4.2. General Conditions for the Synthesis of Compound (1R,4R)-4-((1H-indol-3-yl)methyl)-1-((R)-methyl)-1,2-dihydro-6H-pyrazino[2,1-b]quinazoline-3,6(4H)-dione (1)
To a mixture of anthranilic acid (
i, 287 mg, 2.39 mmol) and TBTU (920 mg, 2.86 mmol, 1.2 equiv) in acetonitrile (20 mL) was added Et
3N (833 µL, 4.78 mmol, 2 equiv) and
d-tryptophan methyl ester (
ii, 521 mg, 2.39 mmol) at room temperature. After stirring for 5 h, the reaction mixture was concentrated under reduced pressure. The residue was dissolved in CH
2Cl
2 and washed with 1 M HCl, extracted with CH
2Cl
2 (3 × 100 mL), dried with Na
2SO
4, filtered, and concentrated. The residue was purified by flash chromatography (eluent 1% MeOH in CH
2Cl
2) to yield
iii as a white solid.
1H NMR and
13C NMR referred to the previous work [
16]. To a solution of
iii (140 mg, 0.416 mmol) in dried CH
2Cl
2 (10 mL)
N-Fmoc-
d-alanine-Cl [
30] (
iv,182 mg, 0.5 mmol) was added. The mixture was stirred for 30 min, followed by the addition of aqueous Na
2CO
3 (1 M, 8 mL, 8 mmol). After continuous stirring for 3 h, the mixture was extracted with CH
2Cl
2 (4 × 100 mL), dried with Na
2SO
4, filtered, and concentrated. The residue was purified by flash chromatography (eluent: 5% MeOH in CH
2Cl
2) to give
v (220.4 mg, 84.2%) as a white solid.
1H NMR (300 MHz, CDCl
3)
δ 11.48 (s, 1H), 8.58 (d, 1H,
J = 8.4 Hz), 8.13 (s, 1H), 7.76 (d, 2H,
J = 7.5 Hz), 7.66 (d, 1H,
J = 7.1 Hz), 7.59 (t, 1H,
J = 7.4 Hz), 7.49-7.26 (m, 8H), 7.17 (t, 1H,
J = 7.2 Hz), 7.06 (t, 1H,
J = 7.6 Hz), 7.00 ( t, 1H,
J = 7.7 Hz), 6.97 (s, 1H), 6.71 (d, 1H,
J = 7.6 Hz), 5.55 (d, 1H,
J = 6.8), 5.03 (dt, 1H,
J = 7.6, 5.3 Hz), 4.44 (m, 2H), 4.36 (1m, 1H), 4.26 (t, 1H,
J = 7.0 Hz), 3.73 (s, 3H), 3.40 (dd, 1H,
J = 15.3, 5.8 Hz), 3.34 (dd, 1H,
J = 15.3, 5.3 Hz), 1.53 (d, 3H,
J = 7.0 Hz) and
13C NMR (75 MHz, CDCl
3) 172.7, 172.2, 168.8, 156.6, 144.2, 143.8, 141.4, 138.6, 136.4, 132.8, 127.8, 127.4, 127.2, 125.3, 123.4, 123.3, 122.1, 121.6, 120.9, 120.0, 119.5, 118.3, 111.7, 109.2, 67.3, 53.6, 52.7, 52.2, 47.3, 27.3, 18.4 (See in [
31]). To a solution of
v (183.2 mg, 0.278 mmol) in dried CH
2Cl
2 (20 mL) Ph
3P (365 mg, 1.39 mmol, 5 equiv), I
2 (345 mg, 1.36 mmol. 4.9 equiv), and
N,
N-diisopropylethylamine (489 µL, 2.81 mmol, 10 equiv) were added. The reaction mixture was stirred at room temperature for 5 h, quenched with aqueous Na
2CO
3, and extracted with CH
2Cl
2 (3 × 100 mL), dried with Na
2SO
4, filtered, and concentrated. Hexane was added to remove an excess of Ph
3P, the precipitate was filtered and was treated with CH
2Cl
2 (10 mL) and piperidine (2.5 mL, 20%) at room temperature for 20 min, followed by solvent evaporation to provide the solid which was triturated with hexane (1 × 200 mL), CH
2Cl
