2848
M. Yamashita et al. / Tetrahedron 60 (2004) 2843–2849
in toluene, 42 ml, 63 mmol) was added dropwise maintain-
ing the reaction temperature below 240 8C and stirred for
1 h. Aqueous HCl (6 M) was added to the reaction mixture,
which was stirred at room temperature for 30 min. The
organic layer was separated, washed with aqueous HCl
(2 M), aqueous NaHCO3 and brine successively, dried over
sodium sulfate and concentrated in vacuo. The residue was
chromatographed on silica gel (AcOEt/n-hexane¼1/19) to
neat) 1514, 1728, 2973; 1H NMR (300 MHz, CDCl3) d 1.44
(6H, s), 2.33 (3H, s), 7.14–7.24 (4H, m), 9.47 (1H, s). 13C
NMR (300 MHz, CDCl3) d 20.8, 22.3, 49.9, 126.5, 129.4,
136.8, 138.0, 202.1; MS (EI): m/z 162 (Mþ); HRMS (EI)
calcd for C11H14O (Mþ) 162.1045. Found: 162.1034.
concentrated. The residue was chromatographed on silica
gel (AcOEt/n-hexane¼1/39) to afford 1 (1.3 g, 75%) as a
white crystalline powder. The spectral data (IR, NMR) were
identical with these of the sample that was prepared by
using method A (Scheme 1). Mp 118–120 8C. Anal. calcd
for C20H24O (280.40): C, 85.67, H, 8.63. Found: C, 85.67,
H, 8.72; MS (EI): m/z 280 (Mþ).
afford 8 (7.8 g, 93% based on 9) as a colorless oil. IR (cm21
,
The following compounds were obtained according to the
general procedure.
4.3.2. 2,2,4,6,7-Pentamethyl-3-(4-methoxylphenyl)-2,3-
dihydrobenzofuran (16). Yield 77%, white crystalline
powder. Mp 105–106 8C. IR (cm21, KBr) 825, 1088, 1248,
1
1510, 2974; H NMR (300 MHz, CDCl3) d 1.02 (3H, s),
The following compounds were obtained according to the
general procedure.
1.48 (3H, s), 1.84 (3H, s), 2.14 (3H, s), 2.23 (3H, s), 3.76
(3H, s), 4.07 (1H, s), 6.48 (1H, s), 6.5–7.2 (4H, m). 13C
NMR (300 MHz, CDCl3) d 11.5, 18.0, 19.3, 24.8, 29.7,
55.0, 57.1, 88.5, 113.4, 115.4, 123.0, 126.3, 129.5, 131.9,
132.5, 136.7, 157.2, 158.2. Anal. calcd for C20H24O2
(296.40): C, 81.04, H, 8.16. Found: C, 80.81, H, 7.92; MS
(EI): m/z 296 (Mþ).
4.2.2. 2-Methyl-2-(4-methoxyphenyl)propanal (15a).
Yield 72%, colorless oil. IR (cm21, neat) 1254, 1514,
1
1724, 2971; H NMR (300 MHz, CDCl3) d 1.43 (6H, s),
3.80 (3H, s), 6.88–6.93 (2H, m), 7.17–7.22 (2H, m), 9.44
(1H, s). 13C NMR (300 MHz, CDCl3) d 22.4, 49.6, 55.1,
114.1, 127.7, 132.9, 158.6, 202.0. Anal. calcd for C11H14O2
(178.23): C, 74.13, H, 7.92. Found: C, 73.77, H, 7.99; MS
(EI): m/z 178 (Mþ).
4.3.3. 2,2,4,6,7-Pentamethyl-3-phenyl-2,3-dihydrobenzo-
furan (17). Yield 90%, white crystalline powder. Mp 89–
1
91 8C. IR (cm21, KBr) 698, 1090, 1456, 2979; H NMR
(300 MHz, CDCl3) d 1.02 (3H, s), 1.50 (3H, s), 1.83 (3H, s),
2.14 (3H, s), 2.23 (3H, s), 4.12 (1H, s), 6.48 (1H, s), 6.5–7.3
(5H, m). 13C NMR (300 MHz, CDCl3) d 11.6, 18.0, 19.3,
24.8, 29.8, 57.9, 88.5, 115.5, 123.0, 126.1, 126.5, 128.1,
128.6, 132.0, 136.9, 140.4, 157.4. Anal. calcd for C19H22O
(266.38): C, 85.67, H, 8.32. Found: C, 85.52, H, 8.19; MS
(EI): m/z 266 (Mþ).
4.2.3. 2-Methyl-2-phenylpropanal (15b). Yield 70%,
1
colorless oil. IR (cm21, neat) 1448, 1496, 1722, 2981; H
NMR (300 MHz, CDCl3) d 1.46 (6H, s), 7.25–7.30 (3H, m),
7.35–7.40 (2H, m), 9.49 (1H, s). 13C NMR (300 MHz,
CDCl3) d 22.3, 50.3, 125.2, 126.5, 127.1, 128.4, 128.7,
141.1, 202.0; MS (EI): m/z 148 (Mþ); HRMS (EI) calcd for
C10H12O (Mþ) 148.0888. Found: 148.0878.
