HETEROCYCLES

Paper | Special issue | Vol. 77, No. 2, 2009, pp. 1249-1259
Received, 6th September, 2008, Accepted, 30th October, 2008, Published online, 31st October, 2008.
DOI: 10.3987/COM-08-S(F)109

■ Preparation and Synthetic Applications of N-(α,β-Unsaturated Acyl)-α-amino Acid Derivatives

Alan R. Katritzky,* Reena Gyanda, Nabin K. Meher, and Yuming Song

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

Abstract

N-(α,β-Unsaturated acyl)-α-amino acids, amides and esters are structural motifs of many biologically active natural products. An alternate and advantageous approach for the synthesis of N-(α,β-unsaturated acyl)-α-amino acid derivatives is developed via acylation of unprotected α-amino acids with stable crystalline N-(α,β-unsaturated acyl)benzotriazole. The proposed methodology provides a new synthesis for compound (9) which is a precursor to a novel cytotoxic agent.

INTRODUCTION
N-(α,β-Unsaturated acyl)-α-amino acids, amides and esters are structural motifs of natural products such as gymnastatin N (1), its 12-hydroxy analog (2)1 and gymnastatin H (3)2. Cis- and trans-tryptophan 4-aminocinnamamides have been isolated from the myxomycete Fuligo aurea.3

N-(α,β-Unsaturated acyl)-α-amino acids and esters have been reported to be biologically active as anticancer agents,1 aldose reductase inhibitors,4 rat kidney enzyme inhibitors,5 pseudo analogs of the naturally occurring antibiotic sparsomycin,6 chromogenic substrates of proteolytic enzymes,7 lysozyme adsorption agents,8 HIV-1 integrase inhibitors9 and inhibitors of mammalian collagenase.10 N-(α,β-unsaturated acyl)-α-amino acids are intermediates in the synthesis of prodrugs for cancer treatment,11 the antibiotic Reutericyclin,12 cell adhesive agents13 used for the synthesis of cyclic-peptide based uPA inhibitors,14 homo-poly(N-acryl α-amino acid) of biological interests15 and 4-hydroxybenzylidene-1-methyl-2-propenyl-imidazolinone (HBMPI) or 4-hydroxybenzylidene- 1-methyl-2-penta-1,3-diene-1-yl-imidazolinone (HBMPDI) models to study the chromophore within Discosoma Red (a red fluorescent protein).16

Published preparations for N-(α,β-unsaturated acyl)-α-amino acids (Scheme 1) from α,β-unsaturated acid and carboxyl - protected α-amino acid units include: (i) coupling mediated by DCC,11,17 1-ethyl-3-(3′-dimethylaminopropyl)carbodiimide hydrochloride (EDCI)9,13 benzotriazol-1-yloxytris (dimethylamino)phosphonium hexafluorophosphate (BOP)18 and tert-butyl 3-(3,4-dihydrobenzotriazine- 4-on)yl carbonate (Boc-ODhbt)19 (ii) via anhydride using isobutyl chloroformate;4 (iii) reactions of α,β-unsaturated acid chlorides with carboxyl - protected or unprotected α-amino acids.12,14,15,20-26

We now report an alternate, advantageous approach to N-(α,β-unsaturated acyl)-α-amino acid derivatives by the acylation of α-amino acids with stable crystalline N-(α,β-unsaturated acyl)benzotriazole acylating agents:27 advantages include the use of stable, crystalline acylating agents and free amino acids in aqueous solutions.

