PYRONES  , in

chemistry, a group of heterocyclic compounds, containing a six-membered ring composed of five carbon atoms and one oxygen atom . Two types are known, namely, the a-pyrones, which may be regarded as the lactones of b-oxydiolefine carboxylic acids, and the 7-pyrones, which may be regarded as anhydrides of diolefine dioxyketogqnes: CH :C~H \CH (3) :CH (y)HC•CO/O (4)OC<CH:CH/O(1) (R) (a) (5) (6) a-Pyrone. y-Pyrone . As a class, the pyrones are rather unstable compounds, the ring being readily broken . When digested with ammonia, the oxygen atom is replaced by the imino (:NH) group, and pyridones or oxypyridines are formed . a-Pyrones.—The coumalic compounds belong to this series, and were first obtained by A . Hantzsch in 1884 (Ann . 222, p . 1) and H. v . Pechmann (Ber., 1884, 17, p . 936) . a-Pyrone or coumalin, C1H402, is obtained by distilling the mercury salt of coumalic acid (from malic acid and sulphuric acid) in a current of hydrogen . It is an oily liquid which boils at 206—209° C., and with alkalis it gives formyl crotonic acid, HO2C•CH:CH•CH2•CHO. a'y-Dimethyl-a-pyrone or mesitene lactone, C7H802, is obtained from iso-dehydracetic acid (from aceto-acetic ester and sulphuric acid) .

Phenylcoumalin or a'-phenyl-a-pyrone, C5H3(

C6H5)02, is found in coto-bark . When heated with alkalis it yields benzoic acid and acetophenone; reduction by hydriodic acid gives 6-phenyl valeric acid, and when heated with ammonium acetate and ammonia it yields phenylpyridone . It forms an addition product with phenol and with aniline; the latter gives diphenylpyridone when boiled with concentrated hydrochloric acid . Paracotoin, C18H804, which also occurs in coto-rind, appears to be a bisoxymethylene phenylpyrone, C5H302•C6H3(CH202) . Various pyronones (keto-dihydropyrones) derived from the compound having formula I . (below) are known, the most important of which is dehydracetic acid, C8H804, first obtained by Geuther (Jena'sche Zeit, 1866, p . 8) . It may be prepared by distilling acetoacetic ester alone, by heating it with acetic anhydride to 200° C. or by heating acetyl chloride with pyridine to 200—220° C . J . N . Collie regards it as having formula II., whilst Feist (Ann . 189o, 257, p .

253) favours formula III . OC•CH :CH OC•CH :C•CH2•CO•CHa OC•CH :C•CHa H2C•CO.O H2C•CO.O CH3•CO•HC•C0.0 (I.) (II.) (III.) It crystallizes in tables which melt at 108—1o9° C., and is a weak acid . Alcoholic potash converts it into aceto-acetic ester, and with concentrated aqueous

caustic potash it is completely decomposed into acetone, acetic acid and carbon monoxide . y-Pyrones.—Many of these compounds are found as naturally occurring substances: thus chelidonic acid is found in Chelidonium majus and meconic acid in opium, and the more complex flavone and flavonol derivatives are also found in various plants . The 7-pyrones may be synthesized by eliminating water from the 1.3.5 triketones: CH2•CO•CO2R CHIC—0O2R OC --j OC / '>O NCH2•CO•CO2R NCH:C-0O2R Acetone dioxalic ester . —> Chelidonic ester . y-Pyrone or pyrocomane, C5H402, melting at 32° C. and boiling at 210—215° C., is obtained by eliminating carbon dioxide from chelidonic acid (obtained as above), or from comanic acid, obtained by heating chelidonic acid. aa'-Dimethyl-y-pyrone, C5H2(CHa)202, is obtained by the action of hydriodic acid on the ester of the corresponding acid (Feist, Ann., 189o, 257, p . 272); by the action of carbonyl chloride on the copper derivative of acetoacetic ester, and by the action of concentrated hydrochloric acid on dehydracetic acid . It forms a barium salt which with an acid yields diacetyl acetone . The most striking property of this compound is that it forms salts with mineral acids (J . N . Collie and Tickle, Journ .

Chem .

