was upfield (6.25 ppm) whereas acylation at N1 showed a significant downfield shift (7.95 ppm).11 Our compounds share this chemical shift pattern where the downfield shift of the amino group indicates acylation of the ring nitrogen (N1) proximal to the primary amino group (Figure 2).

The microwave-assisted reactions of 3a with N-(protected dipeptidoyl)benzotriazoles similarly furnished ring acylated dipeptidoyl 1,2,4-triazole derivatives 5a-c in 76-95% yield. Again, the exocyclic amino group shows a 1H NMR spectra shift in the region of >7ppm as seen in compounds 4a-e.

We observed exclusive ring acylation of triazoles 3a,c with N-(protected α-aminoacyl)benzotriazoles and N-(protected dipeptidoyl)benzotriazoles yielding compounds of type 4 and 5. It was therefore decided to study the acylation of the exocyclic amino group. To this end we required a substrate with a ring-substituted nitrogen to prevent any possible ring acylation. Thus, N5-benzyl-1-methyl-1H-1,2,4-triazole-3,5-diamine 6 was prepared in 51% yield by treatment of isothiourea 2b with 2 equiv. of methylhydrazine according to a literature procedure15 (Scheme 2). The 1-methylsubstituted triazole compound 6 was conveniently acylated at the exocyclic NH2 group using a microwave-assisted protocol (70 °C, 100 W, 30 min) to afford the desired 3,5-diamino-1,2,4-triazole derivatives 7a and 7b in 57% and 71% yield, respectively (Scheme 2).

CONCLUSIONS
In conclusion, we have developed an efficient, fast and convenient method for the microwave-assisted preparation of 1,2,4-triazole substituted amino acids and dipeptides, which can be considered as potential building blocks for peptidomimetics as well as prospective biologically active compounds. Our method offers short reaction times, good to excellent yields and is compatible with a variety of protecting groups.

EXPERIMENTAL
Melting points were determined on a capillary point apparatus equipped with a digital thermometer and are uncorrected. NMR spectra were recorded in CDCl3 or DMSO-d6 on Gemini or Varian NMR operating at 300 MHz for 1H and 75 MHz for 13C with TMS as an internal standard. Elemental analyses were performed on a Carlo Erba-1106 instrument. All microwave assisted reactions were carried out with a single mode cavity Discover Microwave Synthesizer (CEM Corporation, NC). The reaction mixtures were transferred into a 10 mL glass pressure microwave tube equipped with a magnetic stirrer bar. The tube was closed with a silicon septum and the reaction mixture was subjected to microwave irradiation (Discover mode; run time: 60 sec.; PowerMax-cooling mode). All N-(protected α-aminoacyl)- benzotriazoles and N-(protected dipeptidoyl)benzotriazoles used have been prepared according to our previously published methods.25

General Procedure for the Microwave-assisted Preparation of Isothioureas 2a-c. A mixture of dimethyl cyanodithioiminocarbonate (0.44 g, 3 mmol) and the respective primary or secondary amine (3 mmol) in Et2O (5 mL) or EtOH (5 mL) was subjected to microwave irradiation (see Table 1). Compounds 2a,b precipitated from the reaction mixture and were filtered, washed with Et2O (2 x 5 mL) and dried under vacuum. Compound 2c was crystallized from EtOH:hexanes, filtered, washed with hexanes (2 x 5 mL) and dried under vacuum.

Methyl N-cyanomorpholine-4-carbimidothioate (2a). White microcrystals, 76% yield, mp 131-132 °C (lit.28 mp 125-126 °C); 1H NMR (300 MHz, CDCl3) δ 3.82 (t, J = 4.7 Hz, 4H), 3.70 (t, J = 4.7 Hz, 4H), 2.76 (s, 3H); 13C NMR (75 MHz, CDCl3) δ 169.4, 114.9, 66.4, 48.7, 16.3. Anal. Calcd for C7H11N3OS: C 45.39; H 5.99; N 22.68. Found: C 45.24; H 5.96; N 22.64.

