3,592,826 United States Patent Office Patented July 13, 1971 2 3,592,826 MIDAZOLE CATALYSTS FOR REACTIONS OE Erich Marcus, Charleston, and John Kai-Fai Chan, South Charleston, W. Va., assignors to Union Carbide Cor 2 poration -HO CH3C- --CC No Drawing. Filed May 22, 1967, Ser. No. 640,363 Int, C, CO7d 7/16 CHSCH.go -CH3 U.S. C. 260-343.5 13 Claims O O CH, OH ABSTRACT OF THE DISCLOSURE CHC catalyzed reactions for the production of de CHC -CCE hydroacetic acid, 2,4,6-heptanetrione and 2,6-dimethyl-4- CH + pyranone from diketene are disclosed. 5 -CCH O= -Ca The examples illustrate the use of 1-methylimidazole; o/ 1-ethylimidazole; 1-isopropylimidazole; 1-octylimidazole; HO -CH3 1,2-dimethylimidazole; 1,2-diethyl-4-methylimidazole to catalyze the reactions. (C) Tetramer II (B) Tetramer ^ /diketene Dehydroacetic acid can be prepared by the dimeriza tion of diketene in an inert solvent in the presence of a O catalyst and may be illustrated by the following reaction: 25 OCCHCCH O CCs o PH o H. gh O 4N CH CH3C- -CCH -HO CH3C Catalyst CCH -aa-ee CE O 30 O- -CH.." - "...IO-N/ CH3 O (D) Pentamer A number of catalysts such as tertiary amines, The expression “trimer,” “tetramers” and “pentamer" as phenoxides, sodium alkoxides, and sodium acetate have used herein are intended to relate to compounds of the been proposed for the reaction. In using these conven structure A, B, C and D as given above. The formation tional catalysts, it has been observed that the product 35 of the pentamer and two tetramers is undesirable for produced is usually contaminated with a large amount several reasons, one of which is the reduced yield of de of impurities, which make it almost impossible to purify hydroacetic acid. Additionally, once the impurities are the desired product by inexpensive means. As an example precipitated from the mother-liquor together with dehy when dehydroacetic acid is prepared by the dimerization droacetic acid, pure dehydroacetic acid cannot be isolated of diketene in the presence of trimethylamine according 40 by simple crystallization and recrystallization. An ex to U.S. Pat. 2,849,456 impurities are produced that yields pensive codistillation with a high boiling solvent is neces a sodium salt of yellow or orange color and which is also sary to obtain dehydroacetic acid of satisfactory quality. highly insoluble in known solvents and, cannot, be sep arated from dehydroacetic acid by recrystallization. The difficulties encountered in the prior art, especially 45 with regard to separations of impurities are further de These impurities have now been identified as the scribed in U.S. Pat. 2,553,292 and 2,997,482. In one prior tetramers and pentamer of diketene and are formed ac art method of purification substantial decomposition oc cording to the following condensation reactions which curs using a codistillation process, the decomposition also also take place during the conversion of diketene to de hydroacetic acid under certain conditions: resulting in a further reduction of the acid yield. Addi 50 tionally, dehydroacetic acid is sold as an alkali metal salt such as the sodium salt which is white in color. The so CH=C-O O HO dium salts of the two tetramers and the pentamer form CH-CacO - CECCH3CO deep yellow compounds and consequently their presence makes it nearly impossible to obtain sodium dehydro 55 acetate of satisfactory color. |alketene Since both the tetramers and pentamer tend to build up -co, in time as by-products in the mother-liquor in which the CH, CH, CH, CH, vve CH, CHE CH, CH, dehydroacetic acid is produced, it is substantially impos (2,4,6-heptanetrione) OH sible to recycle or re-use the mother-liquor containing diketene and other non-deleterious by-products since the 60 increasing concentration of tetramers and pentamer in the dietene mother-liquor greatly increase the likelihood of subse O OH O O quent contamination of dehydroacetic acid. Accordingly, O C O . a continuous commercial process for the manufacture of / N. -O CC- -CCH3 dehydroacetic acid employing a crystallization step for CH3C-CEI CEI-CCH3. --> 65 separating the acid is not possible since the liquor will O- CH3 always contain tetramers and pentamers as impurities o=d CCH along with trimers that will be subsequently converted to / (A) Trimer tetramers and pentamer. Consequently the mother-liquor, CH if recycled in a continuous process will undergo an incre O mental build-up