W.H. Perkin & V.M. Trikojus
J. Chem. Soc. 1663 (1927)
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No direct synthesis of safrole has been recorded, the most recent attempt being that of Baker and Robinson (JCS, 1925, 127, 1424), who distilled the product of the action of silver oxide and water upon gamma-piperonylpropyltrimethylammonium iodide, but obtained isosafrole, the double bond moving into the position of greater stability. Kawai (Sci. Papers Inst. Phys. Chem. Res., 1925, 3, 263) has shown that the monoallyl ether of pyrocatechol undergoes the Claisen rearrangement to give an oil which he considered to be a mixture of 3- and 4-allyl-1,2-dihydroxybenzene.
This matter has now been reinvestigated and, by repeated fractionation of the product of the rearrangement, both of the above isomerides have been isolated in a pure state as colourless, crystalline solids melting at 28°C and 48°C respectively. As further confirmation of the course of the Claisen migration, it has been found that the properties of the higher-melting isomeride (II) are in accord with those assigned to the "4-allylbrenzcatechin" which Schimmel and Co. (Centr., 1907, II, 1741) isolated in small quantity from betel-leaf oil from Java. The second isomeride, 1,2-dihydroxy-3-allylbenzene (I), has not previously been described.
In order to convert these substances into safrole (III) and o-safrole (IV), respectively, methylenation was carried out, with moderately good yields, by gently refluxing them with methylene iodide and anhydrous potassium carbonate in dry acetone solution. By employing such mild conditions, any change into the isomeric isosafroles during the course of the reaction was inhibited. The identity of the synthetic product with naturally occurring safrole has been established by preparing the pentabromo-derivative (mp 169°C) and the acetamido-derivative (mp 162-163°C) and comparing these with the same derivatives from natural safrole. The properties of the...