Soc., ,1899, p . 710) . For example, hydrochloric acid adds on at the oxygen atom, since the salts so formed are relatively unstable and undergo complete hydrolysis in dilute aqueous solution . The oxygenatom is probably tetravalent, and the salts are to be regarded as oxonium salts (see OXYGEN) . Collie (Journ . Chem . Soc., 1904, 85, p . 971) is of the opinion that both oxygen atoms are to be regarded as tetravalent in these salts and gives the second formula below for the molecule: HC•CO•CH HaC•C•O•C•CH3 H Cl Meconic acid, or oxypyrone tricarboxylic acid (3.2.6) C5H02(OH) (CO2H)2, found in opium, crystallizes in prisms and gives a characteristic deep red colour with ferric chloride . On heating to 20o° it gives comenic acid, C5H2O2(OH)(CO2H), and on distillation pyromeconic acid or /3-oxypyrone . On comenic acid see A . Peratoner, Gazz., 1906, 36 (i.), p .

1 . The tetrahydro-y-pyrones may be obtained by the condensation of

aldehydes with acetone-dicarboxylic ester in the presence of hydrochloric acid . Benzopyrones . Compounds of this type are known in both the a and y series, the former including the coumarins (q.v.) and isocoumarins, and the latter a number of naturally occurring dyestuffs which may be considered as derivatives of flavone (see under) . The isocoumarins (annexed formula) may be prepared by the action of acid chlorides or anhydrides on orthocyanbenzyl cyanide (Ber., 1892, 25, p . 3563); by the molecular rearrange- ment of the benzal or alkylidene phthalides (S . CH Gabriel, Ber., 1885, 18, p . 2443 ; 1887, 20, p . 2363), /\''''' . and by the action of manganese dioxide and hydrochloric acid on ,B-naphthoquinone . 0 The parent substance of the 7-group, namely benzo-y-pyrone (chromone), was obtained in 1900 \/\CO/ by S . Ruhemann (Journ . Chem .

Soc., 77, p . 1179) Isocoumarin. by heating its carboxylic acid (formed by the action of concentrated sulphuric acid on phenoxyfumaric acid) in vacuo . It crystallizes in colourless needles, and its solution in concentrated sulphuric acid is yellow with a

blue fluorescence . The naturally occurring compounds, chrysin, galanzin, quercetin, apigenine, &c., are considered to be derivatives of flavone (or flavonol), which is a phenyl-2-benzo-7-pyrone (S . Kostanecki, Ber., 1898—1906) .. Flavone and flavonol possess the following constitutions, the positions of the substituents being indicated by the numbers: 3/\/ O C— j=am 4~ /\/ 0 _\.—/ 2 LH 6' 5 \/\Co/a. i /\COj 'OH 1 Flavone . Flavonol . Flavone, C15H10O2, is obtained by the action of potassium hydroxide on the acetyl derivative of benzylidene-ortho-oxyacetophenone . It forms colourless needles, which dissolve in concentrated sulphuric acid with a yellow colour and show a faint blue fluorescence . On fusion with caustic alkalis it yields salicylic acid, acetophenone, ortho-oxyacetophenone and benzoic acid, the latter two products being also formed by its hydrolysis. with sodium ethylate . Chrysin or I.3-dioxyflavone, C15H1004, is a yellow dye, which may be obtained from the buds of different varieties of the poplar . On hydrolysis it yields phloroglucin and benzoic and acetic acids .