of these impurities which increases the dietene likelihood of contaminating the acid during separation by 3,592,826 3 4. crystallization and recrystallization. For the same reasons can be converted into a white or nearly white colored so recovery of pure dehydroacetic acid is not possible by dium salt of dehydroacetic acid. When dehydroacetic acid evaporation of the mother-liquor or by processes in which has been made by dimerization of diketene in the presence second and third crop crystals are obtained from the of other tertiary amine catalysts according to procedures mother-liquor. known in the art, even the first crop of dehydroacetic acid It is therefore an object of the present invention to is usually converted into a sodium salt of orange color. overcome these and other difficulties encountered in the Since the formation of impurities rendering a yellow or prior art. orange-colored salt is eliminated or occurs only to a The present invention is directed to these ends and in negligible extent when 1-alkylimidazoles are used as cata its broadest aspect relates to a novel method for the O lysts, the mother-liquor can be used over again repeatedly conversion of diketene to dehydroacetic acid 2,4,6-hep without further purification. tanetrione and 2,6-dimethyl-4-pyranone in which an imid A further aspect of the invention relates to the discovery azole, especially a 1-alkylimidazole is employed as a cat that when the catalysts of the present invention are em alyst. Another aspect of the invention comprises the con ployed with a solvent comprising a ketone containing from version of diketene to dehydroacetic acid in a solvent by 5 three up to about ten carbon atoms and a hydrocarbon means of an imidazole catalyst whereby the tetramers or normally liquid at about room temperature (20° C.) and pentamer impurities produced as a by-product of the having from about five up to about ten carbon atoms and dimerization are substantially reduced or eliminated. It various mixture thereof, increased yields of dehydroacetic is another object of this invention to provide a catalyst acid may be obtained. useful for the production of dehydroacetic acid from dike 20 The present invention also relates to a method for tene in high yields. manufacturing 2,4,6-heptanetrione (diacetylacetone) and According to the present invention, when the imidazole 2,6-dimethylpyranone from diketene and water employ catalyst is used in a process employing a solvent as a ing the novel catalysts described herein. These compounds reaction medium, crystallization, centrifugation or filtra are formed by adding diketene to water containing the tion can be used to obtain high purity dehydroacetic acid 25 catalyst and the time of reaction adjusted so that the directly in high yields, and the mother-liquor which also reaction is stopped when carbon dioxide is no longer contains some acid can be recycled to the reaction maSS given off as a by-product. The amount of water may along with diketene and the imidazole catalyst or the acid vary anywhere from about 0.3 to about 0.9 mole per may be separated by crystallization. This recycling step mole of diketene, more or less and may be readily deter can be employed either in a batch or a continuous process. 30 mined by a person having ordinary skill in the art. A The present invention also relates to a method that per preferred quantity of water comprises from about 0.4 to mits recovery of second and third crop crystals of dehy about 0.6 mole per mole of diketene. droacetic acid. Although the inventors do not wish to be limited by Because recycling of mother-liquor is now possible the any theory it is believed that by employing the catalyst present invention also affords certain economic advantages of the present invention a means has been found for in the recovery of residual dehydroacetic acid from a eliminating or Substantially reducing the production of mother-liquor. Prior to the present invention it was not trimers, tetramers and pentamers of diketene according possible to recycle or re-use the mother-liquor containing to the aforementioned reaction. These undesirable by some residual dehydroacetic acid after the first separation, products are obtained when a tertiary amine catalyst is since a relatively greater quantity of trimers, tetramers 40 employed however the present imidazole catalysts al and pentamer impurities would also be recycled