2/PhMe (1 × 200 mL), and hexane (1 × 200 mL). The vacuum-dried crude residue was dissolved in CH
3CN (10 mL) in the presence of DMAP (64 mg, 0.53 mmol) and refluxed for 19 h. The reaction mixture was purified by preparative TLC (EtOAc:MeOH:CH
2Cl
2, 50:2.5:47.5) to afford
1. Yield: 22.4 mg, 21.11%;
er = 7:93; mp: 105.9–106.5 °C;
= −117.64 (
c 0.034; CHCl
3);
vmax (KBr) 3406, 2924, 2852, 1678, 1473, 1329, and 1261 cm
-1;
1H NMR (300 MHz, CDCl
3):
δ 8. 38 (dd, 1H,
J = 8.0 and 1.1 Hz, CH), 8.18 (br, 1H, NH-Trp), 7.78 (ddd, 1H,
J = 8.5, 7.1, and 1.6 Hz, CH), 7.57 (d, 1H,
J 7.4 Hz, CH), 7.54 (dd, 1H,
J = 8.0 and 1.0 Hz, CH), 7.29 (d, 1H,
J = 3.1 Hz, CH-Trp), 7.27 (d, 1H,
J = 2.0 Hz, CH-Trp), 7.08 (ddd, 1H,
J = 9.5, 7.0 and 0.9 Hz, CH-Trp), 6.83 (ddd, 1H,
J = 8.7, 7.1 and 1.0 Hz, CH-Trp), 6.73 (d, 1H,
J = 2.3 Hz, CH-Trp), 6.70 (d, 1H,
J = 2.0 Hz, NH-amide), 5.54 (dd, 1H
J = 5.2 and 3.6 Hz, CH*-Trp), 4.46 (qd, 1H,
J = 6.9 and 2.8 Hz, CH*-ala), 3.78 (dd, 1H,
J = 14.9 and 5.3 Hz, CH
2-Trp), 3.70 (dd, 1H,
J = 14.9 and 3.4 Hz, CH
2-Trp), 0.58 (d, 3H,
J = 7.0 Hz, CH
3-ala);
13C NMR (75 MHz, CDCl
3):
δ 167.3 (C=O), 161.0 (C=O), 151.2 (C=N), 147.2 (C), 135.7 (C-Trp), 134.9 (CH), 127.9 (C-Trp), 126.9 (CH), 126.8 (CH), 126.7 (CH), 123.7 (CH-Trp), 122.3 (CH-Trp), 120.1 (C), 119.9 (CH-Trp), 118.7 (CH-Trp), 111.1 (CH-Trp), 109.4 (C-Trp), 56.9 (CH*-Trp), 51.9 (CH*-ala), 27.1 (CH
2-Trp), 22.8 (CH
3-ala. (+)-HRMS-ESI
m/z 359.1505 (M + H)
+, (calculated for C
21H
18N
4O
2, 358.1430).
4.3. General Conditions for the Synthesis of Compound (1S,4S)-4-((1H-indol-3-yl)methyl)-1-((S)-sec-butyl) -1,2-dihydro-6H-pyrazino[2,1-b]quinazoline-3,6(4H)-dione (2)
To a mixture of anthranilic acid (
i, 287 mg, 2.39 mmol) and TBTU (920 mg, 2.86 mmol, 1.2 equiv) in acetonitrile (20 mL), Et
3N (833 µL, 4.78 mmol, 2 equiv) and
l-tryptophan methyl ester (
vi, 521 mg, 2.39 mmol) were added at room temperature. After stirring for 5 h, the reaction mixture was concentrated under reduced pressure. The residue was dissolved in CH
2Cl
2 and washed with 1 M HCl, extracted with CH
2Cl
2 (3 × 100 mL), dried with Na
2SO
4, filtered, and concentrated. The residue was purified by flash chromatography (eluent 1% MeOH in CH
2Cl
2) to yield v
ii as a white solid.
1H NMR and
13C NMR referred to the previous work [
16]. To a solution of v
ii (304 mg, 0.901 mmol) in dried CH
2Cl
2 (30 mL),
N-Fmoc-
l-isoleucine-Cl [
30] (
viii, 395 mg, 1.08 mmol) was added. The mixture was stirred for 30 min, followed by addition of aqueous Na
2CO
3 (1 M, 16 mL, 16 mmol). After continuous stirring for 3 h, the mixture was extracted with CH
2Cl
2 (4 × 100 mL), dried with Na
2SO
4, filtered, and concentrated. The residue was purified by flash chromatography (eluent: 5% MeOH in CH
2Cl
2 to give
ix (475.8 mg, 86.2%) as a white solid.