Methyl N-benzyl-N'-cyanocarbamimidothioate (2b). White microcrystals, 86% yield, mp 158-161 °C (lit.29 mp 156-157 °C); 1H NMR (300 MHz, DMSO-d6) δ 8.91 (s, 1H), 7.38-7.25 (m, 5H), 4.50 (s, 2H), 2.63 (s, 3H); 13C NMR (75 MHz, DMSO-d6) δ 170.3, 137.4, 128.4, 127.3, 127.2, 115.8, 46.2, 14.1. Anal. Calcd for C10H11N3S: C 58.51; H 5.40; N 20.47. Found: C 58.13; H 5.29; N 20.68.

Methyl N'-cyano-N-phenylcarbamimidothioate (2c). White microcrystals, 61% yield, mp 195-198 °C (lit.27 mp 194-196 °C); 1H NMR (300 MHz, DMSO-d6) δ 10.16 (s, 1H), 7.52-7.18 (m, 5H), 2.70 (s, 3H); 13C NMR (75 MHz, DMSO-d6) δ 170.2, 137.2, 128.8, 126.4, 124.2, 114.8, 14.9. Anal. Calcd for C9H9N3S: C 56.52; H 4.74; N 21.97. Found: C 56.25; H 4.60; N 21.95.

General Procedure for Microwave Assisted Synthesis of 1,2,4-Triazoles 3a-c. A reaction mixture of the appropriate isothiourea 2a-c (2 mmol) and 72% hydrazine hydrate (0.2 g, 4 mmol) in EtOH (5 mL) was subjected to microwave irradiation (80 °C, 100 W, 5-10 min). On completion of the reaction (TLC), the solvent was removed under reduced pressure and the residue was crystallized from CHCl3:hexanes.

3-Morpholino-1H-1,2,4-triazol-5-amine (3a). White microcrystals, 89% yield, mp 165-166 °C (lit.15 mp 167-168 °C); 1H NMR (300 MHz, DMSO-d6) δ 10.90 (br s, 1H), 5.99 (br s, 2H), 3.66 (t, J = 4.4 Hz, 4H), 3.15 (t, J = 4.4 Hz, 4H); 13C NMR (75 MHz, DMSO-d6) δ 163.1, 156.9, 66.3, 47.4.

N3-Benzyl-1H-1,2,4-triazole-3,5-diamine (3b). White microcrystals, 90% yield, mp 147-148 °C (lit.15 mp 151-153 °C); 1H NMR (300 MHz, DMSO-d6) δ 7.47-7.10 (m, 5H), 6.20 (s, 1H), 5.42 (s, 2H), 4.23 (d, J = 5.7 Hz, 2H); 13C NMR (75 MHz, DMSO-d6) δ 160.1, 157.8, 141.1, 128.0, 127.2, 126.4, 46.2.

N3-Phenyl-1H-1,2,4-triazole-3,5-diamine (3c). White microcrystals, 95% yield, mp 166-169 °C (lit.15 mp 161-162 °C); 1H NMR (300 MHz, DMSO-d6) δ 11.20 (br s, 1H), 8.62 (s, 1H), 7.49 (d, J = 7.9 Hz, 2H), 7.15 (t, J = 7.8 Hz, 2H), 6.71 (t, J = 7.3 Hz, 1H), 5.87 (br s, 2H); 13C NMR (75 MHz, DMSO-d6) δ 157.6, 155.6, 142.6, 128.4, 118.3, 115.5. Anal. Calcd for C8H9N5: C 54.85; H 5.18; N 39.98. Found: C 54.48; H 5.08; N 39.81.