thereby though favoring the production of dehydroacetic acid, resulting in the production of acid with succeedingly also tend to minimize or eliminate the formation of higher quantities of impurities i.e. recycled dehydroacetic trimers, tetramers and pentamers. This is especially un acid would contain some partially separated trimers, tetra expected in view of the fact that similar heterocyclic mers and pentamer impurities obtained from the previous nitrogen compounds such as and pyrroles ex recovery resulting in a build-up of impurities in each Sep hibit a much reduced activity or no activity at all in the aration and additionally new impurities would be formed catalytic oligomerization of . in each cycle by the further conversion of the trimer to It may be theorized that dimerization of diketene lead tetramers and pentamer. Stated otherwise, the Subsequent ing to dehydroacetic acid with an amine catalysts pro re-use of mother-liquor containing unrecoverd dehydro ceeds as follows: acetic acid obtained by prior art methods resulted in the 50 production of increasingly impure acid since the ratio of CH -C-O-g o CH--C-o O 1 complex impurities to unrecovered acid increased during the re CH-C=O -} CH-C=O cycle operation and this increased ratio caused greater (e. quantities of tetramers and pentamer to be separated with NR3 NR3 the dehydroacetic acid. These difficulties have been over 55 come by the present invention since the alkylimidazoles diketone eliminates or substantially reduces the formation of the O tetramers and pentamer. N a CH= isomorization CH=C-o? In another aspect of the present invention and as an Ka- 2:1 complex alternate to recycling the mother-liquor it is now for 60 CE gCHCH &H-c=o Olip the foregoing reasons not only possible to collect a first C S CFI-C-O crop of a high-quality product, but also a second and / S -N3 O N CH2C=O sometimes a third crop of good purity can be obtained e (B. simply by evaporation of solvent from the mother-liquor, 65 Y. NR3 when the mother-liquor is not recycled to the reactants. CH-?y-oO The subsequent recovery of second crop and third crop dehydroacetic acid produced by prior art methods which -- NR3 resulted in the production of increasingly impure acids -CII, can now be avoided or minimized since the increase in the 70 O ratio of impurities to unrecovered acid with each separa OH tion has been substantially reduced or eliminated when Thus in the absence of other nucleophiles the 1:1 com dehydroacetic acid is produced using the catalysts of the plex can react with another mole of diketene to pro present invention. duce the 2:1 complex which can cyclize, expel the cat A material which has been produced in either fashion 75 alyst and form dehydroacetic acid, The condensation 8,592,826 5 6 reactions which lead to the undesirable by-product tetra and a hydrocarbon having from about 5 to about 10 mers and pentamers are also catalyzed by bases (bases carbon atoms and comprises from about 30 to about 60 towards protons) as is the diketene dimerization yield parts of ketone to about 40 to about 70 parts of hydro ing dehydroacetic acid. Therefore it may be theorized carbon, preferably from about 35 to about 55 parts of that an ideal catalyst for the manufacture of dehydro ketone to about 45 to about 65 parts of hydrocarbon on acetic acid should be very nucleophilic with respect to a Weight basis. Suitable ketones in this regard include diketene, but weakly basic with regard to the condensa acetone, methyl ethyl ketone, diisobutyl ketone, diiso tion reaction resulting in the formation of tetramers and propyl ketone, methylisobutyl ketone, cyclohexanone, pentamer. The tertiary amine catalysts of the prior art and the like. Suitable hydrocarbons include, n-hexane, catalyze such condensation reactions; however, the imid O dimethylbutane, methylpentane, methylcyclopentane, hep azole heterocyclic nitrogen compounds of the present tane and its isomers, octane and its isomers, toluene, invention are believed to be so weakly basic with re benzene, Xylene, cumene and tetralin and any mixtures spect to these condensation reactions that the tetramers of said ketones and hydrocarbons. Preferred solvents com and pentamer are not formed or are substantially elimi prise a 50:50 mixture of acetone and hexane or a 60:40 nated. The present catalysts also have the advantage that mixture of acetone and toluene on a weight basis including