RESULTS AND DISCUSSION
To generalize the preparation of N-(α,β-unsaturated acyl)-α-amino acids by the N-acylation of α-amino acids with N-(α,β-unsaturated acyl)benzotriazoles, we optimized conditions for the reaction of N-cinnamoylbenzotriazole (4a) with phenylalanine (5a) with respect to solvent, base and phase-transfer catalyst (Scheme 2). The solvent combination of 1, 2-dimethoxyethane (DME)/water was identified as preferred. In the presence of triethylamine as base, the reaction was complete in 48-72 hr in DME/H2O at 20 oC whereas in MeCN/H2O, 120 hrs were required for approximately the same yield. The use of other amines or bases such as K2CO3 and NaOH in place of NEt3 gave similar results. Tetrabutylammonium bromide (25 mol%) as phase transfer catalyst in CH2Cl2/H2O had no effect with K2CO3 or NaOH systems. We used the above conditions (DME/H2O/Et3N at rt) for the N-acylation of amino acids (5a-f) as described below. Other methods were less useful: although microwave irradiation (60-100 W/75 oC for 75 min.), increased the rate of reaction of 4a with 5b this gave a mixture of the expected product (6ab) and cinnamic acid; the acylation of 5a with 4a in the presence of aqueous LiOH (1N) in DME at room temperature was complete in 120 hr but also gave a mixture of the expected product (6aa) and cinnamic acid.

The N-(α,β-unsaturated-acyl)benzotriazoles (4a-e) required were prepared in 80-90% yields from the corresponding carboxylic acid by reaction with benzotriazole and thionyl chloride.27 Using reagents (4a-e) a variety of (L)-α-amino acids (5a-f) were acylated to give products (6aa-ea) in 34-87% yields (Table 1). Reactions of acylating agent 4a with amino acids (5a-c) were complete in about 48 hr (entries 1-3, Table 1) but for amino acids (5d, 5e) the reactions required 84 hr and 72 hr respectively (entries 4, 5, Table 1). Acylation of sterically hindered L-valine 5f with 4a required 340 hr for complete reaction (entry 6, Table 1). Furylacryloylbenzotriazole 4b reacted with amino acids (5a-5f) in 24-48 hr (entries 7-12, Table 1).

Acylating agent 4c containing a C≡C bond reacted faster (entries 13-15, Table 1) than the corresponding C=C compound (4a). This method has also been successfully applied to prepare 6df and 6ea in good yields under mild conditions (entries 16-17, Table 1). Analysis by HPLC of compound 6ea confirmed the absence of racemization: a single peak was obtained for the enantiomer 6ea at 4.0 min. whereas the racemate gave two peaks at 4.0 and 4.8 min.

Preparation of Boc-protected trans-tryptophan 4-aminocinnamamide (9),3 is outlined in Scheme 3. The amino group of p-aminocinnamic acid was protected to give 7 (98%)28 which was converted into the corresponding acylbenzotriazole 8 (80%).27 On reaction of 8 with tryptophan (5b) the desired product 9 was obtained in 36% yield.

CONCLUSIONS
Using our acyl-benzotriazole methodology, air stable crystalline aromatic and heteroaromatic substituted N-(α,β-unsaturated acyl)benzotriazoles react with free amino acids in aqueous solution at rt to afford potentially bioactive N-(α,β-unsaturated acyl)-α-amino acids derivatives in moderate to good yields without racemization. This methodology provides a new synthesis of compound (9) which is a precursor for a novel cytotoxic agent.3

EXPERIMENTAL
Melting points were determined on a capillary point apparatus equipped with the digital thermometer and are uncorrected. NMR spectra were recorded in CDCl3 or DMSO-d6 with TMS as internal standard for 1H NMR (300 MHz) or a solvent as internal standard for 13C NMR (75 MHz). Elemental and mass spectroscopy analyses were performed by the analytical laboratories, Department of Chemistry, University of Florida. Mass Analysis was performed using a ThermoFinnigan-LCQ ion trap mass spectrometer (San Jose, CA) in electrospray ionization (ESI) mode. Optical rotation values were measured with the use of sodium D line on Perkin-Elmer 241 polarimeter. HPLC analyses were performed on Shimadzu LC -20AT system using chirobiotic T column (4.6 X 250 mm), detection at 254 nm, flow rate 0.5 mL/min. and methanol:water (1:1) as solvent system. Reagents obtained commercially were used without further purification. Column chromatography was conducted with silica gel (200-425 mesh).