General Procedure for Microwave Assisted Synthesis of Compounds 4a-e. A mixture of the respective N-(protected α-aminoacyl)benzotriazole (1 mmol) and 1,2,4-triazole 3a,c (1 mmol) in dry THF (3 mL) was subjected to microwave irradiation (70 °C, 100 W, 30 min). The products were isolated and purified according to the following procedures. The reaction mixtures of compounds 4a,c were quenched with water (2 mL) and extracted with EtOAc (3 x 10 mL). The combined organics were washed with aqueous Na2CO3 solution (10% w/w, 3 x 20 mL), water (3 x 20 mL), dried over MgSO4 and the solvent was removed under reduced pressure. The residues were recrystallized from CH2Cl2:hexanes to give the desired products 4a,c. In case of compounds 4b,d the reaction mixtures were evaporated under reduced pressure and the crude products were recrystallized from MeOH. The reaction mixture of compound 4e was allowed to cool to room temperature and crystallized from a mixture of THF, CH2Cl2 and hexanes. The precipitate was collected, washed with CH2Cl2 (2 x 10 mL) and dried under vacuum.

(S)-Benzyl (1-(5-amino-3-morpholino-1H-1,2,4-triazol-1-yl)-1-oxopropan-2-yl)carbamate (4a). White microcrystals, 95% yield, mp 205-207 °C; 1H NMR (300 MHz, DMSO-d6) δ 7.85 (d, J = 7.2 Hz, 1H), 7.61 (br s, 2H), 7.39-7.31 (m, 5H), 5.04-5.01 (m, 2H), 4.91-4.81 (m, 1H), 3.67-3.62 (m, 4H), 3.31-3.24 (m, 4H), 1.35 (d, J = 7.2 Hz, 3H); 13C NMR (75 MHz, DMSO-d6) δ 172.5, 163.0, 157.2, 155.9, 136.9, 128.4, 127.9, 65.6, 49.4, 45.6, 34.4, 16.2. HRMS calcd. for C17H22N6O4 [M+H]+: 375.1775. Found [M+H]+: 375.1786.

(9H-Fluoren-9-yl)methyl (2-(5-amino-3-morpholino-1H-1,2,4-triazol-1-yl)-2-oxoethyl)carbamate (4b). White microcrystals, 70% yield, mp 211-214 °C; 1H NMR (300 MHz, DMSO-d6) δ 7.90 (d, J = 7.2 Hz, 2H), 7.84-7.67 (m, 3H), 7.58 (br s, 2H), 7.48-7.30 (m, 4H), 4.44-4.09 (m, 5H), 3.63 (br s, 4H), 3.28 (br s, 4H); 13C NMR (75 MHz, DMSO-d6) δ169.3, 163.7, 157.5, 157.3, 144.4, 141.4, 128.3, 127.7, 125.9, 120.8, 66.5, 66.2, 47.3, 46.2, 44.0. Anal. Calcd for C23H24N6O4: C 61.60; H 5.39; N 18.74. Found: C 61.91; H 5.53; N 18.53.

(S)-Benzyl (1-(5-amino-3-morpholino-1H-1,2,4-triazol-1-yl)-1-oxo-3-phenylpropan-2-yl)carbamate (4c). White microcrystals, 95% yield, mp 217-218 °C; 1H NMR (300 MHz, DMSO-d6) δ 7.88 (d, J = 7.2 Hz, 2H), 7.76-7.75 (m, 4H) 7.57 (br s, 2H), 7.45-7.40 (m 4 H), 4.34-4.30 (m, 2H), 4.27-4.21 (m, 2H), 3.63 (d, J = 4.8 Hz, 4H), 3.28 (d, J = 4.8 Hz, 4H); 13C NMR (75 MHz, DMSO-d6) δ 166.7, 163.0, 156.8, 156.6, 143.8, 140.7, 127.6, 127.0, 125.2, 120.1, 65.8, 65.5, 46.6, 45.5, 43.4. Anal. Calcd for C23H26N6O4: C 61.32; H 5.82; N 18.65. Found: C 61.44; H 5.45; N 18.28.