they are strongly nucleophilic with respect to diketene Such mixtures where either component may vary about and favor the formation of dehydroacetic acid. The effect 10%. of the catalyst is also unexpected in view of the fact Selection of a pair or other combination of solvents that one of the products obtained according to the method taken from those specifically enumerated is based on low of the invention, 2,4,6-heptanetrione, is an intermediate 20 cost, Solubility with regard to any impurities in the reac in the formation of the tetramers and pentamer. The tion, poor solubility with regard to dehydroacetic acid, catalyst minimizes formation of these latter compounds good solubility with regard to diketene and generally in and selectively effects the reaction of diketene and water ertness toward diketene. to the trione. Other Solvents inert to diketene may also be useful In summary, the advantages of the invention can be 25 for example, ethyl acetate, isopropyl ether and various obtained on a batch or continuous basis and the build combinations of such solvents. The amount of solvent up of impurities in the reaction mass employing either preferred is about 50 to about 300 parts by volume per method are substantially eliminated or greatly minimized mole of diketene employed. The most preferred amount when compared to the prior art. The quality of a material is about 100 to about 200 parts by volume for each mole produced in this fashion remains high as compared to 30 of diketene used. the case when a mother-liquor obtained from making The preferred temperature range is about 0 to about dehydroacetic acid with an amine catalyst known in 50° C.; the most preferred range is about 15 to about the art is used to make more dehydroacetic acid, whereby 35 C. Since the dimerization reaction is exothermic, it is the quality of the product becomes rapidly poorer. necessary to provide Suitable means to maintain the tem Thus when the catalysts of the present invention are 35 perature in the desirable range. The reaction is conducted employed, the formation of these undesirable impurities satisfactorily under atmospheric pressure; however, sub becomes negligible; a pure product can be isolated from atmospheric and Superatmospheric pressures may be em the reaction mixture simply by crystallization, filtration ployed, the preferred pressure being about 0.1 to about or centrifugation followed by washing with a suitable 0.4 atmosphere where a relatively low boiling solvent is solvent. 40 also employed. The 1-alkylimidazole catalyst generally comprises com When the dimerization is carried out at 25 to 40° C., a pounds represented by the following formula: time of 1 to 5 hours is necessary to complete the reaction. Under the preferred conditions, longer times do not ap III R3 N preciably effect the yields. Good results are obtained when about 0.03 to about NJr. 5.0% by weight of imidazole with respect to diketene and a solvent is employed. A more preferred amount of cata R1 lyst is about 0.1 to about 1% by weight. More catalyst, where R is an alkyl, alkoxyalkyl, hydroxyalkyl, halo although less economical, can be used; less catalyst re alkyl where the halogen is attached to the chains other quires a longer reaction time. than on a or carbon atom, acyloxyalkyl, alkynyl, alkenyl, The following non-limiting examples are given as il or an aralkyl group, containing from one to about twenty lustrations of certain specific embodiments of the in carbon atoms, especially lower alkyl groups of one to vention. about four carbon atoms, and preferably lower alkyl EXAMPLE 1. groups of one to about two carbon atoms. The radicals R. R., and R4 are preferably hydrogen, although they Diketene (186 g., purity 99%) is added dropwise to a can also be represented by the R1 groups as described mixture of toluene (300 ml.) and 1-methylimidazole (1.0 previously, especially the aforementioned alkyl groups g.) at 25 C. over a period of about 2 hours. After all the and preferably the lower alkyl groups. Examples of cat diketene has been added, the reaction mixture is stirred alysts that can be used are 1-methylimidazole, 1-ethyl 60 for 30 min. longer. The mixture is then cooled to 0° C. imidazole, 1-isopropylimidazole, 1-n-propylimidazole, 1 and filtered. The separated product is washed with 200 sec-butylimidazole, 1 - methoxymethylimidazole, 1 - (2- ml. of chilled toluene (0° C.); and then