PYRONES, in chemistry a group of heterocyclic compounds containing a six-membered ring composed of five carbon atoms and one oxygen atom. Substances containing the pyrone nucleus occur in nature; for example, coumarin, the odoriferous principle of the tonka bean, contains a pyrone residue and so also do me conic acid, present in opium, and chelidonic acid, found in the celandine and white hellebore.
Two types of pyrones may be distinguished, the a-pyrones (I.), in which the oxygen atom is adjacent to a carbonyl group, and the 7-pyrones (II.), where the oxygen atom and the carbonyl group are at opposite ends of the ring structure.
a-Pyrone or coumalin (I.) is derived from malic acid (q.v.). This acid on heating with strong sulphuric acid yields coumalic acid (a-pyrone-5-carboxylic acid) and on distilling the mecury salt of this intermediate acid in hydrogen, carbon dioxide is eliminated and a-pyrone is obtained as an oily liquid boiling at 206-209° C.
Phenylcoumalin or a'-phenyl-a-pyrone, colourless crystals melt ing at 68° C, is found in coto-bark. On heating with alkalis it yields benzoic acid and acetophenone. a'13' -Benzo-a-pyrone is discussed under its other name, coumarin (q.v.). Like y-pyrone and dimethy1-7-pyrone (v. infra), coumarin has the property of forming salts with the acids (aurichloric, cobalticyanic, hydriodic and platinichloric acids). 7-Pyrone or pyrocomane (II.) crystal lises in colourless prisms melting at 32° C and boiling at 210— 215° C. It is prepared from chelidonic acid y-pyrone-2-6-dicarb oxylic acid, m.p. 262° C. This acid on being heated loses carbon dioxide in two stages to form comanic acid, y-pyrone-2-carboxylic acid, and finally y-pyrone. The elimination of carbon dioxide is facilitated by the addition of copper powder. Chelidonic acid can
either be obtained from natural sources (v. supra) or prepared synthetically by condensing acetone with ethyl oxalate in presence of sodium ethoxide, when ethyl acetonedioxalate is produced, and this substance on boiling in alcoholic solution loses water to form ethyl chelidonate.
ace'-Dimethylpyrone (III.) is prepared by condensing carbonyl chloride with the copper derivative of ethyl acetoacetate, or by the action of concentrated hydrochloric acid on dehydracetic acid. It forms. white monoclinic plates, melting at 132° C and boiling at 248° C/713 mm. It is readily soluble in water or alcohol to neutral solutions which have a bitter after-taste. Dimethylpyrone has the noteworthy property of giving well-defined crystallisable salts with hydrochloric, platinochloric and tartaric acids.
The salts, which are considerably hydrolysed in aqueous solution, arise from a combination between one of the oxygen atoms (possibly the carbonyl one) and the hydrion of the acid. On the electronic theory of chemical combination either of these oxygens can furnish a pair of electrons to co-ordinate with the hydrion which has already given its single electron to the anion of the acid. Formula IV. represents the dimethylpyrone hydrochloride as having the hydrogen co-ordinated with the carbonyl oxygen. Dimethylpyrone was shown by A. Werner to combine in a similar way with cupric chloride to form the compound C711802,CuC12, the existence of which supports the hypothesis that the oxonium salts of the pyrones are co-ordination compounds arising from the operation of the subsidiary valencies of the oxidic or ketonic oxygen atoms. (G. T. M.)