A process for preparation of 4-hydroxy-pyran-2-one derivative of formula (I), wherein R is, and wherein R1 and R2 are methyl and R3 is hydrogen or methyl, comprising the steps of, heating a compound of formula (II), wherein R is as defined before, and R4 is hydrogen, NH4+ or an alkali metal, in a solvent mixture consisting of an aromatic hydrocarbon and a ketone in an inert atmosphere at a temperature of between 60° C. to 92° C. in the absence or presence of orthophosphoric acid or its alkali dihydrogen salts or alkali hydrogen salts of a dibasic acid, followed by optional neutralization of the reaction mixture with an organic base and obtaining compound of formula (I) in high purity and substantially free of impurities through a step of isolation and crystallization. The process leads to formation of derivatives of formula I in high purity with dimmer impurity (III) less than 0.1% and anhydro impurity (IV) below 0.15%.