References and Notes
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General Procedure
for Synthesis of Tetrasubstituted Furans: To a 5-mL flask,
propargylic alcohols 1 or propargylic acetates 2 (0.5 mmol), 1,3-dicarbonyl compounds 3 (2.0 mmol), toluene (2.0 mL), and FeCl3 (0.025
mmol, 4 mg) were successively added. The reaction mixture was stirred
at reflux, and monitored periodically by TLC. Upon completion, toluene
was removed under reduced pressure by an aspirator, and then the
residue was purified by silica gel column chromatography (EtOAc-hexane)
to afford the corresponding tetrasubstituted furans 5.
Data of selected compounds: 5ab: yellow solid; mp 50-
51 ˚C. ¹H
NMR (400 MHz, CDCl3): δ = 1.08 (d,
6 H, J = 6.0 Hz), 2.18 (s, 3
H), 2.56 (s, 3 H), 5.01 (sept, 1 H, J = 6.4
Hz), 7.22-7.25 (m, 2 H), 7.27-7.30 (m, 1 H), 7.32-7.37
(m, 2 H). ¹³C NMR (100 MHz, CDCl3): δ = 11.7,
14.0, 21.7, 67.2, 113.9, 122.5, 126.7, 127.6, 130.1, 133.4, 146.9,
157.2, 163.9. IR (film): 1704 cm-¹.
MS (ESI): m/z (%) = 281
(100) [M + Na+].
Anal. Calcd for C16H18O3: C, 74.39;
H, 7.02. Found: C, 74.24; H, 7.19. 5ad:
white solid; mp 117-118 ˚C. ¹H
NMR (400 MHz, CDCl3): δ = 2.46 (s,
3 H), 7.12-7.28 (m, 10 H), 7.35-7.39 (m, 1 H),
7.55-7.58 (m, 2 H), 7.80-7.82 (m, 2 H). ¹³C
NMR (100 MHz, CDCl3): δ = 12.3, 121.7, 123.5,
126.2, 126.9, 128.1, 128.2, 128.3, 128.4, 129.0, 129.7, 129.8, 132.2,
133.2, 137.4, 148.1, 150.8, 193.8. IR (film): 1669 cm-¹.
MS (ESI): m/z (%) = 361
(100) [M + Na+]. Anal.
Calcd for C24H18O2: C, 85.18; H,
5.36. Found: C, 85.31; H, 5.14. 5ae: yellow
oil. ¹H NMR (400 MHz, CDCl3): δ = 2.12-2.19
(m, 2 H), 2.30 (s, 3 H), 2.46-2.49 (m, 2 H), 2.87 (t, 2
H, J = 6.4 Hz), 7.26-7.31
(m, 1 H), 7.34-7.39 (m, 4 H). ¹³C
NMR (100 MHz, CDCl3): δ = 12.2, 22.7,
23.9, 38.9, 119.4, 119.9, 127.3, 128.1, 130.0, 131.9, 148.9, 165.9, 194.3.
IR (film): 1675 cm-¹. MS (ESI): m/z (%) = 227
(36) [M + H+], 249
(100) [M + Na+].
Anal. Calcd for C15H14O2: C, 79.62;
H, 6.24. Found: C, 79.79; H, 6.09. 5bb:
pale yellow oil. ¹H NMR (400 MHz, CDCl3): δ = 0.39
(d, 3 H,
J = 6.4 Hz),
0.68 (d, 3 H, J = 6.4 Hz), 2.11
(s, 3 H), 2.65 (s, 3 H), 4.68 (sept, 1 H, J = 6.4
Hz), 7.28-7.48 (m, 4 H), 7.64 (d, 1 H, J = 8.4
Hz), 7.81 (d, 1 H, J = 8.4 Hz),
7.84 (d, 1 H, J = 8.0 Hz). ¹³C
NMR (100 MHz, CDCl3): δ = 11.7, 13.7, 20.7,
21.3, 66.6, 115.3, 119.1, 125.1, 125.5, 125.6, 126.2, 127.4, 127.9,
131.9, 133.3, 133.4, 147.6, 157.6, 163.7. IR (film): 1704 cm-¹.