tert-Butyl (2-(5-amino-3-morpholino-1H-1,2,4-triazol-1-yl)-2-oxoethyl)-carbamate (4d). White microcrystals, 73% yield, mp 212-215 °C; 1H NMR (300 MHz, DMSO-d6) δ 7.55 (br s, 2H), 7.14 (t, J = 6.1 Hz, 1H), 4.14 (d, J = 6.1 Hz, 2H), 3.64 (t, J = 4.7 Hz, 4H), 3.27 (t, J = 4.7 Hz, 4H), 1.39 (s, 9H); 13C NMR (75 MHz, DMSO-d6) δ 169.0, 163.0, 156.8, 155.9, 78.2, 65.6, 45.5, 43.0, 28.2. Anal. Calcd for C13H22N6O4: C 47.84; H 6.79; N 25.75. Found: C 48.17; H 6.97; N 25.95.

tert-Butyl (2-(5-amino-3-(phenylamino)-1H-1,2,4-triazol-1-yl)-2-oxoethyl)-carbamate (4e). White microcrystals, 65% yield, mp 230-233 °C; 1H NMR (300 MHz, DMSO-d6) δ 9.29 (s, 1H), 7.65 - 7.52 (m, 4H), 7.29 - 7.17 (m, 3H), 6.85 (t, J = 7.2 Hz, 1H), 4.27 (d, J = 6.0 Hz, 2H), 1.41 (s, 9H); 13C NMR (75 MHz, DMSO-d6) δ 168.9, 158.4, 155.8, 140.9, 128.6, 120.0, 116.7, 78.3, 43.2, 28.2. Anal. Calcd for C15H20N6O3: C 54.21; H 6.07; N 25.29. Found: C 54.43; H 6.23; N 24.57.

General Procedure for Microwave Assisted Synthesis of Compounds 5a-c. A mixture of the appropriate N-(protected dipeptidoyl)benzotriazole (1 mmol) and 3-morpholino-1H-1,2,4-triazol-5-amine (0.169 g, 1 mmol) in dry THF (3 mL) was subjected to microwave irradiation (70 °C, 100 W, 30 min). The reaction mixtures were allowed to cool to room temperature and evaporated to give crude products. Compound 5a was dissolved in EtOAc (30 mL), washed with aqueous Na2CO3 solution (10% w/w, 3 x 20 mL), water (3 x 20 mL), dried over MgSO4 and the solvent was removed under reduced pressure to give the desired product. Compound 5b was recrystallized from MeOH. Compound 5c was recrystallized from Et2O:hexanes. The precipitates were collected, washed with hexanes (2 x 5 mL) and dried under vacuum.

Benzyl ((S)-1-(((S)-1-(5-amino-3-morpholino-1H-1,2,4-triazol-1-yl)-1-oxopropan-2-yl)amino)-1-oxo-3-phenylpropan-2-yl)carbamate (5a). White microcrystals, 95% yield, mp 188-190 °C; 1H NMR (300 MHz, DMSO-d6) δ 8.36 (d, J = 6.5 Hz, 1H), 7.89 (d, J = 7.6 Hz, 2H), 7.71 (d, J = 7.7 Hz, 2H), 7.57 (m, 3H), 7.46-7.28 (m, 6H), 5.10-5.00 (m, 1H), 4.30-4.17 (m, 3H), 3.68-3.62 (m, 6H), 3.32-3.24 (m, 4H), 1.36 (d, J = 7.0 Hz, 3H); 13C NMR (75 MHz, DMSO-d6) δ 171.9, 169.0, 162.9, 157.1, 156.4, 143.8, 140.7, 127.6, 127.1, 125.2, 120.1, 65.7, 65.6, 47.6, 46.6, 45.5, 42.9, 16.3. Anal. Calcd for C26H31N7O5: C 59.87; H 5.99; N 18.80. Found: C 60.21; H 5.60; N 18.42.

