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General Procedure
for the Synthesis of α-Arylidene-γ-keto Esters
4
A flame-dried round-bottom flask was charged with benzimidazolium
salt 3c (0.25 mmol), aldehyde 1 (1.0 mmol), and THF (5 mL) under positive
pressure of nitrogen followed by addition of DBU (0.25 mmol) with
a syringe. After stirring for 10 min at r.t., BH bromide 2 (1.0 mmol) was added. The reaction mixture
was stirred at r.t. for 12-18 h (Table
[²]
). After completion of
the reaction (the disappearance of 2, monitored
by TLC), the reaction mixture was concentrated under reduce pressure.
The residue was purified by flash column chromatography using hexane-EtOAc
as eluent to afford product 4 in 65-85% yield.
Characterization Data of Representative Compounds
4
Compound 4a: IR (film) : νmax = 1714,
1642 cm-¹. ¹H NMR (400
MHz, CDCl3): δ = 3.84
(s, 3 H, OMe), 5.20 (s, 2 H, CH2),
7.57-7.25 (m, 8 Harom, Ph), 7.88-7.80
(m, 2 Harom, Ph), 8.02 (s, 1 H, C = CH). ¹³C
NMR (100 MHz, CDCl3/TMS): δ = 40.6,
52.5, 126.1, 127.2, 128.1, 128.9, 129.7, 130.6, 133.4, 135.6, 137.2,
138.5, 166.5, 193.4. MS (EI): m/z = 280 [M+].
Anal. Calcd for C18H16O3: C, 77.12;
H, 5.75. Found: 77.44; H, 5.96.
Compound 4c:
IR (film): νmax = 1712, 1640
cm-¹. ¹H NMR (400
MHz, CDCl3): δ = 2.30
(s, 3 H, Me), 3.86 (s, 3 H, OMe), 5.14 (s, 2 H, CH2),
7.50-7.19 (m, 5 Harom, Ph), 7.90-7.77
(m, 4 Harom, 4-MePh), 7.96 (s, 1 H, C = CH). ¹³C
NMR (100 MHz, CDCl3/TMS): δ = 25.4,
41.5, 53.1, 126.2, 127.1, 127.9, 128.7, 129.4, 130.2, 134.34, 135.41,
138.2, 143.3, 167.3, 193.2.MS (EI): m/z = 294 [M+].
Anal. Calcd for C19H18O3: C, 77.53;
H, 6.16. Found: C, 77.86; H, 6.54.
Compound 4d:
IR (film): νmax = 1709,
1638 cm-¹. ¹H NMR (400
MHz, CDCl3): δ = 3.80
(s, 3 H, OMe), 3.84 (s, 3 H, OMe), 5.20 (s, 2 H, CH2),
7.55-7.22 (m, 5 Harom, Ph),
7.38-7.28
(m, 2 Harom, 4-MeOPh), 7.86-7.82 (m, 2 Harom,
4-MeOPh),
7.98 (s, 1 H, C = CH). ¹³C
NMR (100 MHz, CDCl3/TMS): δ = 40.3,
52.8, 55.1, 114.2, 126.5, 127.5, 128.1, 129.1, 129.8, 130.7, 134.9,
137.6, 165.8, 167.3, 191.2. MS (EI): m/z = 310 [M+].
Anal. Calcd for C19H18O4: C, 73.52;
H, 5.85. Found: C, 73.18; H, 5.50.
Compound 4k:
IR (film): νmax = 1711,
1636 cm-¹. ¹H NMR (400
MHz, CDCl3): δ = 3.80
(s, 3 H, OMe), 3.83 (s, 3 H, OMe), 5.15 (s, 2 H, CH2),
7.32-7.22 (m, 4 Harom, 4-ClPh), 7.37-7.30
(m, 2 Harom, 4-MeOPh), 7.85-7.80 (m, 2 Harom,
4-MeOPh),
7.98 (s, 1 H, C = CH). ¹³C
NMR (100 MHz, CDCl3/TMS): δ = 47.3,
52.4, 55.1, 113.8, 127.5, 128.3, 129.1, 129.9, 130.6, 133.1, 134.1,
137.9, 165.9, 167.6, 192.2. MS (EI): m/z = 344 [M+].
Anal. Calcd for C19H17ClO4: C,
66.19; H, 4.97. Found: C, 66.50; H, 4.79.