1H NMR (300 MHz, CDCl
3):
δ 11.43 (s, 1H), 8.59 (d, 1H,
J = 8.3 Hz), 8.19 (s, 1H), 7.76 (d, 2H
J = 7.4 Hz), 7.69-7.56 (m, 2H), 7.53-7.28 (m, 7H), 7.17 (t, 1H,
J = 7.3 Hz), 7.06 (t, 1H,
J = 7.4 Hz), 7.01 (d, 1H,
J = 7.6 Hz), 6.96 (d, 1H,
J = 3.5 Hz), 6.72 (d, 1H,
J = 7.6 Hz), 5.55 (d, 1H,
J = 8.4 Hz), 5.06 (dd, 1H,
J = 12.6 and 5.2 Hz), 4.39 (dd, 2H,
J = 16.4 and 9.1 Hz), 3.73 (s, 3H), 3.37 (m, 2H), 2.11-2.00 (m, 1H), 1.68-1.48 (m, 2H), 1.03 (d, 3H,
J = 6.8 Hz), 0.96 (t, 3H,
J = 7.3 Hz) and
13C NMR (75 MHz, CDCl
3) 172.1, 170.2, 168.6, 144.1, 143.8, 141.3, 139.0, 136.1, 132.9, 127.7, 127.5, 127.1, 127.1, 126.9, 125.3, 125.2, 123.2, 122.8, 122.4, 121.4, 120.2, 120.0, 119.8, 118.5, 111.4, 109.7, 67.2, 61.3, 53.3, 52.6, 47.3, 37.9, 31.4, 27.3, 15.8, 11.7. To a solution of
ix (291.5 mg, 0.432 mmol) in dried CH
2Cl
2 (20 mL) Ph
3P (565 mg, 2.16 mmol, 5 equiv), I
2 (448 mg, 2.12 mmol. 4.9 equiv), and
N,
N-diisopropylethylamine (753 µL, 4.32 mmol, 10 equiv) were added. The reaction mixture was stirred at room temperature for 5 h, quenched with aqueous Na
2CO
3, and extracted with CH
2Cl
2 (3 × 100 mL), dried with Na
2SO
4, filtered, and concentrated. Hexane was added to remove an excess of Ph
3P, the precipitate was filtered and was treated with CH
2Cl
2 (10 mL) and piperidine (2.5 mL, 20%) at room temperature for 20 min, followed by solvent evaporation to provide the solid which was triturated with hexane (1 × 200 mL), CH
2Cl
2/PhMe (1 × 200 mL), and hexane (1 × 200 mL). The vacuum-dried crude residue was dissolved in CH
3CN (10 mL) in the presence of DMAP (158 mg, 1.39 mmol) and refluxed for 19 h. The reaction mixture was purified by preparative TLC (EtOAc: MeOH: CH
2Cl
2, 50:2.5:47.5) to afford
2. Yield: 36 mg, 36.2%; enantiomeric ratio (er) = 73:27; m.p: 181–183 °C;
= +346.40 (
c 0.051; CHCl
3);
vmax (KBr) 3375, 3187, 2880, 1684, 1662, 1472, 1434, and 1261 1 cm
−1;
1H NMR (300 MHz, CDCl
3):
δ 8.38 (dd, 1H,
J = 8.0 and 1.1 Hz, CH), 8.07 (br, 1H, NH-Trp), 7.79 (ddd, 1H,
J = 8.5, 7.1, and 1.6 Hz, CH), 7.62 (dd,
J = 8.2 and 0.5 Hz, CH), 7.54 (ddd, 1H,
J = 8.2, 7.2, and 1.2 Hz, CH), 7.48 (d, 1H,
J 7.9 Hz, CH-Trp), 7.28 (d, 1H,
J = 8.5 Hz, CH-Trp), 7.12 (ddd, 1H,
J = 8.1, 7.1 and 1.1 Hz, CH-Trp), 6.94 (ddd, 1H,
J = 8.0, 7.1 and 1.0 Hz, CH-Trp), 6.87 (d, 1H,
J = 2.3 Hz, CH-Trp), 6.55 (d, 1H,
J = 3.1 Hz, NH-amide), 5.52 (dd, 1H,
J = 6.4 and 3.6 Hz, CH*-Trp), 4.03 (dd, 1H,
J = 8.0 and 3.5 Hz, CH-Ile), 3.83 (dd, 1H,
J = 14.9 and 6.4 Hz, CH
2-Trp), 3.73 (dd, 1H,
J = 14.8 and 3.5 Hz, CH
2-Trp), 0.99–0.85 (m, 1H, CH*-Ile), 0.85–0.69 (m, 2H, CH
2-Ile), 0.66 (d, 3H,
J = 6.5 Hz, CH
3-Ile), 0.58 (t, 3H
J = 7.1 Hz, CH
3-Ile);
13C NMR (75 MHz, CDCl
3):
δ 167.8 (C=O), 161.4 (C=O), 149.4 (C=N), 146.8 (C), 135.9 (C-Trp), 134.7 (CH), 127.9 (C-Trp), 127.1 (CH), 127.1 (CH), 126.8 (CH), 123.5 (CH-Trp), 122.3 (CH-Trp), 120.3 (C), 120.0 (CH-Trp), 119.0 (CH-Trp), 111.0 (CH-Trp), 110.2 (C-Trp), 60.8 (CH*-Ile), 57.6 (CH*-Trp), 40.8 (CH*-Ile), 27.1 (CH
2-Trp), 24.3 (CH
2-Ile), 15.3 (CH
3-Ile), 10.4 (CH
3-Ile); (+)-HREM-ESI
m/z 401.1967 (M + H)
+, 423.1787 ( M + Na)
+ (calculated for C
24H
25N
4O
2, 400.1899).