Benzyl ((S)-1-(((S)-1-(5-amino-3-morpholino-1H-1,2,4-triazol-1-yl)-1-oxo-3-phenylpropan-2-yl)-amino)-1-oxopropan-2-yl)carbamate (5b). White microcrystals, 86% yield, mp 194-195 °C; 1H NMR (300 MHz, DMSO-d6) δ 8.26 (d, J = 7.7 Hz, 1H), 7.56 (br s, 2H), 7.40-7.20 (m, 10H), 7.19-7.11 (m, 1H), 5.28-5.05 (m, 1H), 4.93 (d, J = 2.6 Hz, 2H), 4.12-3.96 (m, 1H), 3.67-3.54 (m, 4H), 3.37-3.22 (m, 4H), 3.14 (dd, J = 13.6, 3.3 Hz, 1H), 2.81 (dd, J = 13.8, 9.8 Hz, 1H), 1.14 (d, J = 7.1 Hz, 3H); 13C NMR (75 MHz, DMSO-d6) δ 172.8, 170.6, 163.0, 157.1, 155.5, 137.6, 137.0, 129.0, 128.3, 128.2, 127.8, 126.5, 65.6, 65.3, 53.8, 49.7, 45.5, 35.7, 18.2. Anal. Calcd for C26H31N7O5: C 59.87; H 5.99; N 18.80. Found: C 59.72; H 6.04; N 18.92.

(S)-Benzyl (2-((1-(5-amino-3-morpholino-1H-1,2,4-triazol-1-yl)-4-methyl-1-oxopentan-2-yl)amino)-2-oxoethyl)carbamate (5c). White microcrystals, 76% yield, mp 102-106 °C; 1H NMR (300 MHz, DMSO-d6) δ 8.26 (d, J = 8.0 Hz, 1H), 7.57 (br s, 2H), 7.43 (t, J = 6.2 Hz, 1H), 7.39-7.29 (m, 5H), 5.18-5.07 (m, 1H), 5.02 (s, 2H), 3.74-3.58 (m, 6H), 3.31-3.22 (m, 4H), 1.79-1.36 (m, 3H), 0.97-0.78 (m, 6H); 13C NMR (75 MHz, DMSO-d6) δ 171.9, 169.4, 162.8, 157.1, 156.4, 137.0, 128.3, 127.7, 65.5, 65.4,50.3, 45.5, 43.1, 24.6, 23.2, 20.9. Anal. Calcd for C22H31N7O3: C 55.80; H 6.60; N 20.71. Found: C 55.86; H 6.55; N 20.33.

N5-Benzyl-1-methyl-1H-1,2,4-triazole-3,5-diamine (6). Methyl N-benzyl-N'-cyanocarbamimidothioate (2.0 g, 10 mmol) and methylhydrazine (1.1 mL, 20 mmol) was refluxed in EtOH (50 mL) for 4 hours. The solvent was removed under reduced pressure and the crude residue was recrystallized from MeCN/hexanes. White microcrystals, 51% yield, mp 159-162 °C (lit.15 mp 159-161 °C); 1H NMR (300 MHz, DMSO-d6) δ 7.40-7.16 (m, 5H), 6.74 (t, J = 6.0 Hz, 1H), 4.90 (br s, 2H), 4.39 (d, J = 6.0 Hz, 2H), 3.34 (s, 3H); 13C NMR (75 MHz, DMSO-d6) δ 160.2, 154.8, 140.4, 128.2, 127.1, 126.7, 46.6, 32.5.