It has been synthesized by heating trimethoxy benzoyl acetophenone (from

ethyl benzoate and phloracetophenone trimethyl ether) with hydriodic acid, and also by the action of hydriodic acid on 2.4-dibrom-1.3-dimethoxyflavonone . Galanzin or a'I.3-trioxyflavone or I.3-dioxyflavonol, C16H1005, crystallizes in yellow needles . It has been synthesized from hydroxydimethoxy-chalkone, C6H5•CH:CH•CO[I]•C6H2(OH)(OCH3)2[2.4.6•], the resulting 1.3-dimethoxy-flavanone compound yielding a nitroso-compound from which galanzin is obtained by the action of concentrated hydriodic acid . Apigenine or r3.4'-trioxyflavone, C15H10O5, found in woad and in parsley, crystallizes in pale yellow needles . On fusion at moderate temperatures with caustic alkalis it gives phloroglucin and para-oxyacetophenone, whilst at higher temperatures it yields protocatechuic and para-oxybenzoic acids and phloroglucin . It is obtained synthetically by brominating I.3.4'-trimethoxyflavonone, the resulting tribromo-compound by the consecutive reactions of alcoholic potash and hydriodic acid yielding apigenine . Kaempferol or I.3.4'-trioxyflavonol, C15H10O5, is found in the blossoms ofpDelphinium consolida and D. zazil . It is obtained by the action of hydriodic acid on kaempherid, and crystallizes in yellowish needles, which on fusion with caustic alkalis give para-oxybenzoic acid and phloroglucin . It is obtained synthetically from hydro~xcyy-trimethoxychalkone,CH30[4]•C6H4[I]•CH CH•CO•[i]C6H2(OH)(OCHa)2[2.4.6]by a method similar to that used for galanzin . Kaempferid occurs together with galanzin and alpinin in galganta root . It crystallizes in pale yellow needles, which dissolve in the caustic alkalis with an intense yellow colour, and in concentrated sulphuric acid with a HC---C=----CH II CI.O•H H3C• '—0— •CH3 as loose grains in detritus due to the disintegration of the matrix, The grains may be surrounded by a chloritic rind, or by a crust of a fibrous mineral called by A . Schrauf kelyphite (from the Gr, KiXv4,os, a nut-shell), which seems in some cases to be an amphibole .

In the

serpentine of Zoblitz and of Greifendorf near Leipzig, in Saxony, pyrope is characteristically developed; and the Saxon garnets, found loose in gravels, were referred to by G . Agricola as far back as 1546 . Several localities in Bohemia are famous for yielding pyrope, and from its characteristic occurrence here it is often known, even when found elsewhere, as Bohemian garnet . The garnet-bearing district is a tract of about 70 square kilometres in the north of Bohemia, the chief locality being Meronitz near Bilin . It is notable that the pyrope is found at Meronitz in a clayey calcareous tufa or conglomerate, with opal and serpentine, products of the decomposition of a peridotite . It occurs also in sands and gravels near Chrastian, Lobositz, Triblitz, Podseditz, Chodolitz, and at several other localities in the Mittel Gebirge, between Teplitz and Leitmeritz . It is believed that the original pyrope-bearing rocks resulted from the eruptive activity which gave rise to Linhorka Hill, near Starrey . The garnets in the detritus are accompanied by zircon, spinet, corundum, cyanite, tourmaline, olivine, &c . Though generally very small, they are abundant, and are used not only as ornamental stones, but as a counterpoise in delicate weighing and as an abrasive agent . To obtain the stones the detritus is washed, and the garnets picked out by hand and then sized through sieves . The pyrope is generally rose-cut or step-cut, and often mounted with a foil . Beads are faceted all over .

Some pyrope is cut en cabochon, forming, like

almandine, carbuncle, and if very dark the stone is hollowed at the back so as to form a " garnet-shell." The industry of cutting Bohemian garnets is centred in Turnau on the Iser, near Reichenberg; but there are also works at other localities . Large stones are very rare, but a Bohemian pyrope as large as a hen's egg is preserved in the Imperial treasury at Vienna; and another the size of a pigeon's egg in the Grune Gewolbe of Dresden . Pyrope occurs in many localities in the western part of the United States, especially in Colorado, Arizona and New Mexico, where it is often called `. ruby." It is found loose in sand accompanied by olivine, and has resulted from the alteration of a peridotite . The Navajo Indians of New Mexico collect the garnet from the sands of the ant-hills and scorpion-holes . Very fine pyrope occurs in the diamond-fields of South Africa, having been derived from olivine-bearing rocks . It occurs in the blue-ground and in the detritus of the river-diggings . The Cape garnets have usually a rich colour, but some stones incline to an orange hue . The finest pyrope is often cut as a brilliant, and passes under the misleading name of " Cape ruby." A pyrope-bearing rock, rather like that of South Africa, occurs in Elliott county, Kentucky, U.S.A . ; it is notable, too, that pyrope is found near Elie in Fife, in Scotland, where it occurs in volcanic agglomerates and in basaltic dikes . Sir A . Geikie has pointed out the suggestive resemblance of the occurrence there to that in South Africa . See " Bohemian Garnets," by G .

F . Kunz, Trans . Amer . Inst .

Mining Eng . (1893), xxi . 241; and " Die bohmischen Granatlagerstatten," by Dr Hans Oehmichen, Zeil. f. prakt . Geol . (1900), viii . I . Both papers contain bibliographical lists . (F .

W .