Organic Syntheses, CV 9, 741
UBIQUINONE-1
Submitted by Yoshinori Naruta and Kazuhiro Maruyama
1.
Checked by Steven W. Elmore and Leo A. Paquette.
1. Procedure
CAUTION! This experiment should be performed with gloves in an efficient hood in order to avoid the contact of toxic
tributyltin derivatives with the skin and to avoid their unpleasant odor.
A
500-mL, three-necked, round-bottomed flask fitted with a low-temperature thermometer, 125-mL pressure-equalizing dropping funnel, and nitrogen gas inlet is flame-dried under vacuum, cooled to room temperature, and flushed with
nitrogen. The flask is charged with
5.00 g (27.4 mmol) of 2,3-dimethoxy-5-methyl-1,4-benzoquinone (Note
1) as a solid followed by
100 mL of dry dichloromethane (Note
2) via syringe. The addition funnel is charged with
11.8 g (32.9 mmol) of tributyl(3-methyl-2-butenyl)tin (Note
3) in
100 mL of dry dichloromethane. The flask is immersed in an
acetone/dry ice bath, the solution cooled to −78°C, and
10.1 mL (82.3 mmol) of boron trifluoride etherate (Note
4) is added dropwise from a syringe. Next, the
stannane solution is added dropwise over a 30-min period. Following completion of the addition, the cooling bath is removed and the reaction mixture is allowed to warm to room temperature. At −40°C, the color of the solution turns from deep red to yellow. When the reaction mixture reaches 0°C, it is treated with
100 mL of 10% hydrochloric acid, stirred for 5 min, and allowed to stand. The aqueous phase that separates is washed with
dichloromethane (2 × 20 mL) and the combined organic layers are washed with water (2 × 20 mL) and
brine (2 × 20 mL), dried over
anhydrous magnesium sulfate, and concentrated. The residual yellow oil is dissolved in
200 mL of ether and this solution is added to
37.1 g (137 mmol) of ferric chloride hexahydrate (FeCl3 · 6 H2O) in 200 mL of water in a
500-mL conical flask. The resulting mixture is stirred for 8 hr at room temperature. The layers are then separated, the aqueous phase is extracted with
ether (2 × 50 mL), and the combined organic solutions are stirred with
100 mL of 10% aqueous potassium fluoride for 2 hr (Note
5). The insoluble tin salts are separated by filtration (Note
6). The organic phase in the filtrate is washed with water (3 × 20 mL) and
brine (3 × 20 mL), dried over
anhydrous magnesium sulfate, and concentrated to give a deep red oil, that is purified by column chromatography (Note
7). The red-orange band is collected (1600–3000 mL) to give
6.15 g (
90%) of
2,3-dimethoxy-5-methyl-6-(3-methyl-2-butenyl)-1,4-benzoquinone as a deep red oil (Note
8).
2. Notes
4.
Boron trifluoride etherate was purchased from Nacalai Tesque or Aldrich Chemical Company, Inc. and used without further purification. Since the purity of the BF
3 complex affects the yield of the
quinone, reagent stored for a long period should be distilled prior to its use.
5. This facile method for removing organotin impurities was developed by Keck.
2
6. Tin wastes are collected and disposed of in an environmentally safe manner. (See Waste Disposal Information below.)
7. A gravity column (7 cm × 15 cm) was packed with 60-230 mesh silica gel. Elution was effected with 10%
ethyl acetate in petroleum ether.
8. Spectroscopic data are as follows:
1H NMR (400 MHz, CDCl
3) δ: 1.68 (s, 3 H, cis-CH
3), 1.74 (s, 3 H, trans-CH
3), 2.20 (s, 3 H, ring CH
3), 3.17 (d, 2 H, J = 7.0, CH
2), 3.98 (s, 3 H, CH
3O), 3.99 (s, 3 H, CH
3O), 4.94 (t, 1 H, J = 7.0, C=CH); IR (neat) cm
−1: 2790, 1650, 1617, 1455, 1329, 1262, 1555, 1102, 1013.
Waste Disposal Information
A
chloroform solution of toxic
tributyltin fluoride was placed in a
round-bottomed flask equipped with a reflux condenser, and
bromine (3 equiv per Bu3SnF) was added all at once. After the mixture was stirred for 2–3 days at room temperature, aqueous
sodium thiosulfate was added until the brown color of
bromine disappeared. The aqueous layer containing inorganic tin compounds was disposed of in accordance with "Prudent Practices in the Laboratory"; National Academy Press; Washington, DC, 1995.
3. Discussion
Ubiquinones are synthesized by the introduction of an isoprenyl side chain to the corresponding quinone or hydroquinone using several compounds: Lewis acid-isoprenyl alcohols,
3 π-allylnickel bromides,
4,5 isoprenyltrialkyltins,
6 N-sulfinylamideisoprenyl alcohols,
7 amalgamated zinc-isoprenyl bromides,
8 and Claisen rearrangement of isoprenyl aryl ethers.
9 Among these procedures, the isoprenyltin method is superior to the others with respect to yield, simplicity of manipulation, and purity of product. This method can be also applied to the synthesis of the higher homologues of ubiquinone-n (n = 2–10). In general, synthesis of allylated quinones or hydroquinones can be performed by the reaction with allylborane,
10 π-allylnickel complexes,
11 allyltrimethylsilane,
12 or allyltrialkyltins.
6,13,14 Direct allylation to the quinone is considered to be the best procedure compared with stepwise methods including protection/deprotection of quinone.
15 The present method is applicable to a broad range of quinones and allylated trialkyltins to give the corresponding hydroquinone (or quinone after oxidation) in good to excellent yields. Several other examples are shown in the Table. In this reaction,
boron trifluoride plays two roles: activation of the quinones resulting in 1,2-addition and acceleration of dienone-phenol rearrangement of the allylic group.
8 In addition this method can be applied to the synthesis of many other polyprenylated quinones,
6,13 including naturally occurring quinones,
plastoquinone-n,
phylloquinone (vitamin K1), and
menaquinone-n (vitamin K2(n)). The polyprenyl side chain can be stereoselectively introduced into a quinone nucleus without formation of the corresponding chromanol or other side chain-cyclized products. Other methods of polyprenyl group introduction to quinones or their protected forms are known: a Friedel-Crafts alkylation,
16 a free-radical alkylation,
17 and coupling reactions with organometallic reagents including lithium,
18 magnesium,
19,20 and
tin.
21
TABLE
PREPARATION OF ALLYLATED QUINONES FROM QUINONES AND ALLYLATED TRIBUTYLTINS
|
|
Yield (%) |
|
Yield (%) |
|
|
68 |
|
81 |
|
45a |
|
90 |
|
55a |
|
82 |
|
58a |
|
42a |
|
72 |
|
78a |
|
aisolated after oxidation in ether with aqueous FeCl3 solution
|
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