(S)-Benzyl (1-((1-methyl-5-(phenylamino)-1H-1,2,4-triazol-3-yl)amino)-1-oxopropan-2-yl)-carbamate (7a). A mixture of Cbz-L-Ala-Bt (0.32 g, 1 mmol) and N5-benzyl-1-methyl-1H- 1,2,4-triazole-3,5-diamine (0.20 g, 1 mmol) in dry THF (3 mL) was subjected to microwave irradiation (70 °C, 100 W, 30 min). The reaction was quenched with water (2 mL), and extracted with EtOAc (3 x 10 mL). The combined organics were washed with aqueous Na2CO3 solution (10% w/w, 3 x 20 mL), water (3 x 20 mL) and dried over MgSO4. The solvent was then removed under reduced pressure and the residue was recrystallized from CH2Cl2/hexanes. White microcrystals, 57% yield, mp 184-185 °C; 1H NMR (300 MHz, DMSO-d6) δ 7.47 (d, J = 7.6 Hz, 1H), 7.38-7.29 (m, 11H), 7.28-7.18 (br s, 1H), 5.00 (s, 2H), 4.41 (d, J = 5.9 Hz, 2H), 4.14 (d, J = 1.3 Hz, 1H), 3.48 (s, 3H), 1.21 (d, J = 7.1 Hz, 3H); 13C NMR (75 MHz, DMSO-d6) δ 155.6, 154.7, 140.0, 128.3, 128.2, 127.7, 127.0, 126.7, 65.3, 46.4, 33.0, 18.0. Anal. Calcd for C21H24N6O3: C 61.75; H 5.92; N 20.57. Found: C 61.37; H 5.92; N 20.63.

Benzyl ((S)-1-(((S)-1-((5-(benzylamino)-1-methyl-1H-1,2,4-triazol-3-yl)amino)-1-oxo-3-phenylpropan-2-yl)amino)-1-oxopropan-2-yl)carbamate (7b). A mixture of Cbz-L-Ala-L-Phe-Bt (0.48 g, 1 mmol) and N5-benzyl-1-methyl-1H-1,2,4-triazole-3,5-diamine (0.20 g, 1 mmol) in dry THF (3 mL) was subjected to microwave irradiation (70 °C, 100 W, 30 min). The reaction was quenched with water (2 mL), and extracted with EtOAc (3 x 10 mL). The combined organics were washed with aqueous Na2CO3 solution (10% w/w, 3 x 20 mL), water (3 x 20 mL) and dried over MgSO4. The solvent was then removed under reduced pressure and the residue was recrystallized from CH2Cl2/hexanes. White microcrystals, 71% yield, mp 107-109 °C; 1H NMR (300 MHz, DMSO-d6) δ 10.27 (br s, 1H), 7.95 (d, J = 7.9 Hz, 1H), 7.45-7.14 (m, 16H), 7.07 (br s, 1H), 5.06-4.91 (m, 2H), 4.72-4.51 (m, 1H), 4.42 (d, J = 6.0 Hz, 2H), 4.10-3.94 (m, 1H), 3.50 (s, 3H), 3.09-2.93 (m, 1H), 2.89-2.68 (m, 1H), 1.12 (d, J = 7.1 Hz, 3H); 13C NMR (75 MHz, DMSO-d6) δ 172.9, 169.5, 156.2, 155.3, 152.4, 140.6, 138.1, 137.6, 130.0, 129.0, 128.9, 128.6, 128.4, 127.7, 127.4, 126.9, 66.1, 54.7, 50.7, 47.1, 38.2, 33.7, 18.9. Anal. Calcd for C30H33N7O4: C 64.85; H 5.99; N 17.65. Found: C 64.55; H 6.04; N 17.55.

