1870
Q. Tan et al.
SHORT PAPER
Anal. Calcd for C16H32O2Si2: C, 61.47; H, 10.32. Found:C,61.30; H,
10.27.
camphorquinones were prepared from d- and l-camphor
via d- and l-camphoric acid by the same method.
In summary, by using the easily available camphoric acid
(2) as starting material, we have accomplished an efficient
and economic synthesis of optically pure camphorquino-
ne (1) through three steps. The yield is high and the reac-
tion can be carried out on a large scale. This offers a good
method for the preparation of camphorquinone (1) instead
of using SeO2 and other selenium-containing reagents.
Camphorquinone (1)
To a stirred solution of 4 (2.86 g, 0.01 mol) in CCl4 (15 mL) was
added dropwise a solution of Br2 in CCl4 (10 mL) until no further
decolorization of bromine occurred and the system became bright
yellow. The solvent was then removed under reduced pressure and
the residue crystallized immediately. The bright crystals were re-
crystallized from petroleum ether; yellow needles; yield: 1.36 g
(81%); mp 198 °C; [ ]D –105 (c = 2, toluene) (Lit.8 mp 198–
20
199 °C; [ ]D20 –102.5 (c = 2, toluene)).
1H NMR (300 MHz, CDCl3): = 0.95 (s, 3 H), 1.05 (s, 3 H), 1.11
(s, 3 H), 1.55–1.69 m, 2 H), 1.82–1.96 (m, 1 H), 2.11–2.22 (m, 1 H),
2.58–2.64 (m, 1 H).
13C NMR (300 MHz, CDCl3): = 8.87, 17.53, 21.19, 22.40, 30.08,
42.70, 58.14, 58.78, 202.90, 204.89.
Melting points were determined on a Thomas-Hoover melting point
apparatus and are uncorrected. NMR spectra were recorded in
CDCl3 on Bruker DRX-500 (500 MHz) and AV-300 (300 MHz)
spectrometers. Elemental analyses were performed by Elementar
Vario ElIII equipment. Optical rotations were measured on Wzz-1
apparatus. TMSCl, camphoric acid (2) and other reagents were pur-
chased from Aldrich and used as received. The boiling point range
of petroleum ether used is 60–90°C.
Acknowledgment
This work was supported by the National Natural Science Founda-
tion of China (No. 29872035).
Diethyl Camphorate (3)
Camphoric acid (2) (19.0 g, 0.095 mol), EtOH (60 mL) and benzene
(30 mL) were introduced into a 1-L flask. Conc. H2SO4 (10 mL) was
added dropwise to the mixture. The flask was equipped with a water
separator and a reflux condenser. The mixture was brought to reflux
until no additional H2O was collected. EtOH and benzene were re-
moved and the residue was poured into ice (50 g). The mixture was
neutralized with NaHCO3 and extracted with Et2O (3 × 30 mL). The
Et2O extract was washed with brine and dried (Na2SO4). The sol-
vent was removed and the residue was distilled under reduced pres-
sure to afford 3 as a colorless liquid; bp 135 °C/5 mmHg); yield:
20.6 g (85%); [ ]D20 +33.6 (c = 1, EtOH).
References
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1H NMR (300 MHz,CDCl3): = 0.66 (s, 3 H),1.09 (s, 3 H), 1.13–
1.18 (m, 9 H),1.30–1.42 (m, 1 H), 1.65–1.72 (m, 1 H), 2.04–2.09
(m, 1 H), 2.40–2.49 (m, 1 H), 2.67 (t, J = 9.3 Hz, 1 H), 3.97–4.10
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13C NMR (300 MHz, CDCl3): = 13.8, 14.0, 20.8, 21.2, 22.2, 22.6,
32.2, 46.4, 52.4, 55.7, 59.6, 59.7, 173.2, 174.9.
Anal. Calcd for C14H24O4: C, 65.60; H, 9.44. Found: C, 65.41; H,
9.22.
1,7,7-Trimethyl-2,3-bis(trimethysiloxy)bicyclo[2.2.1]hept-2-ene
(4)
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A 500 mL three-necked flask fitted with a stirrer, a reflux condenser
and an addition funnel was charged with toluene (100 mL) and
freshly cut Na (4.2 g, 0.18 mol) under nitrogen. The toluene was
brought to gentle reflux. The stirrer was operated at a speed so that
the Na could be fully dispersed. A mixture of 3 (11.2 g, 0.044 mol)
and TMSCl (22.5 mL, 0.18 mol) in toluene (80 mL) was added
dropwise over 2 h. The mixture was maintained at gentle reflux dur-
ing and after the addition. After 13 h, TLC showed the absence of
starting ester 3 and completion of the reaction. The mixture was
cooled and filtered. The precipitate was washed several times with
petroleum ether. The filtrate was transferred to a flask and the sol-
vent was removed. The residue was directly used in the next reac-
tion without further purification. For further analyses it was purified
by column chromatography over silica gel with petroleum ether–
Et2O (20:1) as eluent to give 4 (9.4g, 75%) as a colorless liquid; bp
125 °C/5 mmHg); [ ]D20 +23.3 (c = 0.2, hexane).
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1H NMR (300 MHz, CDCl3): = 0.16–0.21 (m,18 H), 0.69 (s, 3 H),
0.89 (s, 3 H), 0.97 (s, 3 H),1.28 (m, 2 H), 1.52 (m, 1 H), 1.81 (m, 1
H), 2.10 (m, 1 H).
13C NMR (500 MHz, CDCl3): = 1.16, 10.73, 19.82, 19.96, 26.22,
33.20, 51.99, 52.80, 53.34, 133.77, 135.10.
Synthesis 2003, No. 12, 1869–1871 © Thieme Stuttgart · New York