Organic Syntheses, CV 5, 251
Submitted by Albert W. Burgstahler and Leonard R. Worden
1.
Checked by Wayland E. Noland, William E. Parham, and Carol Wong.
1. Procedure
A.
o-Formylphenoxyacetic acid. A solution of
80.0 g. (2 moles) of sodium hydroxide pellets in 200 ml. of distilled water is added to a mixture of
106 ml. (122 g., 1 mole) of salicylaldehyde (Note
1),
94.5 g. (1 mole) of chloroacetic acid (Note
1) and (Note
2), and 800 ml. of water. The mixture is stirred slowly and heated to boiling. The resulting black solution (Note
3) is heated under reflux for 3 hours (Note
4). The solution is acidified with
190 ml. of concentrated hydrochloric acid (sp. gr. 1.19) and is steam-distilled to remove unchanged
salicylaldehyde (40.0–40.5 g.) (Note
5). The residual acidic mixture is cooled to 20°, and the precipitated product is collected on a
Büchner funnel and rinsed with water. The light tan solid when dry weighs
99–100 g. (
82–83% based on recovered
salicylaldehyde), m.p.
130.5–133.0° (Note
6).
B.
Coumarone. A mixture of
90.0 g. (0.5 mole) of crude (Note 7), dry o-formylphenoxyacetic acid,
180 g. of anhydrous, powdered sodium acetate,
450 ml. of acetic anhydride, and
450 ml. of glacial acetic acid (Note
8) in a
2-l. flask is heated under gentle reflux with stirring for 8 hours. The hot black solution (total volume
ca. 1.2 l.) (Note
3) is poured into 2.5 l. of ice water and extracted with one
600-ml. portion of ether (Note
9). The
ether layer is washed with one 600-ml. portion of water and then with several portions of cold dilute
5% sodium hydroxide solution (Note
10) until the aqueous layer is basic. The
ether layer is washed successively with water and saturated
sodium chloride solution and is partially dried over anhydrous granular
sodium sulfate. The
ether is removed at water-bath temperature and the product is distilled, b.p.
166.5–168.0° (735 mm.) or
97.5–99.0° (80 mm.). The water-white
benzofuran weighs
37.5–40.0 g. (
63.5–67.8%,
52–56% overall from
salicylaldehyde),
n20D 1.5672; λ
max 245 (log ε 4.08), 275 (3.45), and 282 mμ (3.48).
2. Notes
1. Matheson, Coleman and Bell practical grade material was used.
3. At no time did the checkers observe a black solution. The color of the solution changed from yellow to red-brown.
4. The yield is not increased by longer reflux periods.
5. Removal of unchanged
salicylaldehyde by steam distillation (followed conveniently by testing the distillate with
2,4-dinitrophenylhydrazine reagent) provides a product sufficiently pure for use in the next step. Also, the recovered
salicylaldehyde can be used again without further purification.
6. Three crystallizations of
36 g. of the crude o-formylphenoxyacetic acid from 360 ml. of water with
10 g. of activated carbon give 18 g. of glistening colorless plates, m.p.
133.0°–133.5°.
9. An additional extraction does not increase the yield appreciably.
10. About 250 ml. of this solution is required.
3. Discussion
4. Merits of the Preparation
Although the high-temperature catalytic dehydrocyclization of
o-ethylphenol11 gives
benzofuran in fair yield, these conditions are not convenient in the laboratory and cannot be applied easily to functionally substituted
o-formylphenoxyacetic acids. The other methods of preparation give unsatisfactory yields, are unnecessarily lengthy, or require expensive starting materials. The method of Rössing,
2 on which the present procedure is based, gives good yields in its original form only in the case of
o-acylphenoxyacetic acids;
o-formylphenoxyacetic acids give principally the corresponding coumarilic acids. Often these can be decarboxylated only in very poor yield.
12 In the preparation described here,
benzofuran is obtained directly in fair overall yield from readily available and inexpensive starting materials without the necessity of a separate decarboxylation step.
Copyright © 1921-2002, Organic Syntheses, Inc. All Rights Reserved