4.4. General Conditions for the Synthesis of Quinazolinone-3,6-(4H)-Diones Compound 4, 5, 6, 7, 8, and 9
In a closed vial anthranilic acid (i, 28 mg, 200 µmol), N-Boc-l-isoleucine (x, 44 mg, 200 µmol) for 4 and 5, or N-Boc-l-methionine (xi, 46 mg, 200 µmol) for 6 and 7, or N-Boc-o-Bn-Tyrosine (xii, 74 mg, 200 µmol) for 8 and 9, and triphenylphosphite (63 µL, 220 µmol) were added along with 1 mL of dried pyridine. The vial was heated in heating block with stirring at 55 °C for 16–24 h. After cooling the mixture to room temperature, d-tryptophan methyl ester hydrochloride (ii) for 5, and 7, L-tryptophan methyl ester hydrochloride (vi) for 4, 6, 8 and 9 (51 mg, 200 µmol) was added, and the mixture was irradiated in the microwave at the constant temperature at 220 °C for 1.5 min. Reaction mixtures were prepared in the same conditions and treated in parallel. After removing the solvent with toluene, the crude product was purified by flash column chromatography using hexane:EtOAc (60:40) as a mobile phase. The preparative TLC was performed using CH2Cl2:Me2CO (95:5) as mobile phase. The major compound appeared as a black spot with no fluorescence under the UV light (366 nm). The desirable compounds 4, 5, 6, 7, 8, and 9 were collected as yellow solids. Before analysis, compounds were recrystallized from methanol.
(1R,4S)-4-((1H-indol-3-yl)methyl)-1-((S)-sec-butyl)-1,2-dihydro-6H-pyrazino[2,1-b]quinazoline-3,6(4H)-dione(4). Yield: 29.2 mg, 7.1%; er = 3:97; m.p: 220–221 °C; = + 484.7 (c 0.037; CHCl3); vmax (KBr) 3373, 3059, 2880, 1684, 1662, 1472, 1434, and 1261 cm−1; 1H NMR (300 MHz, CDCl3): δ 8. 38 (dd, 1H, J = 7.9 and 1.2 Hz, CH), 8.07 (br, 1H, NH-Trp), 7.78 (ddd, 1H, J = 8.4, 7.1, and 1.6 Hz, CH), 7.57 (d, J = 8.0 Hz, CH), 7.52 (d, 1H, J = 6.0 Hz, CH), 7.47 (d, 1H, J = 8.0 Hz, CH-Trp), 7.28 (d, 1H, J = 8.2 Hz, CH-Trp), 7.12 (t, 1H, J = 7.1 Hz, CH-Trp), 6.96 (t, 1H, J = 7.5 Hz, CH-Trp), 6.57 (d, 1H, J = 2.4 Hz, CH-Trp), 5.68 (dd, 1H, J = 5.2 and 2.7 Hz, CH*-Trp), 5.64 (s, 1H, NH-amide), 3.76 (dd, 1H, J = 14.8 and 2.7 Hz, CH2-Trp), 3.63 (dd, 1H, J = 14.9 and 5.3 Hz, CH2-Trp), 2.80 (d, 1H, J = 2.4 Hz, CH*-Ile), 2.36 (dt, 1H, J 14.9 and 7.5 Hz, CH*-Ile), 0.98 (m, 2H, CH2-Ile), 0.88 (d, 3H, J = 6.5 Hz, CH3-Ile), 0.64 (t, 3H J = 6.4 Hz, CH3-Ile); 13C NMR (75 MHz, CDCl3): δ 169.5 (C=O), 160.9 (C=O), 150.7 (C=N), 147.1 (C), 136.1 (C-Trp), 134.7 (CH), 127.2 (CH), 127.1 (CH), 127.0 (C-Trp), 126.9 (CH), 123.6 (CH-Trp), 122.8 (CH-Trp), 120.2 (C), 120.1 (C-Trp), 118.8 (CH-Trp), 111.1 (CH-Trp), 109.4 (C-Trp), 56.8 (CH*-Trp), 55.1 (CH*-Ile), 35.8 (CH*-Ile), 27.4 (CH2-Trp), 25.8 (CH2-Ile), 13.2 (CH3-Ile), 11.0 (CH3-Ile; (+)-HRMS-ESI m/z 401.1964 (M + H)+ (calculated for C24H25N4O2, 400.1899).
(1S,4R)-4-((1H-indol-3-yl)methyl)-1-((S)-sec-butyl)-1,2-dihydro-6H-pyrazino[2,1-b]quinazoline-3,6(4H)-dione (5). Yield: 27.2 mg, 6.6%; er = 3:97; m.p: 218–220 °C; = −372.6 (c 0.034; CHCl3); vmax (KBr) 3373, 3059, 2880, 1684, 1662, 1472, 1434, 1261 cm−1; 1H NMR (300 MHz, CDCl3): δ 8. 37 (dd, 1H, J = 7.9 and 1.2 Hz, CH), 8.07 (br, 1H, NH-Trp), 7.78 (ddd, 1H, J = 8.4, 7.1, and 1.6 Hz, CH), 7.57 (d, J = 8.0 Hz, CH), 7.52 (d, 1H, J = 6.0 Hz, CH), 7.47 (d, 1H, J = 8.0 Hz, CH-Trp), 7.28 (d, 1H, J = 8.2 Hz, CH-Trp), 7.12 (t, 1H, J = 7.1 Hz, CH-Trp), 6.96 (t, 1H, J = 7.5 Hz, CH-Trp), 6.57 (d, 1H, J = 2.4 Hz, CH-indole), 5.68 (dd, 1H, J = 5.2 and 2.7 Hz, CH*-Trp), 5.52 (s, 1H, NH-amide), 3.76 (dd, 1H, J = 14.8 and 2.7 Hz, CH2-Trp), 3.63 (dd, 1H, J = 14.9 and 5.3 Hz, CH2-Trp), 2.80 (d, 1H, J = 2.4 Hz, CH*-Ile), 2.37 (dt, 1H, J = 14.9 and 7.5 Hz, CH*-Ile), 0.88 (m, 2H, J = 6.7 Hz, CH2-Ile), 0.62 (d, 3H, J = 6.5 Hz, CH3-Ile), 0.46 (t, 3H J = 6.4 Hz, CH3-Ile); 13C NMR (75 MHz, CDCl3): δ 169.4 (C=O), 160.9 (C=O), 150.1 (C=N), 147.1 (C), 136.1 (C-Trp), 134.7 (CH), 127.2 (CH), 127.2 (CH), 127.0 (CH-Trp), 126.9 (CH), 123.6 (CH-Trp), 122.7 (CH-Trp), 120.2 (C), 120.1 (C-Trp), 118.8 (CH-Trp), 111.1 (CH-Trp), 109.4 (C-indol), 56.8 (CH*-Trp), 55.5 (CH*-Ile), 35.6 (CH*-Ile), 27.4 (CH2-Trp), 25.9 (CH2-Ile), 13.2 (CH3-Ile), 11.0 (CH3-Ile; (+)-HRMS-ESI m/z 401.1973 (M + H)+ (calculated for C24H25N4O2, 400.1899).
