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CDCl3): d¼2.37 (s, 3H), 6.17 (d, J¼9.9 Hz, 1H), 6.77 (d,
J¼9.9 Hz, 1H), 7.15–7.55 (m, 10H), 7.79 (d, J¼2.3 Hz,
1H), 7.92 (d, J¼2.3 Hz, 1H). 13C NMR (75 MHz, CDCl3):
d¼15.8 (CH3), 83.0 (OC), 120.1 (C), 123.8 (CH]), 125.0
(C), 126.8 (4ꢂCH]), 127.5 (2ꢂCH]), 128.1 (5ꢂCH]),
132.2 (CH]), 138.6 (CH]), 145.2 (2ꢂC), 154.5 (C).
FTIR (KBr): n¼3058, 2969, 2920, 1637, 1598, 1489, 1445,
1381, 1338, 1207, 1167, 1102, 1055, 987, 905, 735,
J¼10.0 Hz, 1H), 6.63 (s, 1H), 6.87 (d, J¼10.0 Hz, 1H),
7.24 (tt, J¼1.7, 8.4 Hz, 2H), 7.30 (dd, J¼6.9, 8.4 Hz, 4H),
7.40 (dd, J¼1.7, 6.9 Hz, 4H); 13C NMR (75 MHz, CDCl3):
d¼18.2 (CH3), 21.4 (CH3), 81.9 (OC), 115.0 (CH]),
117.0 (C), 120.4 (CH]), 127.0 (4ꢂCH]), 127.4 (CH]),
127.5 (2ꢂCH]), 128.1 (4ꢂCH]), 135.5 (C), 141.0 (C),
145.2 (2ꢂC), 152.5 (C). FTIR (KBr): n¼3480–3104, 3085,
3058, 3025, 2956, 2919, 2850, 1632, 1613, 1596, 1562,
1491, 1462, 1446, 1372, 1313, 1224, 1160, 1141, 1092,
695 cmꢀ1
.
1073, 1031, 979, 941, 907, 844, 756, 723, 696 cmꢀ1
.
4.4.3. 6-Dihydroxyborane-7,8-dimethyl-2,2-diphenyl-
2H-1-benzopyran (20). This compound was obtained by
general procedure (2), starting from 14 (1.07 g,
2.75 mmol). The product 20 was isolated as a white solid
(754 mg, 2.12 mmol, 77%). Mp>260 ꢁC; 1H NMR
(300 MHz, CDCl3): d¼2.29 (s, 3H), 2.63 (s, 3H), 6.11 (d,
J¼9.6 Hz, 1H), 6.67 (d, J¼9.6 Hz, 1H), 7.25 (tt, J¼1.5,
8.5 Hz, 2H), 7.31 (dd, J¼1.5, 7.0 Hz, 4H), 7.45 (dd,
J¼7.0, 8.5 Hz, 4H), 7.68 (s, 1H); 13C NMR (75 MHz,
CDCl3): d¼11.7 (CH3), 19.0 (CH3), 83.1 (OC), 117.7 (C),
123.9 (CH]), 124.4 (C), 126.8 (4ꢂCH]), 127.2 (CH]),
127.4 (2ꢂCH]), 128.1 (4ꢂCH]), 133.4 (CH]), 145.4
(2ꢂC), 147.2 (C), 153.5 (C). FTIR (KBr): n¼3059, 3027,
2923, 1632, 1598, 1569, 1445, 1379, 1326, 1217, 1167,
4.5. General procedure (3) for the synthesis of nitrated
compounds 24–29
A 50 mL two-necked flask equipped with a magnetic stirrer
was charged with boronic acid (1 mmol) and dry acetonitrile
(20 mL), purged with argon, and cooled to ꢀ35 ꢁC. With
rapid stirring, trifluoroacetic anhydride (1 mL) was added
dropwise over 10 min with a syringe. Stirring was continued
at ꢀ35 ꢁC. Under argon atmosphere, in a separate flask, am-
monium nitrate (88 mg, 1.1 mmol) and dry acetonitrile
(10 mL) were mixed and the resulting mixture was cooled
to 0 ꢁC. To this vigorously stirred suspension, trifluoroacetic
anhydride (6–8 mL) was carefully and slowly added until all
solids had dissolved. The nitrating agent, thus prepared, was
transferred by syringe into the reaction flask within 10 min.
Stirring was continued for 2 h at ꢀ35 ꢁC and the reaction
mixture was allowed to warm to room temperature. The sol-
vent was removed under reduced pressure and the residue
was diluted with Et2O (30 mL). The resulting mixture was
washed with water (2ꢂ20 mL) and brine (3ꢂ20 mL), dried
with MgSO4, filtered and concentrated to dryness under re-
duced pressure. The crude material was purified by column
chromatography (SiO2; eluent: cyclohexane/Et2O gradient
100:0 to 70:30) to yield the ipso-nitrated product.
1098, 965, 907, 739, 696, 638 cmꢀ1
.
