MECHANISM OF ORGANIC REACTIONS: III. THE NATURE OF THE MECHANISM OF MIGRATION OF THE ACYL RADICAL

Abstract

The mechanism first proposed by Emil Fischer to explain the migration of acyl radicals in the case of glycerides of fatty acids receives strong confirmatory support in the isolation by the authors of a product analogous to the intermediate dioxolane derivative postulated by him. From the theoretical consideration it seemed highly probable that with any glycol monoester containing a carbonyl group of very pronounced negative polarity, the tendency to undergo cyclisation would be so pronounced as to permit of the isolation of the corresponding dioxolane ring.It is shown that all attempts to synthesize glycol monotrichloroacetate gave, instead, the ring isomer, namely, 2-hydroxy-2′-trichloromethyl-1,3-dioxolane. The latter is quite stable at temperatures below 90 °C., but at 100–110 °C. decomposes into ethylene carbonate, chloroform, and a certain amount of higher-boiling, unidentified products. In presence of a trace of pyridine the decomposition into chloroform and ethylene carbonate takes place smoothly and apparently quantitatively. Similarly, when ethylene oxide is treated with dichloroacetic acid, 2-hydroxy-2′-dichloromethyl-1,3-dioxolane is produced. On the other hand, acetic and monochloroacetic acids react to give the normal half-esters. Epichlorohydrin and glycidol on treatment with trichloroacetic acid give the corresponding ring compounds, namely 2-hydroxy-2′-trichloromethyl-4-chloromethyl-1,3-dioxolane and 2-hydroxy-2′-trichloromethyl-4-hydroxymethyl-1,3-dioxolane, respectively. The last-named compound thus represents the cyclic derivative postulated by Fischer as the intermediate product formed in acyl migration occurring in the case of glycerol derivatives. Trimethylene glycol treated with trichloroacetyl chloride gives 2-hydroxy-2′-trichloromethyl-metadioxane. They are all mobile, pleasant-smelling, transparent liquids which decompose when heated at ordinary pressure.The reaction of acetic, mono-, di-, and trichloroacetic acids on 1,2-anhydromannopyranose and 1,2-triacetyl mannopyranose is under investigation. Preliminary work appears to indicate the formation of the corresponding 1,3-dioxolane derivative of mannose in each case. The conclusion is drawn that all organic monoesters of polyvalent alcohols, carbohydrates, polysaccharides, o-amino phenols, etc., containing a free hydroxyl group, the hydrogen atom of which is spatially in close proximity to the carbonyl group of the acyl radical, tend to pass over into a ring isomer. Theoretically, at least, there must exist in all of these cases an equilibrium between the open-chain and its corresponding cyclic isomer.Investigations of the structure of the monoacyl derivatives of glycerol, of carbohydrates, polysaccharides, amino phenols, etc., should permit of the isolation, in many cases, of the intermediate dioxolane or other ring concerned in the migration of the acyl radical. The existence is postulated of a definite equilibrium between the open-chain ester and the corresponding ring isomer in all monoesters of poly-alcohols in which spatial proximity of a hydroxyl to the carbonyl group exists, and the prediction is made of the probable isolation of many new cyclic isomers under investigation when appropriate consideration is given to their physical and chemical properties, such as stability, behavior towards acids, alkalies, etc.