Organic Syntheses, Coll. Vol. 5, p.251 (1973); Vol. 46, p.28 (1966).
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.
overall from salicylaldehyde
245 (log ε 4.08), 275 (3.45), and 282 mμ (3.48).
Matheson, Coleman and Bell practical grade material was used.
At no time did the checkers observe a black solution. The color of the solution changed from yellow to red-brown.
The yield is not increased by longer reflux periods.
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.
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°
An additional extraction does not increase the yield appreciably.
About 250 ml. of this solution is required.
4. Merits of the Preparation
Although the high-temperature catalytic dehydrocyclization of o-ethylphenol11
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.
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