General method for the preparation of N-(α,β-unsaturated acyl)-α-amino acids (6): A solution of cinnamoylbenzotriazole (0.125 g, 0.5 mmol) in DME (30 mL) was added at rt to a solution of phenylalanine (0.083 g, 0.5 mmol) in DME (14 mL) and water (6 mL) in the presence of triethylamine (0.17 mL, 1.2 mmol). The reaction mixture was then stirred at rt for 56 h; 6 N HCl (1 mL) was added and the solvent was removed under reduced pressure. The residue was extracted with EtOAc (20 mL), washed with 6 N HCl (5 mL) and brine (10 mL), and then dried over MgSO4 (anhyd.). Filtration and evaporation of the solvent under reduced pressure gave (2S)-3-phenyl-2-{[(E)-3-phenyl-2-propenoyl]amino}- propanoic acid (6aa) (0.13 g, 87%) as yellow microcrystals. Mp 191-193 oC (Lit.,26 mp 198-199.5 oC); 1H NMR (DMSO-d6): δ 2.89-2.97 (m, 1H), 3.10-3.17 (m, 1H), 4.54-4.62 (m, 1H), 6.71 (d, J = 15.9 Hz, 1H), 7.18-7.31 (m, 5H), 7.37-7.42 (m, 4H), 7.54-7.57 (m, 2H), 8.45 (d, J = 8.1 Hz, 1H); 13C NMR (DMSO-d6): 36.8, 53.7, 121.7, 126.5, 127.6, 128.3, 129.0, 129.1, 129.6, 134.8, 137.6, 139.3, 164.9, 173.1. HRMS [M+H]+ Calcd for C18H18NO3: 296.1281. Found: 296.1270

(2S)-3-(1H-Indol-3-yl)-2{[(E)-3-phenyl-2-propenoyl]amino}propanoic acid (6ab): (50%) brown microcrystals (MeOH/water); mp 228-230 oC; 1H NMR (DMSO-d6): δ 3.03-3.27 (m, 2H), 4.59-4.66 (m, 1H), 6.73 (d, J = 15.9 Hz, 1H), 6.95-6.99 (m, 1H), 7.03-7.08 (m, 1H), 7.15-7.18 (m, 1H), 7.31-7.41 (m, 5H), 7.53-7.56 (m, 3H), 8.39 (d, J = 7.7 Hz, 1H), 10.85 (s, 1H), 12.74 (br s, 1H); 13C NMR (DMSO-d6): 27.2, 53.1, 109.8, 111.4, 118.2, 118.4, 120.9, 121.8, 123.6, 127.2, 127.5, 128.9, 129.5, 134.8, 136.1, 139.1, 164.9, 173.4. Anal. Calcd for C20H18N2O3: C, 71.84; H, 5.43; N, 8.38. Found: C, 71.62; H, 5.56; N, 8.44.

2-{[(E)-3-Phenyl-2-propenoyl]amino}acetic acid (6ac): (44%) white microcrystals (EtOAc/hexane); mp 198-201 oC (Lit.,29 mp 194.7-197.3 oC); 1H NMR (DMSO-d6): δ 3.88 (d, J = 5.8 Hz, 2H), 6.72 (d, J = 15.7 Hz, 1H), 7.35-7.47 (m, 4H), 7.56-7.59 (m, 2H), 8.43 (t, J = 5.8 Hz, 1H), 12.63 (br s, 1H); 13C NMR (DMSO-d6): 40.8, 121.7, 127.6, 129.0, 129.6, 134.8, 139.2, 165.3, 171.4. Anal. Calcd for C11H11NO3: C, 64.38; H, 5.40; N, 6.83. Found: C, 64.43; H, 5.34; N, 6.91.