References

1. S. Caddick and R. Fitzmaurice, Tetrahedron, 2009, 65, 3325. CrossRef
2. C. O. Kappe, Angew. Chem. Int. Ed., 2004, 43, 6250. CrossRef
3. B. Wathey, J. Tierney, P. Lidström, and J. Westman, Drug Discovery Today, 2002, 7, 373. CrossRef
4. P. Lidström, J. Tierney, B. Wathey, and J. Westman, Tetrahedron, 2001, 57, 9225. CrossRef
5. X. Collin, A. Sauleau, and J. Coulon, Bioorg. Med. Chem. Lett., 2003, 13, 2601. CrossRef
6. S. Papakonstantinou-Garoufalias, N. Pouli, P. Marakos, and A. Chytyroglou-Ladas, Il Farmaco 2002, 57, 973. CrossRef
7. C. Chen, R. Dagnino, C. Q. Huang, J. R. McCarthy, and D. E. Grigoriadis, Bioorg. Med. Chem. Lett., 2001, 11, 3165. CrossRef
8. C. Gespach, I. Menez, and S. Emami, Biosci. Rep., 1983, 3, 871. CrossRef
9. H. J. Wadsworth, S. M. Jenkins, B. S. Orlek, F. Cassidy, M. S. G. Clark, F. Brown, G. J. Riley, D. Graves, J. Hawkins, and C. B. Naylor, J. Med. Chem., 1992, 35, 1280. CrossRef
10. S. M. Jenkins, H. J. Wadsworth, S. Bromidge, B. S. Orlek, P. A. Wyman, G. J. Riley, and J. Hawkins, J. Med. Chem., 1992, 35, 2392. CrossRef
11. J. P. Malerich, J. S. Lam, B. Hart, R. M. Fine, B. Klebansky, M. J. Tanga, and A. D'Andrea, Bioorg. Med. Chem. Lett., 2010, 20, 7454. CrossRef
12. S. Huang, P. J. Connolly, R. Lin, S. Emanuel, and S. A. Middleton, Bioorg. Med. Chem. Lett., 2006, 16, 3639. CrossRef
13. E. A. Steck, R. P. Brundage, and L. T. Fletcher, J. Am. Chem. Soc., 1958, 80, 3929. CrossRef
14. M. T. Wu, J. Heterocycl. Chem., 1977, 14, 44.
15. J. Reiter, L. Pongó, T. Somorai, and P. Dvortsák, J. Heterocycl. Chem., 1986, 23, 401. CrossRef
16. R. L. Webb and C. S. Labaw, J. Heterocycl. Chem., 1982, 19, 1205. CrossRef
17. R. L. Webb, D. S. Eggleston, C. S. Labaw, J. J. Lewis, and K. Wert, J. Heterocycl. Chem., 1987, 24, 275. CrossRef
18. P. J. Garratt, S. N. Thorn, and R. Wrigglesworth, Tetrahedron, 1993, 49, 165. CrossRef
19. A. R. Dunstan, H.-P. Weber, G. Rihs, H. Widmer, and E. K. Dziadulewicz, Tetrahedron Lett., 1998, 39, 7983. CrossRef
20. V. J. Hruby and P. M. Balse, Curr. Med. Chem., 2000, 7, 945.
21. V. J. Hruby, Nat. Rev. Drug Discovery, 2002, 1, 847. CrossRef
22. A. Grauer and B. Koenig, Eur. J. Org. Chem., 2009, 5099. CrossRef
23. G. A. Patani and E. J. LaVoie, Chem. Rev., 1996, 96, 3147. CrossRef
24. D. Boeglin, S. Cantel, A. Heitz, J. Martinez, and J.-A. Fehrentz, Org. Lett., 2003, 5, 4465. CrossRef
25. A. R. Katritzky, P. Angrish, and E. Todadze, Synlett, 2009, 15, 2392. CrossRef
26. J. Bonjean and W. Schunack, Arch. Pharm. (Weinheim), 1987, 320, 608. CrossRef
27. C. G. McCarty, J. E. Parkinson, and D. M. Wieland, J. Org. Chem., 1970, 35, 2067. CrossRef
28. M. Yokoyama, M. Kurauchi, and T. Imamoto, Tetrahedron Lett., 1981, 22, 2285. CrossRef
29. T. Suyama, N. Suzuki, M. Nishimura, Y. Saitoh, H. Ohkoshi, and J.-i. Yamaguchi, Bull. Chem. Soc. Jpn., 2005, 78, 873. CrossRef