Products formed in the radiation synthesis of 5-oxo-tetrahydrofuran-3-carboxylic acid derivatives

After the radiation-induced addition of 2-propanol to the diesters of cis-2-butenedioic (maleic) acid at 20 °C the reaction mixture contains not only the expected lactone-type products, i.e., the derivatives of 2,2-dimethyl-5-oxotetrahydrofuran-3-carboxylic acid (II) but also the corresponding hydroxy derivatives, i.e., the derivatives of 3-carboxy-4-methyl-4-hydroxypentanoic acid (IV). These hydroxy derivatives are formed as the primary products, however, they are not stable in the solution even in the form of their diesters. Their lactonization accompanied by the splitting off of alcohols proceeds rather slowly. The lactonization rate constant of the di-isopropyl ester of the acid IV is 1.61 . 10-5 s-1, of the di-n-octyl ester of IV it is 7.41 . 10-6 s-1 at 25 °C. At a higher temperature, e.g., during the esterification, the hydroxy derivatives of IV are partly lactonized, partly they form the respective unsaturated diesters by splitting off a molecule of water. A series of new esters of the acid II and of the triesters of its 4-(1,2-dicarboxyethyl) derivative (telomer 1 : 2) are described and the wavenumbers of the characteristic lactone bands in the region 1 777 - 1 786 cm-1 in the IR spectra of both types of lactone-esters are given.