(2S)-2-{[(E)-3-Phenyl-2-propenoyl]amino}propanoic acid (6ad): (40%) white microcrystals (EtOAc/hexane); mp 193.4-194.5 oC; 1H NMR (DMSO-d6): δ 1.32 (d, J = 7.1 Hz, 3H), 4.30-4.35 (m, 1H), 6.71 (d, J = 15.9 Hz, 1H), 7.35-7.46 (m, 4H), 7.55-7.57 (m, 2H), 8.42 (d, J = 6.9 Hz, 1H); 13C NMR (DMSO-d6): 17.3, 47.7, 121.7, 127.5, 129.0, 129.5, 134.8, 139.1, 164.6, 174.2. [α]D25 +10.8 (c 1.0, EtOH). HRMS [M+H]+ Calcd for C12H14NO3: 220.0968. Found: 220.0948.

(2S,3S)-3-Methyl-2-{[(E)-3-phenyl-2-propenoyl]amino}pentanoic acid (6ae): (46%) pale yellow microcrystals (EtOAc/hexane); mp 80.0-82.5 oC; 1H NMR (DMSO-d6): δ 0.88-0.91 (m, 6H), 1.20-1.30 (m, 1H), 1.39-1.50 (m, 1H), 1.75-1.90 (m, 1H), 4.31-4.36 (m, 1H), 6.86 (d, J = 15.7 Hz, 1H), 7.40-7.57 (m, 6H), 8.22 (d, J = 8.2 Hz, 1H), 12.63 (br s, 1H); 13C NMR (DMSO-d6): 11.4, 15.7, 24.8, 36.6, 56.5, 122.0, 127.6, 129.0, 129.6, 135.0, 139.2, 165.1, 173.1. Anal. Calcd for C15H19NO3: C, 68.94; H, 7.33. Found: C, 67.12; H, 7.43.

(2S)-3-Methyl-2-{[(E)-3-phenyl-2-propenoyl]amino}butanoic acid (6af): (53%) white microcrystals (EtOAc/hexane); mp 192.0−194.0 oC (Lit.,26 mp 187−188 oC); 1H NMR (DMSO-d6): δ 0.92 (d, J = 6.6 Hz, 6H), 2.08-2.14 (m, 1H), 4.27-4.32 (m, 1H), 6.87 (d, J = 15.7 Hz, 1H), 7.37-7.46 (m, 4H), 7.55-7.57 (m, 2H), 8.21 (d, J = 8.2 Hz, 1H), 12.65 (br s, 1H); 13C NMR (DMSO-d6): 18.0, 19.2, 29.9, 57.3, 121.9, 127.5, 129.0, 129.5, 134.9, 139.1, 165.1, 173.0. [α]D25 +18.2 (c 1.0, EtOH). Anal. Calcd for C14H17NO3: C, 68.00; H, 6.93; N, 5.66. Found: C, 67.37; H, 7.42; N, 6.00.

(2S)-2-{[(E)-3-(2-Furyl)-2-propenoyl)amino]-3-phenylpropanoic acid (6ba): (54%) yellow microcrystals (EtOAc/hexane); mp 138-140 oC (Lit.,30 mp 119-121 oC); 1H NMR (DMSO-d6): δ 2.85-2.95 (m, 1H), 3.08-3.14, (m, 1H), 4.50-4.58 (m, 1H), 6.47 (d, J = 15.7 Hz, 1H), 6.56-6.60 (m, 1H), 6.76-6.77 (m, 1H), 7.16-7.25 (m, 6H), 7.77 (s, 1H), 8.50 (d, J = 8.0 Hz, 1H), 12.79 (br s, 1H); 13C NMR (DMSO-d6): 36.9, 53.8, 112.5, 114.1, 118.9, 126.5, 126.6, 128.3, 129.1, 137.7, 144.9, 150.9, 164.8, 173.1. [α]D25 -15.0 (c 1.0, EtOH). Anal. Calcd for C16H15NO4.EtOAc (4:1): C, 66.44; H, 5.58; N, 4.56. Found: C, 66.07; H, 5.32; N, 5.00.