(1R,4S)-4-((1H-indol-3-yl)methyl)-1-(2-(methylthio)ethyl)-1,2-dihydro-6H-pyrazino[2,1-b]quinazoline-3,6(4H)-diones (6). Yield: 27 mg, 6.1%; m.p: 198–200.7 °C; = +74.1 (c 0.045; CHCl3); vmax (KBr) 3295, 3067, 2915, 1682, 1600, 1470, 770, and 697 cm−1; 1H NMR (300 MHz, CDCl3): δ 8. 37 (dd, 1H, J = 8.0 and 1.2 Hz, CH), 8.07 (br, 1H, NH-Trp), 7.78 (ddd, 1H, J = 8.4, 7.0, and 1.5 Hz, CH), 7.57 (d, 1H, J = 2.5 Hz, CH), 7.53 (dd, 1H, J = 8.2 and 1.1 Hz, CH), 7.41 (d, 1H, J = 8.0 Hz, CH-Trp), 7.30 (d, 1H, J = 8.3 Hz, CH-Trp), 7.13 (t, 1H, J = 7.7 Hz, CH-Trp), 6.93 (t, 1H, J = 7.0 Hz, CH-Trp), 6.71 (d, 1H, J = 2.4 Hz, CH-Trp), 6.37 (s, 1H, NH-amide), 5.67 (dd, 1H, J = 5.2 and 3.0 Hz, CH*-Trp), 3.74 (dd, 1H, J = 15.0 and 3.0 Hz, CH2-Trp), 3.65 (dd, 1H, J = 14.9 and 5.3 Hz, CH2-Trp), 2.99 (dd, 1H, J = 6.6 and 3.6 Hz, CH*-Met), 2.49-2.13 (m, 4H, CH2-Met), 1.96 (s, 3H, CH3-Met); 13C NMR (75 MHz, CDCl3): δ 169.3 (C=O), 161.1 (C=O), 150.3 (C=N), 147.1 (C), 136.0 (C-Trp), 134.7 (CH), 127.2 (CH), 127.2 (C-Trp), 126.9 (CH), 123.5 (CH-Trp), 122.7 (CH-Trp), 120.2 (C), 120.2 (C-Trp), 118.7 (CH-Trp), 111.1 (CH-Trp), 109.5 (C-Trp), 57.2 (CH*Trp), 52.8 (CH*-Met), 30.7 (CH2-S-Met), 29.7 (CH2-Met), 27.2 (CH2-Trp), 15.3 (CH3-Met); HRMS-ESI m/z 419.1544 (M + H)+ (calculate for C23H23N4O2S, 418.1463).