4.3.4. 2,2,4,6,7-Pentamethyl-3-(4-chlorophenyl)-2,3-
dihydrobenzofuran (18). Yield 85%, white crystalline
powder. Mp 110–112 8C. IR (cm21, KBr) 831, 1084, 1489,
2976; 1H NMR (300 MHz, CDCl3) d 1.02 (3H, s), 1.49 (3H,
s), 1.83 (3H, s), 2.13 (3H, s), 2.23 (3H, s), 4.08 (1H, s), 6.48
(1H, s), 6.5–7.3 (4H, m). 13C NMR (300 MHz, CDCl3) d
11.5, 18.0, 19.3, 24.8, 29.7, 57.2, 88.3, 115.6, 123.1, 125.7,
128.3, 129.9, 131.8, 132.3, 137.2, 139.0, 157.3. Anal. calcd
for C19H21OCl (300.82): C, 75.86, H, 7.04, Cl, 11.79.
Found: C, 75.82, H, 6.99, Cl, 11.66; MS (EI): m/z 300 (Mþ).
4.2.4. 2-Methyl-2-(4-chlorophenyl)propanal (15c). Yield
87%, colorless oil. IR (cm21, neat) 1105, 1495, 1724, 2976;
1H NMR (300 MHz, CDCl3) d 1.45 (6H, s), 7.18–7.23 (2H,
m), 7.32–7.36 (2H, m), 9.47 (1H, s). 13C NMR (300 MHz,
CDCl3) d 22.4, 49.9, 128.0, 128.8, 133.1, 139.6, 201.4; MS
(EI): m/z 182 (Mþ); HRMS (EI) calcd for C10H11ClO (Mþ)
182.0499. Found: 182.0481.
4.2.5. 1-Phenylcyclopropanecarbaldehyde (31). Yield
90%, colorless oil. IR (cm21, neat) 1446, 1498, 1711,
1
2823; H NMR (300 MHz, CDCl3) d 1.37–1.41 (2H, m),
4.3.5. 2,2,5-Trimethyl-3-(4-methylphenyl)-2,3-dihydro-
benzofuran (19). Yield 52%, colorless oil. IR (cm21
,
1.54–1.58 (2H, m), 7.29–7.36 (5H, m), 9.28 (1H, s). 13C
NMR (300 MHz, CDCl3) d 16.0, 37.3, 127.5, 128.5, 129.9,
137.4, 200.8; MS (EI): m/z 146 (Mþ); HRMS (EI) calcd for
C10H10O (Mþ) 146.0732. Found: 146.0726.
neat) 1251, 1491, 2974; 1H NMR (300 MHz, CDCl3) d 0.96
(3H, s), 1.56 (3H, s), 2.24 (3H, s), 2.33 (3H, s), 4.26 (1H, s),
6.71 (1H, d, J¼8.1 Hz), 6.83 (1H, s), 6.95–6.99 (3H, m),
7.07–7.15 (2H, m). 13C NMR (300 MHz, CDCl3) d 20.7,
20.9, 24.0, 28.8, 58.1, 89.7, 109.2, 126.3, 128.7, 128.8,
128.9, 129.5, 130.3, 130.6, 136.5, 136.7, 156.8; MS (EI):
m/z 252 (Mþ); HRMS (EI) calcd for C18H20O (Mþ)
252.1514. Found: 252.1511.
4.3. General procedure for the preparation of 3-aryl-2,2-
dialkyl-2,3-dihydrobenzofurans using phenols and
2-aryl-2,2-dialkylacetaldehydes
4.3.1. 2,2,4,6,7-Pentamethyl-3-(4-methylphenyl)-2,3-
dihydrobenzofuran (1). (Scheme 6). A suspension of
phenol 2 (0.84 g, 6.2 mmol), aldehyde 8 (1.0 g, 6.2 mmol)
and CF3SO3H (0.09 g, 0.62 mmol) in toluene (6.2 ml) was
refluxed for 1 h. The reaction mixture was cooled to room
temperature, and aqueous NaOH (1 M) was added before it
was stirred for 30 min. The organic layer was separated,
washed with water, dried over sodium sulfate and
4.3.6. 2,2,5-Trimethyl-3-(4-methoxyphenyl)-2,3-
dihydrobenzofuran (20). Yield 60%, pale yellow oil. IR
(cm21, neat) 1250, 1491, 1512, 2974; 1H NMR (300 MHz,
CDCl3) d 0.96 (3H, s), 1.55 (3H, s), 2.25 (3H, s), 3.79 (3H,
s), 4.25 (1H, s), 6.71 (1H, d, J¼8.1 Hz), 6.82–6.86 (3H, m),
6.95–7.02 (3H, m). 13C NMR (300 MHz, CDCl3) d 20.7,
24.0, 28.7, 55.1, 57.7, 89.8, 109.2, 113.6, 126.2, 128.7,