(2S)-2-{[(E)-3-(2-Furyl)-2-propenoyl]amino}-3-(1H-indole-3-yl)propanoic acid (6bb): (52%) yellow microcrystals (EtOAc/hexane); mp 65 oC (Lit.,30 mp 98-100 oC); 1H NMR (DMSO-d6): δ 3.01-3.26 (m, 2H), 4.50-4.70 (m, 1H), 6.51 (d, J = 15.8 Hz, 1H), 6.58 (s, 1H), 6.76 (s, 1H), 6.96-7.00 (m, 1H), 7.04-7.08 (m, 1H), 7.15-7.22 (m, 2H), 7.33 (d, J = 7.7 Hz, 1H), 7.55 (d, J = 7.4 Hz, 1H), 7.77 (s, 1H), 8.47 (d, J = 7.1 Hz, 1H), 10.85 (s, 1H); 13C NMR (DMSO-d6): 27.3, 53.2, 109.9, 111.4, 112.4, 114.0, 118.2, 118.4, 119.1, 121.0, 123.6, 126.5, 127.2, 136.1, 144.9, 151.0, 164.8, 173.5. HRMS [M+H]+ Calcd for C18H17N2O4: 325.1183. Found: 325.1200

2-{[(E)-3-(2-Furyl)-2-propenoyl]amino}acetic acid (6bc): (34%) white microcrystals (EtOAc/hexane); mp 221-223 oC (Lit.,18 mp 245 oC); 1H NMR (DMSO-d6): δ 3.86 (d, J = 6.0 Hz, 2H), 6.48 (d, J = 15.4 Hz, 1H), 6.58-6.60 (m, 1H), 6.79 (d, J = 3.3 Hz, 1H), 7.25 (d, J = 15.7 Hz, 1H), 7.78 (s, 1H), 8.49 (t, J = 6.0 Hz, 1H), 12.61 (br s, 1H); 13C NMR (DMSO-d6): 40.8, 112.4, 114.1, 118.8, 126.6, 144.9, 150.9, 165.2, 171.4. Anal. Calcd for C9H9NO4: C, 55.38; H, 4.65; N, 7.18. Found: C, 55.17; H, 4.60; N, 7.13.

(2S)-2-{[(E)-3-(2-Furyl)-2-propenoyl]amino}propanoic acid (6bd): (63%) yellow microcrystals (EtOAc/hexane); mp 143−146.5 oC (Lit.,30 mp 112-113 oC); 1H NMR (DMSO-d6): δ 1.30 (d, J = 7.1 Hz, 3H), 4.28-4.33 (m, 1H), 6.47 (d, J = 15.7 Hz, 1H), 6.58-6.59 (m, 1H), 6.78 (d, J = 3.3 Hz, 1H), 7.32 (d, J = 15.7 Hz, 1H), 7.78 (s, 1H), 8.45 (d, J = 7.1 Hz, 1H), 12.55 (br s, 1H); 13C NMR (DMSO-d6): 17.3, 47.7, 112.4, 114.0, 118.9, 126.5, 144.9, 150.9, 164.5, 174.2. Anal. Calcd for C10H11NO4: C, 57.41; H, 5.30; N, 6.70. Found: C, 57.20; H, 5.43; N, 7.09.

(2S)-2-{[(E)-3-(2-Furyl)-2-propenoyl]amino}propanoic acid (6be): (42%) yellow microcrystals (EtOAc/hexane); mp 45-50 oC (Lit.,18 mp 215 oC); 1H NMR (DMSO-d6): δ 0.83-0.89 (m, 6H), 1.16-1.26 (m, 1H), 1.38-1.47 (m, 1H), 1.75-1.86 (m, 1H), 4.28-4.33 (m,1H), 6.57-6.59 (m, 1H), 6.63 (d, J = 15.7 Hz, 1H), 6.76-6.77 (m, 1H), 7.23 (d, J = 15.7 Hz, 1H), 7.78 (s, 1H), 8.29 (d, J = 8.5 Hz, 1H), 12.63 (s, 1H); 13C NMR (DMSO-d6): 11.3, 15.7, 24.8, 36.5, 56.5, 112.4, 113.9, 119.2, 126.5, 144.9, 151.0, 165.0, 173.1. Anal. Calcd for C13H17NO4: C, 62.14; H, 6.82; N, 5.57. Found: C, 62.43; H, 7.13; N, 5.47.

