It is obtained by the oxidation of xanthene (methylene diphenylene oxide) with chromic acid; by the action of phosphorus oxychloride on disodium salicylate; by heating 2 2'-dioxybenzophenone with concentrated sulphuric acid; by distilling fluoran with lime; by the oxidation of xanthydrol (R.
The most important is the normal salt, Na 4 P 2 0 7, which is readily obtained by heating disodium orthophosphate, Na 2 HPO 4.
Soc., 1888, 53, p. 1 94): C2H4Br2+2NaCH(C02R)2--->(CH 2) 2 C(CO 2 R) 2 +CH 2 (CO 2 R) 2; ethyl butane tetracarboxylate is also formed which may be converted into a tetramethylene carboxylic ester by the action of bromine on its disodium derivative (W.
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