(1S,4R)-4-((1H-indol-3-yl)methyl)-1-(2-(methylthio)ethyl)-1,2-dihydro-6H-pyrazino[2,1-b]quinazoline-3,6(4H)-diones (7). Yield: 34.8 mg, 7.9%; er = 49: 51; m.p.: 197–200 °C; = −56.9 (c 0.041; CHCl3); vmax (KBr) 3290, 3058, 2918, 2854, 1684, 1670, 1602, 773, and 695 cm−1; 1H NMR (300 MHz, CDCl3): δ 8. 37 (dd, 1H, J = 8.0 and 1.1 Hz, CH), 8.12 (br, 1H, NH-Trp), 7.78 (ddd, 1H, J = 8.4, 7.2, and 1.5 Hz, CH), 7.57 (d, J = 8.3 Hz, CH), 7.54 (d, 1H, J = 7.0 Hz, CH), 7.40 (d, 1H, J = 8.0 Hz, CH-Trp), 7.30 (d, 1H, J = 8.2 Hz, CH-Trp), 7.12 (t, 1H, J = 7.1 Hz, CH-Trp), 6.92(t, 1H, J = 7.0 Hz, CH-Trp), 6.71 (d, 1H, J = 2.4 Hz, CH-Trp), 5.67 (dd, 1H, J = 5.4 and 3.1 Hz, CH*-Trp), 6.54 (s, 1H, NH-amide), 3.73 (dd, 1H, J = 15.0 and 2.9 Hz, CH2-Trp), 3.65 (dd, 1H, J = 15.1 and 5.5 Hz, CH2-Trp), 3.01 (dd, 1H, J = 6.6 and 3.6 Hz, CH*-Met), 2.48-2.20 (m, 4H, CH2-Met), 1.96 (s, 3H, CH3-Met); 13C NMR (75 MHz, CDCl3): δ 169.5 (C=O), 161.0 (C=O), 150.6 (C=N), 146.9 (C), 136.1 (C-Trp), 134.7 (CH), 127.2 (C-Trp), 127.2 (CH), 126.9 (CH), 123.5 (CH-Trp), 122.6 (CH-Trp), 120.2 (C), 120.1 (C-Trp), 118.7 (CH-Trp), 111.2 (CH-Trp), 109.5 (C-Trp), 57.2 (CH*-Trp), 52.8 (CH*-Met), 30.7 (CH2-S-Met), 29.7 (CH2-Met), 27.2 (CH2-Trp), 15.3 (CH3-Met); (+)-HRMS-ESI m/z 419.1526 (M + H)+, (calculated for C23H23N4O2S, 418.1463).
(1R,4S)-4-((1H-indol-3-yl)methyl)-1-(4-(benzyloxy)benzyl)-1,2-dihydro-6H-pyrazino[2,1-b]quinazoline-3,6(4H)-diones (8). Yield: 81.9 mg, 14.8%; er = 63:37; m.p.: 226.9–227.9 °C; = +46.7 (c 0.05; CHCl3); vmax (KBr) 3393, 3268, 2954, 1671, 1611, 1511, 1465, 1240, 772, and 697 cm−1; 1H NMR (300 MHz, CDCl3): δ 8. 39 (dd, 1H, J = 8.0 and 1.2 Hz, CH), 8.05 (br, 1H, NH-Trp), 7.80 (ddd, 1H, J = 8.5, 7.2 and 1.5 Hz, CH), 7.62 (d, J = 8.0 Hz, CH), 7.56 (dt, 1H, J = 7.3, 7.7, and 1.1 Hz, CH), 7.45-7.39 (m, 5H, CH-Bz), 7.32 (d, 2H, J = 8.0 Hz, CH-Trp), 7.17 (t, 1H, J = 7.5 Hz, CH-Trp), 6.88 (t, 1H, J = 7.5 Hz, CH-Trp), 6.76 (d, 2H, J = 9.0 Hz, CH-Tyr), 6.61 (d, 1H, J = 3.0 Hz, CH-Trp), 6.39 (d, 2H, J = 8.5 Hz, CH-Tyr), 5.64 (dd, 1H, J = 5.2 and 2.7, CH*-Trp), 5.35 (s, 1H, NH-amide), 5.05 (s, 2H, CH2-Bz), 3.76 (dd, 1H, J = 14.9 and 2.6 Hz, CH2-Trp), 3.67 (dd, 1H, J = 15.0 and 5.3 Hz, CH2-Trp), 3.52 (dd, 1H, J = 14.7 and 3.6 Hz, CH2-Tyr), 2.89 (dd, J = 11.1 and 3.6 Hz, CH*-Tyr), 2.46 (dd, 1H, J = 14.7 and 11.2 Hz, CH2-Tyr); 13C NMR (75 MHz, CDCl3): δ 169.2 (C=O), 160.7 (C=O), 157.9 (C-Tyr), 151.0 (C=N), 147.0 (C), 137.0 (C-Trp), 136.1(C-Bz), 134.8 (CH), 128.7 (CH-Tyr (2)), 128.6 (CH-Bz (2)), 128.0 (C-Tyr), 127.4 (CH-Bz), 127.3 (CH-Trp), 127.2 (CH), 127.1 (CH-Bz (2)), 127.0 (CH), 126.9 (CH), 123.8 (CH-Trp), 122.8 (CH-Trp), 120.6 (C), 120.4 (CH-Trp), 119.0 (CH-Trp), 115.5 (CH-Tyr (2H)), 111.1 (CH-Trp), 109.7 (C-Trp), 70.1 (CH2-Bz), 57.4 (CH*-Trp), 52.9 (CH*-Tyr), 37.1 (CH2-Tyr), 29.7 (CH2-Trp); (+)-HRMS-ESI m/z 541.2232 (M + H)+, (calculated for C34H29N4O3, 540.2161).
