novel compounds having the formula : ## str1 ## or mixtures thereof , wherein r 1 and r 2 are c 1 - 20 hydrocarbyl groups which may be the same or different , or one of r 1 and r 2 may be hydrogen , and r 3 and r 4 are methyl or hydrogen , and may be the same or different , can be prepared by the reaction of an alkyl ester , preferably the methyl ester , of polymethylmuconic acid with 2 pssh , the reaction product of ethanol and phosphorus pentasulfide .