air-dried. The ethylhexyl)imidazole, 1-octylimidazole, 1-hexadecylimid weight is 137 g., M.P. 110-11. C. azole, 1,2-dimethylimidazole, 1,2-diethyl-4-methylimid Concentration of the mother-liquor and the toluene azole, 1,2,5-trimethylimidazole, 2-ethyl-1-methylimidaz wash give additional product (9.3 g), M.P. 108-110° C. ole, 2-ethyl-1,4-dimethylimidazole, and the like. The combined yield is 79% based on diketene. The reaction is preferably carried out in a solvent The process of Example 1 is repeated using trimethyl liquid medium employing times, temperatures and pres amine (1.0 g.) as a catalyst, the reaction conditions and sures or any combination thereof. Sufficient to produce recovery procedure being otherwise the same. The first from diketene, dehydroacetic acid, or from diketene and 70 crop yield of dehydroacetic acid obtained is 76%, M.P. water, 2,6-dimethylpyranone and 2,4,6-heptanetrione. The 108-111° C. The product, when converted into sodium reaction conditions are easily determined by a person dehydroacetate gives an orange colored salt. The sodium having ordinary skill in the art. salt of the dehydroacetic acid obtained using trimethyl As noted before the preferred solvent comprises a mix amine as a catalyst may be used as an illustration to in ture of a ketone having from 3 to about 10 carbon atoms 75 dicate the amount of impurities formed. 3,592,826 7 8 The dehydroacetic acid prepared according to the acid, is recycled to the reactor together with 50 ml. of method of the present invention when dissolved in 8 mil fresh toluene containing 0.6 g. of 1-methylimidazole. A liliters of a solution of 10 grams of sodium hydroxide in further amount of diketene (179 g. 95% purity) is then 100 milliliters of pure redistilled absolute ethanol and added over a period of about 2 hours, maintaining the stirred for 10 to 15 minutes until a salt is formed, yielded temperature at 20 to 25 C. during the addition. a white to cream-white sodium salt. Dehydroacetic acid is recovered as before and the resi The above example illustrates that by employing the dual liquid is recycled to the reactor again with the same process of the present invention a substantially purer de of fresh solvent and catalyst; further amounts of diketene hydroacetic acid is obtained as compared to the dimeriza are then added under the same conditions. This process tion of diketene in the presence of trimethylamine as a IO is repeated 7 times. A total yield of 71% is obtained. The catalyst. product from each run, when converted into sodium de EXAMPLE 2. hydroacetate, yields a creamy white colored sodium salt. Under the same conditions as described in Example 1, The process of Example 7 is carried out with trimethyl when diketene (179 g, 95% purity) is added to a mixture amine (1.g.) as catalyst. The mother-liquor obtained from 5 the first run with trimethylamine is used once again. The of toluene (300 ml.) and 1-ethylimidazole (1.0 g.), 113 overall yield of dehydrodacetic acid is 76%. The product, g. (66%) of a first crop dehydroacetic acid, M.P. 109 when converted into sodium dehydroacetate however gave 111° C., is obtained. an organe colored salt. EXAMPLE 3 The method of Example 7 illustrates that the mother Under similar conditions as in Example 1, when dike 20 liquid obtained from a first run production of dehydroace tene (179 g, 95% purity) is added to a mixture contain tic acid may be recycled to the reaction mass along with ing 1-octylimidazole (2.0 g.) in toluene (300 ml.), 114 added diketene and catalyst without affecting the purity g. (66%) of a first crop dehydroacetic acid, M.P. 109 of the subsequent runs of dehydroacetic acid. This is at 111 C., is obtained. tributable to the fact that high purity dehydroacetic acid It may be seen by comparison of Example 3 with Ex is obtained, i.e. the quantity of impurities produced em amples 1 and 2 that as the alkyl group R1 of Formula ploying the catalyst of the present invention is minimized; III increases in molecular weight the catalytic activity of consequently, recycling the mother-liquor does not result the imidazole for the reaction decreases slightly and con in building up or concentrating the impurities in the reac sequently an increased amount of catalyst, as employed tion mass. in Example 3, has to be used in order to obtain com 30 EXAMPLE 8 parable conversions. The sodium salts of Examples 2 and Under similar conditions as