(1S,4S)-4-((1H-indol-3-yl)methyl)-1-(4-(benzyloxy)benzyl)-1,2-dihydro-6H-pyrazino[2,1-b]quinazoline-3,6(4H)-diones (9). Yield: 119.9 mg, 21.7%; er = 29:71; m.p.: 165.9–166.6 °C; = +205.8 (c 0.076; CHCl3); νmax (KBr) 3489, 3364, 2923, 1674, 1612, 1512, 1467, 1249, 774, and 695 cm−1; 1H NMR (300 MHz, CDCl3): δ 8. 42 (dd, 1H, J = 8.0 and 1.1 Hz, CH), 8.08 (br, 1H, NH-Trp), 7.83 (ddd, 1H, J 8.5, 7.1, and 1.5 Hz, CH), 7.66 (d, J 7.7 Hz, CH), 7.62-7.52 (m, 2H, CH), 7.39 (t, 4H, J = 2.6 Hz, CH-Bz), 7.35 (ddd, J = 6.2, 3.4, and 1.5 Hz, 1H, CH-Bz), 7.30 (d, 1H, J = 8.1 Hz, CH-Trp), 7.20 (td, 1H, J = 7.6 and 1.1 Hz, CH-Trp), 6.10 (td, 1H, J = 7.5 and 1.0 Hz, CH-Trp), 6.74 (d, 2H, J = 8.7 Hz, CH-Tyr), 6.60 (d, 1H, J = 2.3 Hz, CH-Trp), 6.23 (d, 2H, J = 8.6 Hz, CH-Tyr), 5.56 (t, 1H, J = 4.2 Hz, CH*-Trp), 5.55 (s, 1H, NH-amide), 4.99 (s, 2H, CH2-Bz), 4.33 (dt, 1H, J = 11.7 and 2.8 Hz, CH*-Tyr), 3.86 (dd, 1H, J =14.9 and 3.0 Hz, CH2-Trp), 3.80 (dd, 1H, J = 14.9 and 4.4 Hz, CH2-Trp), 2.95 (dd, 1H, J = 13.3 and 3.1 Hz, CH2-Tyr), 0.53 (dd, J = 13.1 and 11.9 Hz, CH2-Tyr); 13C NMR (75 MHz, CDCl3): δ 166.5 (C=O), 160.9 (C=O), 158.0 (C-Tyr), 150.2 (C=N), 147.2 (C), 136.9 (C-Trp), 135.8 (C-Bn), 134.9 (CH), 130.28 (CH-Tyr (2)), 128.6 (CH-Bz (2)), 128.1 (C-Tyr), 128.0 (CH-Bz), 127.7 (C-Trp), 127.4 (CH), 127.0 (CH-Bz (2)), 126.9 (CH), 123.5 (CH-Trp), 122.8 (CH-Trp), 120.5 (C), 120.2 (C-Trp), 119.5 (CH-Trp), 115.2 (CH-Tyr (2)), 111.4 (CH-Trp), 109.7 (C-Trp), 70.0 (CH2-Bz), 57.9 (CH*-Tyr), 56.8 (CH*-Trp), 42.0 (CH2-Tyr), 26.6 (CH2-Trp(+)-HRMS-ESI m/z 541.2221 (M + H)+, (calculated for C34H29N4O3, 540.2161).
4.5. General Conditions for the Synthesis of (1S)-4-((1H-indol-3-yl)methyl)-1-(4-hydroxybenzyl)-1,2-dihydro -6H-pyrazino[2,1-b]quinazoline-3,6(4H)-dione (10 and 11)
In an oven-dried round-bottomed flash equipped with Teflon-coated magnetic stir bar, a rubber septum, a glass stopper, and nitrogen gas inlet compound 8 or 9 (50 mg, 0.092 mmol) dissolved with anhydrous CH2Cl2 (5 mL) was added. After cooled the mixture to −78 °C, 1M boron trichloride in CH2Cl2 (190 µL, 0.19 mmol, 2eq) was added dropwise over 5 min at −78 °C. After stirring for 45 min at -78 °C, the mixture was quenched by syringe addition of CHCl3/MeOH (10/1, 10 mL) at -78 °C and was warmed to ambient temperature. The solvent was evaporated, and the content was purified by flash column chromatography using hexane: EtOAc (6:4) as mobile phase. Compounds 10 or 11 were obtained as pale yellow solids. Before analysis, compounds were recrystallized from methanol.
