Improved method of synthesizing 2,2-dimethyl-4,6-dioxo-1,3-dioxane (Meldrum's Acid)

Abstract

In the current report we attempt to study in detail the principal stages of the process, from the synthesis of the starting acid II to its cyclization into Meldnnn's acid, with a view to establishing a production technique that would be viable on an industrial scale. Of the numerous malonic acid synthesis methods [7] only those based on the hydrolysis of malonic acid diethyl ester (III) are acceptable in this respect. Acid hydrolysis is distinctly more viable, since in alkaline processing, malonic acid salts are formed which, because of the acid's high water-solubility, are not readily converted into II on an industrial scale (separation of II from inorganic salts is extremely difficult). In the past, hydrochloric acid [8, 9] was used predominantly to saponify the ester groups of compound III, with sulfuric acid [10] being employed less frequently. The use of either acid is fraught with considerable technological difficulties. For example, aqueous hydrochloric acid (when heated) causes substantial apparatus corrosion, which is very difficult to avoid as it has to be distilled off at the end of the saponification process. Apart from its corrosive effects a major obstacle when using sulfuric acid is the separation of malonic acid from the bottom (comprising a suspension of II in concentrated sulfuric acid) left over after evaporation of water. We were able to overcome all these technological problems by using orthophosphoric acid to saponify the ester groups of compound III. Firstly, it is well-known that normal stainless steels resist the action of orthophosphoric acid solutions over a much wider range of temperature and concentration than they do for hydrochloric or sulfuric acids [1]. Secondly, we found that orthophosphoric acid was readily soluble in malonic ester III, while malonie acid did not dissolve in III, and could be easily filtered off. Because of the properties of acid II and ester III outlined above, we were able to create an almost waste-free, environmentally clean method of synthesizing II based on the saponification of malonic ester in the presence of H3PO 4. It was found that when III had been heated in aqueous H3PO 4, then the water had been evaporated off, the mass had been diluted with ester III, and malonic acid had been filtered off, the mother liquor could be recycled after dilution with water. After six cycles of the operation, the mother liquor is decolorized with carbon and reused in the hydrolysis process. By distilling off and regenerating ethyl alcohol formed at the saponification stage, substantial losses are eliminated and the disposal of waste waters avoided (see Experimental). The cyclic process for synthesizing acid II is shown in schematic form below