The present invention relates to optically active heteroaromatic β -hydroxy esters of formula (I) useful in the synthesis of epothilone derivatives, to beta-keto esters of formula (IV) used to produce these intermediates by means of an asymmetric hydrogenation with a chiral metal catalyst or microbiological or enzymatic reduction, as well as to processes for their production (I), wherein: A is a bicyclic heteroaromatic residue of formula (A), wherein “heteroaromatic” stands for a 5-or 6-membered heteroaromatic ring having up to 2 heteroatoms selected from oxygen, nitrogen or sulphur, which is optionally substituted with one or two substituents selected from alkyl, optionally protected hydroxyalkyl, halo-alkyl, halogen or CN, and R is a straight-chain or branched, optionally saturated alkyl chain, which optionally contains 1-3 oxygen atoms; phenyl: cyclohexyl; or benzyl residue (IV).