4.4.4. 6-Dihydroxyborane-5,8-dimethyl-2,2-diphenyl-
2H-1-benzopyran (21). This compound was obtained by
general procedure (2), starting from 15 (1.07 g,
2.75 mmol). The product 21 was isolated as a white solid
(656 mg, 1.84 mmol, 67%). Mp>260 ꢁC; 1H NMR
(300 MHz, CDCl3): d¼2.31 (s, 3H), 2.69 (s, 3H), 6.19 (d,
J¼10.1 Hz, 1H), 6.95 (d, J¼10.1 Hz, 1H), 7.23 (tt, J¼1.5,
8.5 Hz, 2H), 7.31 (dd, J¼1.5, 7.0 Hz, 4H), 7.45 (dd, J¼7.0,
8.5 Hz, 4H), 7.79 (s, 1H). 13C NMR (75 MHz, CDCl3):
d¼15.9 (CH3), 17.1 (CH3), 81.9 (OC), 119.6 (C), 121.1
(CH]), 122.3 (C), 126.8 (4ꢂCH]), 127.4 (2ꢂCH]),
127.9 (CH]), 128.1 (4ꢂCH]), 139.9 (CH]), 140.5 (C),
145.3 (2ꢂC), 154.0 (C). FTIR (KBr): n¼3059, 3026, 2921,
2854, 1632, 1573, 1447, 1390, 1372, 1322, 1219, 1167,
4.5.1. 2,2-Diphenyl-6-nitro-2H-1-benzopyran (24). This
compound was obtained by general procedure (3), starting
from 18 (328 mg, 1 mmol). The product 24 was isolated as
a white powder (162 mg, 0.49 mmol, 49%). Mp 134 ꢁC;
1H NMR (300 MHz, CDCl3): d¼6.30 (d, J¼9.9 Hz, 1H),
6.67 (d, J¼9.9 Hz, 1H), 6.97 (d, J¼8.8 Hz, 1H), 7.24–7.41
(m, 10H), 7.94 (d, J¼2.7 Hz, 1H), 8.04 (dd, J¼2.7, 8.8 Hz,
1H); 13C NMR (75 MHz, CDCl3): d¼84.5 (OC), 116.8
(CH]), 120.9 (C), 121.8 (CH]), 122.4 (CH]), 125.6
(CH]), 126.9 (4ꢂCH]), 128.1 (2ꢂCH]), 128.4
(4ꢂCH]), 130.7 (CH]), 142.0 (C), 143.9 (2ꢂC), 158.0
(C). Anal. Calcd for C21H15NO3: C, 76.53; H, 4.59; N,
4.25. Found: C, 76.49; H, 4.71; N, 4.19. FTIR (KBr):
n¼3054, 3027, 1647, 1614, 1576, 1509, 1475, 1446, 1335,
1067, 1022, 969, 906, 739, 695, 638 cmꢀ1
.
4.4.5. 8-Dihydroxyborane-5,6-dimethyl-2,2-diphenyl-
2H-1-benzopyran (22). This compound was obtained by
general procedure (2), from 16 (1.07 g, 2.75 mmol). The
product 22 was isolated as a white solid (637 mg,
1.79 mmol, 65%). Mp>260 ꢁC; 1H NMR (300 MHz,
CDCl3): d¼2.18 (s, 3H), 2.22 (s, 3H), 6.12 (d, J¼10.1 Hz,
1H), 6.88 (d, J¼10.1 Hz, 1H), 7.20–7.40 (m, 10H), 7.45 (s,
1H); 13C NMR (75 MHz, CDCl3): d¼14.9 (CH3), 19.7
(CH3), 83.0 (OC), 118.9 (C), 120.9 (CH]), 127.0 (4ꢂ
CH]), 127.8 (2ꢂCH]), 128.1 (CH]), 128.4 (4ꢂCH]),
129.6 (C), 136.7 (C), 137.1 (CH]), 144.1 (2ꢂC), 156.0
(C). FTIR (KBr): n¼3500–3100, 3058, 2957, 2922, 2854,
1647, 1595, 1490, 1443, 1391, 1326, 1256, 1219, 1053,
1248, 1224, 1124, 1086, 983, 902, 832, 747, 695 cmꢀ1
.
4.5.2. 2,2-Diphenyl-8-methyl-6-nitro-2H-1-benzopyran
(25). This compound was obtained by general procedure
(3), starting from 19 (343 mg, 1 mmol). The product 25
was obtained as a dark brown solid (243 mg, 0.71 mmol,
934, 904, 758, 734, 699 cmꢀ1
.
1
71%). Mp 98 ꢁC; H NMR (300 MHz, CDCl3): d¼2.33 (s,
4.4.6. 6-Dihydroxyborane-5,7-dimethyl-2,2-diphenyl-
2H-1-benzopyran (23). This compound was obtained
by general procedure (2), starting from 17 (1.07 g,
2.75 mmol). The product 23 was isolated as a white powder
(343 mg, 0.962 mmol, 35%). Mp>260 ꢁC; 1H NMR
(300 MHz, CDCl3): d¼2.26 (s, 3H), 2.29 (s, 3H), 6.12 (d,
3H), 6.26 (d, J¼10.1 Hz, 1H), 6.65 (d, J¼10.1 Hz, 1H),
7.20–7.50 (m, 10H), 7.79 (d, J¼2.3 Hz, 1H), 7.91 (d,
J¼2.3 Hz, 1H); 13C NMR (75 MHz, CDCl3): d¼15.9
(CH3), 84.3 (OC), 120.1 (CH]), 120.3 (C), 122.3 (CH]),
126.3 (CH]), 126.7 (4ꢂCH] and C), 127.9 (2ꢂCH]),
128.4 (4ꢂCH]), 130.1 (CH]), 141.2 (C), 144.1 (2ꢂC),