HRMS: m/z calcd
for C20H20O3: 308.1412; found:
308.1413. 5bd: pale yellow oil. ¹H
NMR (400 MHz, CDCl3): δ = 2.27 (s,
3 H), 7.03 (dd, app. t, 2 H,
J = 7.6,
7.6 Hz), 7.17-7.32 (m, 6 H), 7.39-7.41 (m, 2 H), 7.62-7.67
(m, 5 H), 7.73-7.75 (m, 1 H), 7.84-7.86 (m, 1
H). ¹³C NMR (100 MHz, CDCl3): δ = 12.5,
121.5, 123.2, 125.1, 125.7, 125.76, 126.1, 126.5, 127.8, 127.9,
128.2, 128.3, 128.4, 128.5, 129.4, 129.7, 129.9, 132.2, 132.7, 133.5, 137.4,
149.2, 151.9, 193.3. IR (film): 1658 cm-¹.
MS (ESI): m/z (%) = 389
(100) [M + H+]. Anal.
Calcd for C28H20O2: C, 86.57; H,
5.19. Found: C, 86.39; H, 5.34. 5da: white
solid; mp 96-97 ˚C. ¹H NMR
(400 MHz, CDCl3): δ = 1.15 (t, 3 H, J = 7.2 Hz), 2.19 (s, 3 H),
2.57 (s, 3 H), 4.15 (q, 2 H, J = 7.2 Hz),
7.14 (d, 2 H, J = 8.4 Hz), 7.49
(d, 2 H, J = 8.4 Hz). ¹³C NMR
(100 MHz, CDCl3): δ = 11.7, 14.0, 14.1,
59.9, 113.2, 120.4, 120.9, 130.8, 131.7, 132.3, 147.2, 157.7, 164.1.
IR (film): 1701 cm-¹. MS (ESI): m/z (%) = 345
(100), 347 (90) [M + Na+].
Anal. Calcd for C15H15BrO3: C,
55.75; H, 4.68. Found: C, 55.51; H, 4.82. 5dc:
yellow oil. ¹H NMR (400 MHz, CDCl3): δ = 2.00
(s, 3 H), 2.17 (s, 3 H), 2.55 (s, 3 H), 7.14 (d, 2 H, J = 8.8 Hz), 7.55 (d, 2 H, J = 8.4 Hz). ¹³C
NMR (100 MHz, CDCl3): δ = 11.6, 14.3,
30.8, 119.8, 121.5, 122.8, 131.5, 131.6, 132.7, 147.2, 156.4, 195.5.
IR (film): 1623 cm-¹. MS (ESI): m/z (%) = 315
(100), 317 (86) [M + Na+].
Anal. Calcd for C14H13BrO2: C,
57.36; H, 4.47. Found: C, 57.47; H, 4.29. 5ea:
yellow oil. ¹H NMR (400 MHz, CDCl3): δ = 1.09
(t, 3 H, J = 7.2 Hz), 2.20 (s,
3 H), 2.58 (s, 3 H), 3.93 (s, 3 H), 4.12 (q, 2 H, J = 7.2
Hz), 7.33 (d, 2 H, J = 8.4 Hz),
8.03 (d, 2 H, J = 8.4 Hz). ¹³C
NMR (100 MHz, CDCl3): δ = 11.8, 13.9,
14.1, 52.1, 59.9, 113.3, 120.6, 128.4, 128.9, 130.0, 138.4, 147.5,
157.9, 164.1, 167.1. IR (film): 1723 cm-¹. HRMS: m/z calcd
for C17H19O5H+:
303.1236; found: 303.1227. 5ec: yellow
solid; mp 100-101 ˚C. ¹H NMR
(400 MHz, CDCl3): δ = 1.97 (s, 3 H),
2.18 (s, 3 H), 2.54 (s, 3 H), 3.94 (s, 3 H), 7.33 (d, 2 H, J = 8.4 Hz), 8.08 (d, 2 H, J = 8.4 Hz). ¹³C
NMR (100 MHz, CDCl3): δ = 11.6, 14.2,
30.7, 52.2, 120.1, 122.8, 129.0, 129.6, 129.8, 138.7, 147.3, 156.4, 166.8,
195.4. IR (film): 1725, 1671 cm-¹.
MS (ESI): m/z (%) = 273
(18) [M + H+], 295
(100) [M + Na+].