(2S)-2-{[(E)-3-(2-Furyl)-2-propenoyl]amino}-3-methylbutanoic acid (6bf): (61%) yellow microcrystals (EtOAc/hexane); mp 96-98 oC (Lit.,30 mp 136-138 oC ); 1H NMR (DMSO-d6): δ 0.93-0.95 (m, 6H), 2.0-2.20 (m, 1H), 4.28-4.32 (m, 1H), 6.63-6.71 (m, 2H), 6.81-6.82 (m, 1H), 7.27 (d, J = 15.5 Hz, 1H), 7.83 (s, 1H), 8.31 (d, J = 8.4 Hz, 1H), 12.68 (br s, 1H); 13C NMR (DMSO-d6): 18.0, 19.2, 29.9, 57.4, 112.4, 113.9, 119.2, 126.5, 144.9, 151.0, 165.1, 173.1. [α]D25 +5.2 (c 1.0, EtOH). Anal. Calcd for C12H15NO4: C, 60.75; H, 6.37; N, 5.90. Found: C, 60.04; H, 7.05; N, 5.65.

(2S)-3-Phenyl-2-[(3-phenyl-propynoyl)amino]propanoic acid (6ca): (38%) white microcrystals (EtOAc/hexane); mp 112.5-114 oC; 1H NMR (DMSO-d6): δ 2.88-2.95 (m, 1H), 3.10-3.14 (m, 1H), 4.40-4.49 (m, 1H), 7.20-7.40 (m, 5H), 7.40-7.60 (m, 5H), 9.18 (d, J = 7.4 Hz, 1H), 12.89 (br s, 1H); 13C NMR (DMSO-d6): 36.1,53.9, 83.6, 83.8, 119.7, 126.5, 128.3, 129.0, 130.4, 132.2, 137.6, 172.4. [α]D25 -47.0 (c 1.0, EtOH). Anal. Calcd for C18H15NO3: C, 73.70; H, 5.15; N, 4.78. Found: C, 73.38; H, 5.33; N; 4.71.

(2S)-3-(1H-Indol-3-yl)-2-[(3-phenyl-2-propynoyl)amino]propanoic acid (6cb): (69%) pale yellow microcrystals (EtOAc/hexane); mp 142.2-144.0 oC; 1H NMR (DMSO-d6): δ 3.07-3.21 (m, 2H), 4.40-4.60 (m, 1H), 6.99 (t, J = 7.1 Hz, 1H), 7.07 (t, J = 7.7 Hz, 1H), 7.18 (s, 1H), 7.33-7.36 (m, 1H), 7.45-7.47 (m, 3H), 7.53-7.60 (m, 3H), 9.17 (d, J = 7.7 Hz, 1H), 10.88 (s, 1H), 12.76 (br s, 1H); 13C NMR (DMSO-d6): 26.7, 53.4, 83.8, 109.9, 111.5, 118.1, 118.5, 119.8, 121.1, 123.7, 127.0, 129.0, 130.4, 132.2, 132.6, 136.2, 152.4, 172.8. [α]D25 -39.5 (c 1.0, EtOH). HRMS [M+H]+ Calcd for C20H17N2O3: 333.1234. Found: 333.1230

(2S, 3S)-3-Methyl-2-[(3-phenyl-2-propynoyl)amino]pentanoic acid (6ce): (70%) yellow gel; 1H NMR (DMSO-d6): δ 0.85-0.90 (m, 6H), 1.10-1.30 (m, 1H), 1.30-1.50 (m, 1H), 1.70-1.90 (m, 1H), 4.20-4.30 (m, 1H), 7.40-7.50 (m, 3H), 7.60 (d, J = 5.8 Hz, 2H), 9.05 (d, J = 7.4 Hz, 1H), 12.7 (br s, 1H); 13C NMR (DMSO-d6): 11.3, 15.6, 24.8, 35.9, 57.0, 83.8, 83.9, 119.9, 129.0, 130.3, 132.2, 152.6, 172.5. [α]D25 +24.5 (c 1.0, EtOH). Anal. Calcd for C15H17NO3: C, 69.48; H, 6.61; N, 5.40. Found: C, 69.22; H, 6.64; N, 5.63.