(1R,4S)-4-((1H-indol-3-yl)methyl)-1-(4-hydroxybenzyl)-1,2-dihydro-6H-pyrazino[2,1-b]quinazoline-3,6(4H)-dione (10). Yield: 12.5 mg, 30%; er = 36:65; m.p.: 134–136 °C; = +39.5 (c 0.034; CH3OH); vmax (KBr) 3427, 1677, 1603, 1515, 1468, 1159, 1025, 998, and 765 cm−1; 1H NMR (300 MHz, DMSO-d6): δ 10.48 (s, 1H, NH-Trp), 8.89 (d, 1H, J = 4.9 Hz, OH-Tyr), 8.32 (dd, 1H, J = 8.0 and 1.2 Hz, CH), 7.81 (dd, 1H, J = 7.7 and 1.6 Hz, CH), 7.61 (d, J = 7.9 Hz, CH), 7.56 (dd, 1H, J = 12.7 and 7.2 Hz, CH), 7.39 (dd, 1H, J = 8.0 and 5.4 Hz, CH-Trp), 7.34 (dt, 1H, J = 8.1 Hz, CH), 7.13 (dd, 1H, J = 13.4 and 6.7 Hz, CH-Trp), 6.84 (dd, 1H, J = 13.4 and 6.7 Hz, CH-Trp), 6.60 (d, 1H, J = 3.4 Hz, CH-Trp), 6.58(dd, 2H, J = 8.5 and 6.0 Hz, CH-Tyr), 6.53 (d, 1H, J = 4.4 Hz, NH-amide), 6.46 (dd, 2H, J = 7.9 and 5.7 Hz, CH-Tyr), 5.44 (dd, 1H, J = 5.0 and 2.8 Hz, CH*-Trp), 3.64 (dd, 1H, J = 14.8 and 2.7 Hz, CH2-Trp), 3.56 (dd, 1H, J = 14.9 and 5.3 Hz), 3.26 (dt, J = 13.4 and 3.6 Hz, CH2-Tyr), 3.03 (dt, 1H, J = 8.9 and 4.7 Hz, CH*-Tyr), 2.67 (dt, 1H, J = 13.3 and 10.5 Hz); 13C NMR (75 MHz, CDCl3): δ 168.1 (C=O), 160.0 (C=O), 150.5 (C-Tyr), 146.3 (C), 135.8 (C-Trp), 134.1 (CH), 129.4 (CH-Tyr (2)), 126.7 (C-Trp), 126.6 (CH), 126.4 (CH), 126.1 (CH), 124.9 (C-Tyr), 123.7 (CH-Trp), 121.2 (CH-Trp), 119.6 (CH-Trp), 118.9 (CH-Trp), 115.0 (C-Tyr), 111.2 (CH-Trp), 108.0 (C-Trp), 56.6 (CH*-Trp), 52.8 (CH*-Tyr), 36.3 (CH2-Tyr), 26.5 (CH2-Trp); (+)-HRMS-ESI m/z 451.1766 (M + H)+, 473.1576 (M + Na)+ (calculated for C27H23N4O3, 450.1692).
(1S,4S)-4-((1H-indol-3-yl)methyl)-1-(4-hydroxybenzyl)-1,2-dihydro-6H-pyrazino[2,1-b]quinazoline-3,6(4H)-dione(11). Yield: 25.7 mg, 68.7%; er = 61:39; m.p.: 102–103 °C; = +75.9 (c 0.079; CH3OH); vmax (KBr) 3428, 2927, 1667, 1610, 1592, 1474, 1337, 1232, 772, and 699 cm–1; 1H NMR (300 MHz, DMSO-d6): δ 10.54 (s, 1H, NH-Trp), 8.86 (s, 1H, OH-Tyr), 8.34 (dd, 1H, J = 8.0 and 1.2 Hz, CH), 7.83 (dd, 1H, J = 7.7, and 1.6Hz, CH), 7.62 (d, J = 7.9 Hz, CH), 7.56 (dt, 1H, J = 7.6, 7.6, and 0.6 Hz, CH), 7.49 (d, 1H, J = 7.9 Hz, CH-Trp), 7.30 (d, 1H, J = 8.1 Hz, CH), 7.20 (d, 1H, J = 3.3 Hz, NH-amide), 7.09 (dt, 1H, J = 7.6 and 0.6 Hz, CH-Trp), 6.94 (dt, 1H, J = 7.5 and 0.5 Hz, CH-Trp), 6.66 (d, 1H, J = 2.3 Hz, CH-Trp) 6.57(d, 2H, J = 5.6 Hz, CH-Tyr), 6.45 (d, 2H, J = 8.4 Hz, CH-Tyr), 5.37 (dd, 1H, J = 5.2 and 3.3 Hz, CH*-Trp), 4.36 (dt, 1H, J = 10.5 and 3.4 Hz, CH*-Tyr), 3.63 (dd, 1H, J = 14.8 and 3.1 Hz, CH2-Trp), 3.55 (dd, 1H, J = 14.9 and 5.4 Hz, CH2-Trp), 2.69 (dd, J = 13.4 and 3.6 Hz CH2-Tyr), 0.86 (dd, 1H, J = 13.3 and 10.5 Hz, CH2-Tyr); 13C NMR (75 MHz, CDCl3): δ 165.8 (C=O), 160.2 (C=O), 155.8 (C-Tyr), 150.4 (C=N), 146.7 (C), 135.6 (C-Trp), 134.2 (CH), 129.9 (CH-Tyr (2)), 127.4 (CH-Trp), 126.3 (CH), 126.2 (CH), 126.1 (CH-Trp), 125.9 (C-Tyr), 123.7 (CH-Trp), 121.1 (CH-Trp), 119.5 (CH-Trp), 118.7 (CH-Trp), 114.9 (CH-Tyr (2)), 111.2 (CH-Trp), 108.0 (C-Trp), 57.2 (CH*-Trp), 56.3 (CH*-Tyr), 41.8 (CH2-Tyr), 26.1 (CH2-Trp); (+)-HRMS-ESI m/z 451.1771 (M + H)+, 473.1564 (M + Na)+ (calculated for C27H23N4O3, 450.1692).