Anal. Calcd for C16H16O4: C, 70.57;
H, 5.92. Found: C, 70.85; H, 5.81. 5ga: yellow
oil. ¹H NMR (400 MHz, CDCl3): δ = 0.84
(t, 3 H,
J = 6.8 Hz),
1.07 (t, 3 H, J = 7.2 Hz), 1.20-1.27
(m, 4 H), 1.54-1.62 (m, 2 H), 2.49 (t, 2 H, J = 7.6 Hz), 2.58 (s, 3 H), 4.09
(q, 2 H, J = 7.2 Hz), 7.22-7.24
(m, 2 H), 7.27-7.30 (m, 1 H), 7.31-7.36 (m, 2
H). ¹³C NMR (100 MHz, CDCl3): δ = 13.8,
13.9, 14.1, 22.3, 25.8, 28.2, 31.2, 59.7, 113.5, 121.2, 126.7, 127.5,
130.0, 133.4, 151.3, 157.4, 164.4. IR (film): 1712 cm-¹.
MS (ESI): m/z (%) = 323
(100) [M + Na+].
Anal. Calcd for C19H24O3: C, 75.97;
H, 8.05. Found: C, 75.80; H, 8.23. 5hc:
yellow oil. ¹H NMR (400 MHz, CDCl3): δ = 0.84 (t,
3 H,
J = 6.8 Hz),
1.18-1.28 (m, 4 H), 1.53-1.61 (m, 2 H), 1.90 (s, 3
H), 2.43 (t, 2 H, J = 8.0 Hz),
2.54 (s, 3 H), 3.75 (s, 3 H), 6.93 (d, 1 H, J = 8.4
Hz), 6.90-7.02 (m, 1 H), 7.16 (dd, 1 H, J = 7.6,
1.6 Hz), 7.32-7.36 (m, 1 H). ¹³C
NMR (100 MHz, CDCl3): δ = 13.9, 14.3,
22.3, 25.9, 28.1, 29.3, 31.2, 55.2, 110.6, 116.6, 120.6, 122.7,
123.2, 129.2, 131.4, 151.1, 155.8, 157.3, 196.2. IR (film): 1673
cm-¹. HRMS: m/z [M + H+] calcd
for C19H25O3: 301.1806; found:
301.1798. 5ic: yellow oil. ¹H
NMR (400 MHz, CDCl3): δ = 0.77 (t,
3 H,
J = 7.2 Hz),
1.09-1.22 (m, 4 H), 1.44-1.52 (m, 2 H), 1.88 (s, 3
H), 2.36 (t, 2 H, J = 7.6 Hz),
2.46 (s, 3 H), 7.09 (d, 2 H,
J = 8.4
Hz), 7.29 (d, 2 H, J = 8.4 Hz). ¹³C
NMR (100 MHz, CDCl3): δ = 13.9, 14.4,
22.2, 25.7, 28.1, 30.8, 31.1, 119.5, 122.7, 128.6, 131.2, 132.3,
133.3, 151.3, 156.5, 195.5. IR (film): 1675 cm-¹.
MS (ESI): m/z (%) = 305
(24) [M + H+], 327
(100) [M + Na+].
Anal. Calcd for C18H21ClO2: C,
70.93; H, 6.94. Found: C, 70.81; H, 7.09.
<A NAME="RW10508ST-9">9</A> Gabriele has also reported that 4-ethyl-2-methylfuran
could be obtained directly by treatment of (Z)-2-ethyl-5-trimethyl-silylpent-2-en-4-yn-1-ol
with TBAF without added solvent followed by transfer distillation;
however, PdI2 as the catalyst is needed in the absence
of trimethylsilyl group (Scheme 3). See:
Gabriele B.
Salerno G.
Lauria E.
J.
Org. Chem.
1999,
64:
7687
For synthetic applications of the γ-effect
and β-effect of silicon, see:
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Sakurai H.
Imai T.
Hosomi A.
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<A NAME="RW10508ST-11">11</A> The percentages of the enol content
of 1,3-dicarbonyl compounds in CCl4 follow the order:
dibenzoylmethane (3d; 96%) > acetylacetone
(3c; 80%) > ethyl acetoacetate
(3a; 7.5%) > diethyl
malonate (3f; 0.007%). See:
Burdett JL.
Rogers MT.
J.
Am. Chem. Soc.
1964,
86:
2105
