Abstract: A process for the separation of at least one isomer from a mixture of isomers of a tetrahydropyran-2-one, having at least two chiral centres which comprises selective reaction of at least one isomer with a reagent catalysed by a hydrolase enzyme whereby at least one isomer is preferentially converted into a distinct chemical species from the other isomers so that it is susceptible of separation by an appropriate chemical or physical separation process in which the tetrahydropyran-2-one is of Formula (1): ##STR1## wherein: Z is --H or a protecting group susceptible of reaction with the reagent under the influence of the enzyme; andY is formyl or protected formyl.