Dehydroacetic acid

Anhydride manufactured by acetic acid pyrolysis sometimes contains ketene polymers, eg, acetylacetone, diketene, dehydroacetic acid, and particulate carbon, or soot, is occasionally encountered. Polymers of aHene, or its equilibrium mixture, methylacetylene—aHene, are reactive and refractory impurities, which if exposed to air, slowly autoxidize to dangerous peroxidic compounds. 

The higher, long-chain dimers as weH as the tetramer dehydroacetic acid are far more stable and can be handled safely. The alkylketene dimers (AKDs) are shipped to the paper industry in tank tmcks in the form of ready-to-use aqueous emulsions with a total soHds content of 12—25% and a guaranteed shelf life of 30 days, as they have good hydrolytic stabHity. In this form they are not combustible Hquids, and are Hsted in the Canadian Domestic Substances List. 

Pyrans and related compounds react with ammonia to give pyridines. A commercially useful example is the reaction of dehydroacetic acid (derived from diketene) with ammonia to give 2,6-dimethyl-4-pyridinone [7516-31 -6] via 2,6-dimethyl-4-pyridinone-3-carboxyhc acid [52403-25-5]. Chlorination of the pyridone gives clopidol [2971-90-6] (56), a coccidiostat (72,73). 

DEHYDROACETIC ACID Incompatible with cationics and some nonionics... 

The pH of the finished product may have a strong influence on the type of preservative used. A good example of this can be seen with the use of organic acids which may exist in a predominantly dissociated or an undissociated form as a consequence of the product pH. The undissociated form is considered to confer the antimicrobial activity and the effect of pH on benzoic, sorbic and dehydroacetic acid is described in the graph below. It can be seen that, at the normal pH of most personal care products ie. 5.5 to 7.0, there is little activity remaining. Hence organic acids would be suitable preservatives for predominantly acidic products, such as astringent washes made with lemons. 

Diketene residues in a tank trailer awaiting incineration decomposed violently on standing, blew off the dome cover and ignited [1]. The risk of autoignition during the exothermic dimerisation of diketene to dehydroacetic acid is eliminated by operating in a non-flammable solvent [2], It produces a moderate pressure rise on exothermic decomposition at 125°C [3],... 

In the 250-cc. flask 100 g. (0.78 mole) of freshly vacuum-distilled ethyl acetoacetate and 0.05 g. of sodium bicarbonate (Note 3) are placed and heated so that the toluene is just kept boiling until the liquid in the flask has reached 200-2 io° (Note 4). The time for heating is usually seven to eight hours, during which period 27 g. of distillate boiling at 720 (mostly alcohol) is collected, and the color of the reaction mixture becomes dark brown. The dehydroacetic acid, while still hot (Note 5), is transferred to a 200-cc. distilling flask (Note 6) and distilled vn vacuo. After a... 

Dehydroacetic acid has been prepared by the action of acetic... 

Degradation, 14 Dehydration, 48, 74 Dehydroacetic acid, 26 Diacetylmethane, 6 Diazotization, 16, 73... 

The frustrating and sometimes capricious nature of simple reactions of 2-amino-4(5H)-oxazolones is exemplified in the following reports. Ramsh and co-workers acetylated 68 with acetyl chloride in benzene but isolated both a poor yield and a poor mass balance of 131 together with dehydroacetic acid 132. Attempts to transaminate 131 with diethylamine in the absence of a solvent or with aniline in benzene failed. The authors recovered 68, which is in stark contrast to an earlier report from Hansen and Masch that acetylation of 68 with acetic anhydride gave 131 in 87% yield. In addition, these same authors reported that reaction of 131 with diethylamine at room temperature gave 134 in excellent yield (Scheme 6.35). 

The only four- - six-membered ring interconversions of any real synthetic significance are those involving diketene. Base-catalyzed dimerization of diketene is a long-established and efficient method for the preparation of dehydroacetic acid (equation 161), while mild treatment with water in the presence of tertiary amine bases gives 2,6-dimethyl-4-pyrone (equation 162). 1,3-Dioxins are obtained from the acid-catalyzed condensation of diketene with ketones (equation 163). 

Bromo-2-pyrones and 3-bromocoumarins give furan- and benzofuran-2-carboxylic acids by ring fission and subsequent closure, e.g. (228) — (229) (116) or (117) — (230). Pyrone rings are opened by aqueous acid in some cases, probably by successive protonation and attack of a water molecule, e.g. dehydroacetic acid (231) gives (233) which immediately forms (232) or (234) with HC1 or H2S04, respectively. 

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