(2S)-3-Methyl-2-{[(E)-3-(3,4,5-trimethoxyphenyl)-2-propenoyl]amino}butanoic acid (6df): (52%) pale yellow microcrystals (EtOAc/hexane); mp 198-200 oC (Lit.,31 mp 196-198 oC); 1H NMR (DMSO-d6): δ 0.92 (d, J = 4.1 Hz, 6H), 2.1-2.30 (m, 1H), 3.68 (m, 3H), 3.81 (m, 6H), 4.20-4.30 (m, 1H), 6.79-6.90 (m, 3H), 7.37 (d, J = 14.8 Hz, 1H), 8.13 (d, J = 7.4 Hz, 1H), 12.65 (br s, 1H); 13C NMR (DMSO-d6): 18.0, 19.2, 30.0, 55.8, 57.3, 60.1, 104.9, 121.3, 130.6, 138.6, 139.2, 153.1, 165.2, 173.1. [α]D25 +28.2 (c 1.0, EtOH). Anal. Calcd for C17H23NO6: C, 60.52; H, 6.87; N, 4.15. Found: C, 60.64; H, 7.33; N, 4.45.

(2S)-3-Phenyl-2-{[(E)-3-(2-thienyl)-2-propenoyl]amino}propanoic acid (6ea): (61%) white microcrystals (EtOAc/hexane); mp 164-165 oC (Lit.,32 mp 172-173 oC); 1H NMR (DMSO-d6): δ 2.85-2.93 (m, 1H), 3.08-3.13 (m, 1H), 4.45-4.62 (m, 1H), 6.44 (d, J = 15.4 Hz, 1H), 7.08-7.11 (m, 1H), 7.20-7.31 (m, 5H), 7.32-7.38 (m, 1H), 7.53 (d, J = 15.4 Hz, 1H), 7.60-7.61 (m, 1H), 8.43 (d, J = 8.0 Hz, 1H), 12.79 (br s, 1H); 13C NMR (DMSO-d6): 36.8, 53.7, 120.3, 126.5, 128.1, 128.3, 128.4, 129.1, 130.9, 132.3, 137.6, 139.8, 164.7, 173.1. [α]D25 -31.1 (c 1.0, EtOH). Anal. Calcd for C16H15NO3S: C, 63.77; H, 5.02; N, 4.65. Found: C, 63.41; H, 5.04; N, 4.99. HPLC analyses: Rt (E, S): 4.0 min., Rt (E, R): 4.8 min.

Preparation of (E)-3-{4-[(tert-butoxycarbonyl)amino]phenyl}-2-propenoic acid (7): p-Aminocinnamic acid (1.63 g, 10 mmol) was dissolved in dioxane (15 mL) and 1 M aqueous NaOH (10 mL) and stirred at 0 oC for 30 min. A solution of di-t-butyldicarbonate (2.4 g, 11 mmol) in dioxane (10 mL) was added at 0 oC and the mixture was stirred at 0 oC for 18 h. The reaction mixture was acidified with citric acid and extracted with EtOAc (3 × 30 mL). The combined organic layer was dried over anhydrous Na2SO4. Filtration and evaporation of solvent under reduced pressure gave 7 (2.33 g, 98%) as yellow microcrystals (from EtOAc/hexane). Mp 196-197.5 oC (lit.,28 mp 204 oC); 1H NMR (DMSO-d6): δ 1.49 (s, 9H), 6.41 (d, J = 16.1 Hz, 1H), 7.51-7.56 (m, 3H), 7.59-7.62 (m, 2H), 9.62 (br s, 1H), 12.25 (s, 1H); 13C NMR (DMSO-d6): 28.1, 79.5, 116.8, 118.0, 127.9, 129.1, 141.6, 143.8, 152.6, 167.9. Anal. Calcd for C14H17NO4: C, 63.86; H, 6.51; N, 5.32. Found: C, 63.55; H, 6.76; N, 5.04.