described in Example 1 3 also were prepared according to the method of Example diketene (179 g, 95.5% purity) is added to a mixture of 1 and white to cream-white colored sodium salts of de 1-methylimidazole (1.0 g.) and a binary solvent (300 ml.) hydroacetic acid are obtained. consisting of 50% of acetone and 50% of hexane by EXAMPLE 4 35 weight. After filtration, the solid is washed with 200 ml, of the same solvent mixture chilled to 0° C. One hundred Under the same conditions as in Example 1, when dike forty grams (140 g., 81.5%) of a first crop dehydroacetic tene (179 g., 96% purity) is added to a mixture of acid, M.P. 109-110° C., is obtained. toluene (300 ml.) and 1,2-dimethylimidazole (1 g.), 85 Concentration of the mother-liquor to dryness gives 43 g. (49%) of a first crop dehydroacetic acid, M.P. 108.5- 40 g. of residue, which, upon recrystallization from methanol 111° C., is obtained. yields (10.8 g.) dehydroacetic acid, M.P. 108-110° C. The EXAMPLE 5 combined yield is 88% based on diketene. Under the same conditions as in Example 1, when Example 8 is similar to Example 7, however, the diketene (179 g., 96.7% purity) is added to a mixture mother-liquor is not recycled but subsequently treated to of toluene (300 ml.) and 1,2-diethyl-4-methylimidazole obtain second crop crystals of dehydroacetic acid so as to (2 g.), 62 g. (37%) of a first crop dehydroacetic acid, improve the recovery. The dehydroacetic acid obtained is M.P. 108-11 1 C., is obtained. converted into its sodium salt according to method of Ex Both Examples 4 and 5 when compared to Example ample 1 and a white to cream-white salt is obtained thus 1 also illustrate that 2 and 4 substitution of the imidazole illustrating as in Example 7 that impurities are produced by an alkyl group results in reducing the catalytic activity in either negligible quantities or not at all and are not of the compound. The preferred substituent in these posi carried over to any great extent in the mother-liquor when tions is therefore hydrogen. The sodium salt of the de a second crop yield of crystals is obtained. Example 8 also hydroacetic acid obtained from Examples 4 and 5 was illustrates the effect of the acetone-hexane solvent on the prepared according to the method of Example 1 and white reaction to increase the yields of dehydroacetic acid. to cream-white salts were obtained indicating a compound EXAMPLE 9 having high purity. The method of Example 8 is repeated substituting for EXAMPLE 6 the Solvent a 40:60 (weight basis) acetone:toluene mix The process of Example 1 is repeated using 4 g. of 1 ture and a first crop of dehydroacetic acid is obtained in methylimidazole, the reaction conditions and recovery 60 67% yield. procedure being otherwise the same. The combined yield EXAMPLE 10 of dehydroacetic acid M.P. 108-110° C. is 85% based on The method of Example 8 is repeated, however, 1-iso diketene. propyl-imidazole is substituted for the 1-methylimidazole, The dehydroacetic acid obtained according to the and a first crop of dehydroacetic acid is obtained in 80% method of Example 6 is converted into its sodium salt 65 yield. according to the method of Example 1 and a white to EXAMPLE 11 cream-white salt is obtained. The method of Example 2 is repeated substituting 1-n- EXAMPLE 7 propylimidazole for the 1-ethylimidazole and substan The process of Example 1 is repeated by adding diketene O tially the same results are obtained. (179 g, 95% pure) to a mixture of 1-methylimidazole (1.0 g.) and toluene (300 ml.), and a 66% yield of a EXAMPLE 12 first crop dehydroacetic acid is obtained. After recovering The method of Example 3 is repeated substituting 1-sec the dehydroacetic acid, the residual liquid (250 ml.), con butylimidazole for the 1-octylimidazole and substantially sisting mostly of toluene and unrecovered dehydroacetic the same results are obtained. 3,592,826 9 10 EXAMPLE 13 where R is a one to about 8 carbon atom alkyl or Procedure: 1-methylimidazole (2.0 g.) is dissolved in alkoxyalkyl, R2, R3, and R4 are selected from a member 18 g. of water. Diketene (175 g., 96.7% pure) is added of the group consisting of hydrogen and R1 at from about slowly at 20-30°C. over a period of about 3 hours. After 0 to about 50° C. the addition is completed, the reaction mixture is kept at 2. The method of claim 1 where said imidazole com 30-60° C. for several hours longer until carbon dioxide prises 1-methylimidazole. ceases to evolve. Any low-boiling products formed are 3. The method of claim 1 where said imidazole com removed on a rotary evaporator at 25 C./2 mm. Gas prises 1-ethylimidazole. chromatographic analyses indicate that the reaction mix 4. The method of claim 1 where said imidazole com ture contains 95.0 g. of diacetylacetone (2,4,6-heptanetri O prises 1-octylimidazole. one) in a 67% yield, and 7.3 g. of 2,6 - dimethyl-4- 5. The method of claim 1 where said imidazole com pyranone in a 5.9% yield. prises 1,2-dimethylimidazole. 