Org. Synth. 1943, 23, 25
DOI: 10.15227/orgsyn.023.0025
2,3-DIHYDROPYRAN
[Pyran, dihydro-]
Submitted by R. L. Sawyer and D. W. Andrus.
Checked by Nathan L. Drake and Charles M. Eaker.
1. Procedure
The reaction is carried out in a heated tube similar to that described by Herbst and Manske
[Org. Syntheses Coll. Vol. 2, 389 (1943)] except that the receiving chamber B (
loc. cit.,
Fig. 2) does not have a side arm, and that a
10-cm. water-cooled condenser is attached to the exit end of the tube leading directly to the
receiver. The tube is packed with
activated alumina (Note 1) held in place at the ends by plugs of glass wool.
The furnace is heated to 300–340° (Note 2), and 204 g. (195 ml., 2 moles) of tetrahydrofurfuryl alcohol (Note 3) is introduced from the dropping funnel at the rate of 50 ml. per hour. The product, collected in an Erlenmeyer flask which contains 30 g. of anhydrous potassium carbonate, consists of a light-brown oil and a lower aqueous layer. When the reaction tube has drained, the lower aqueous layer is separated and discarded. The upper layer is fractionated through a short column, and a fraction boiling at 70–86° is collected. This consists of a mixture of water and dihydropyran, most of which distils at 83–86°. The residue (25–35 g.) is mainly unchanged tetrahydrofurfuryl alcohol (Note 4).
The water-dihydropyran fraction separates into two layers. The lower aqueous layer is separated and discarded. The upper layer, consisting of fairly pure dihydropyran, is dried over 5–6 g. of anhydrous potassium carbonate, decanted, refluxed for 1 hour with 2–3 g. of metallic sodium, and then distilled from sodium (Note 5). The yield is 110–118 g. of dihydropyran, boiling at 84–86° (66–70%).
2. Notes
1.
Several varieties of technical
activated alumina were used. After three or four runs the catalyst becomes covered with a brown tar and the yield of
dihydropyran decreases. The catalyst may be regenerated by igniting it at red heat until the tar is burned off. The checkers used 8–14 mesh activated alumina from the Aluminum Ore Company of America. The catalyst was reactivated at 450° by drawing a slow stream of air through it until the tar was burned off.
2.
The temperature of the furnace is measured by a
thermometer placed alongside the glass tube inside the furnace. The temperature should be 330–340° except in the regions about 10 cm. from each end of the furnace; here the temperatures will be 300–340°, depending on the construction of the furnace.
3.
According to the submitters, Eastman's practical grade of
tetrahydrofurfuryl alcohol must be purified by distillation; the fraction boiling at
79–80°/20 mm. was used. The checkers used, without purification,
tetrahydrofurfuryl alcohol obtained from the Quaker Oats Company. The yields were equally good.
4.
The recovered
tetrahydrofurfuryl alcohol turns yellow on standing and is unsuitable for further runs. If it is used, the yield of
dihydropyran drops to
36–38%, and the catalyst must be regenerated after each run.
5.
Dihydropyran is very difficult to dry. Even after this treatment the product often contains traces of water.
3. Discussion
The procedure given above is essentially that of Paul.
1,2 Aluminum silicate,
titanium oxide, and
basic aluminum phosphate have also been used as catalysts in this rearrangement.
3 The action of
sodium amide upon
tetrahydrofurfuryl bromide gives chiefly
1,4-epoxy-4-pentene and a small amount of
dihydropyran.
1,4-Epoxy-4-pentene undergoes rearrangement at 380° in the presence of alumina to yield
dihydropyran.
4This preparation is referenced from:
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
activated alumina
basic aluminum phosphate
potassium carbonate (584-08-7)
sodium (13966-32-0)
tetrahydrofurfuryl alcohol (97-99-4)
sodium amide (7782-92-5)
titanium oxide
2,3-Dihydropyran,
Pyran, dihydro-,
dihydropyran
Tetrahydrofurfuryl bromide (1192-30-9)
1,4-epoxy-4-pentene
Aluminum silicate
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