Preparation of tert-butyl N-{4-[(E)-3-(1H-1,2,3-benzotriazole-1-yl)-3-oxo-1-propenyl]phenyl}carbamate (8): To a solution of benzotriazole (0.60 g, 5.06 mmol) in CH2Cl2 (20 mL) was added SOCl2 (0.16 g, 1.37 mmol) at rt with stirring. After 30 min., tert-Boc-aminocinnamic acid (0.36 g, 1.37 mmol) was added in one portion and stirring was continued for 2 h. The yellow precipitate was filtered off and washed with CH2Cl2 (2 × 50 mL). The filtrate was washed with aqueous 2 N NaOH (3 × 60 mL), dried over Na2SO4 and the solvent was removed under reduced pressure to give 8 in 80% yield as yellow needles (from CHCl3/hexane); mp 124-126 oC; 1H NMR (CDCl3): δ 1.54 (s, 9H), 6.97 (s, 1H), 7.48-7.55 (m, 3H), 7.65-7.71 (m, 3H), 8.05 (d, J = 12.9 Hz, 2H), 8.15 (d, J = 8.5 Hz, 1H), 8.42 (d, J = 8.2 Hz, 1H); 13C NMR (CDCl3): 28.3, 81.1, 113.8, 114.8, 118.3, 120.1, 126.1, 128.6, 130.2, 130.2, 131.5, 141.7, 146.3, 148.4, 152.3, 164.1. Anal. Calcd for C20H20N4O3: C, 65.92; H, 5.53; N, 15.37. Found: C, 65.65; H, 5.48; N, 15.20.

Preparation of (S)-[{(E)-3-{4-[(tert-Butoxycarbonyl)amino]phenyl}-2-propenoyl)amino]-3-(1H-indol-3-yl)propanoic acid (9): A solution of tert-butyl N-{4-[(E)-3-(1H-1,2,3-benzotriazole-1-yl) -3-oxo-1-propenyl]phenyl}carbamate (0.091 g, 0.25 mmol) in DME (14 mL) was added at rt to a solution of L-tryptophan (0.051 g, 0.25 mmol) in DME (1.5 mL), water (1.65 mL) and Et3N (0.168 mL, 1.2 mmol). The reaction mixture was then stirred at rt for 15d. 6 N HCl (4 mL) was then added and the solvent was removed under reduced pressure. The residue was extracted with EtOAc (20 mL), washed with brine (10 mL), and then dried over MgSO4. After removal of the solvent the crude product was purified by column chromatography (silica gel) using EtOAc/hexane (6:4) to give the product 9 in 36% yield. Yellow microcrystals (from EtOAc/hexane); mp 145 oC; 1H NMR (DMSO-d6): δ 1.51 (s, 9H), 3.06-3.13 (m, 2H), 4.60-4.65 (m, 1H), 6.62 (d, J = 15.7 Hz, 1H), 7.01 (t, J = 7.2 Hz, 1H), 7.09 (t, J = 7.6 Hz, 1H), 7.19 (s, 1H), 7.31-7.38 (m, 2H), 7.46-7.54 (m, 4H), 7.57-7.60 (m, 1H), 8.35 (d, J = 7.7 Hz, 1H), 9.61 (s, 1H), 10.89 (s, 1H), 12.71(br s, 1H); 13C NMR (CDCl3): 27.0, 28.3, 53.6, 81.1, 109.4, 111.4, 117.9, 118.4, 119.6, 122.1, 123.5, 127.7, 128.9, 129.1, 136.1, 139.9, 141.7, 152.2, 167.1, 174.5. HRMS [M+Na]+ Calcd for C25H27N3O5Na: 472.1843. Found: 472.1838.


Submitted in honor of the 75th anniversary of Professor Emeritus Keiichiro Fukumoto.

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