6. The method of claim 1 where said imidazole com EXAMPLE 1.4 prises 1,2-diethyl-4-methylimidazole. Example 13 is repeated using 1-ethylimidazole as a 7. The method of claim 1 where said imidazole com catalyst and substantially the same results are obtained. prises 1-propylimidazole. 8. The method of claim 1 where said imidazole com EXAMPLE 1.5 prises 1-butylimidazole. Example 13 is repeated using 1,2-diethyl-4-methyl 9. The method of claim where said reaction is con imidazole as a catalyst; again, diacetylacetone and di 20 ducted in the presence of a solvent comprising about 30 methylpyranone are identified as products of the reac to about 60 parts by weight of a ketone having from 3 tion. to about 10 carbon atoms and about 70 to about 40 parts EXAMPLE 16 by weight of a normally liquid hydrocarbon having from Example 13 is repeated using 1-propylimidazole as a about 5 to about 10 carbon atoms. catalyst and substantially the same results obtained. 25 10. The method of claim where said reaction is con When the other specifically enumerated ducted in the presence of acetone and hexane. herein are employed according to the methods of Ex 1. The method of claim 10 where said reaction is con amples 1, 13, 7 and 8, substantially the same results are ducted in the presence of acetone and toluene. obtained. By employing the various reactants, solvents 12. The method of claim 1 where said dimerization is and dimerization conditions noted above substantially 30 conducted continuously in a solvent by separating dehy the same results are obtained as in the foregoing examples droacetic acid from said solvent and recycling said solvent in which imidazole catalysts are used. to said diketene and said catalyst and continuously The dehydroacetic acid obtained has utility as an inter replenishing said diketene and said catalyst at a rate com mediate in biologically active compounds and products. mensurate with the formation of said dehydroacetic acid, Other utilities are given in U.S. Pats, 3,246,001 and 35 and depletion of said catalyst. 3,206,253. The 2,4,6-heptanetrione may be used to manu 13. The method of claim where said dimerization is facture the tetramers and pentamers previously noted, by conducted in a solvent and said dehydroacetic acid is reaction with an amine and diketene. The tetramers and separated therefrom by crystallization and recrystalliza pentamers may be used as indicators in acid base reac tion. tions. The 2,6-dimethylpyranone has biological activity 40 References Cited and may be used in antifungus and antibacterial applica UNITED STATES PATENTS tions. Although the invention has been described by reference 2,229,204 1/1941 Boese ------260-343.5 to one or more embodiments it is not intended that the 2,729,652 1/1956 Nordt ------260-343.5 broad scope of the novel catalyst comprising imidazoles, 45 3,173,954 3/1965 Clark ------260-343.5 especially 1-alkylimidazoles and the method for convert ing diketene to dehydroacetic acid or 2,6-dimethylpyran ALEXMAZEL, Primary Examiner one or 2,4,6-heptanetrione in the presence of such catalysts A. M.T.TIGHE, Assistant Examiner be limited thereby, but that some modifications are 50 intended to be included within the spirit and the broad U.S. C. X.R. scope of the following claims. 260-309, 343.2, 479, 526, 592, 595, 999 What is claimed is: 1. A method for the manufacture of dehydroacetic acid comprising dimerizing diketene in the presence of an 85 imidazole of the structure: R3-i-NU. 99. UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,592,826 Dated July 13, 1971 Inventor(s) E. Marcus, et all It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below: - - In claim 1, change the structural formula to read as follows :

N J. R1 Signed and sealed this 28th day of November l972.

(SEAL) Attest : EDWARD M.FLETCHER, JR. ROBERT GOTESCHALK Attesting Officer Commissioner of Patents L- -