The invention relates to a process for preparing optically active 5-hydroxy-3-ketoesters of formula A1 or A2, ##STR1##

or one of the tautomers thereof,

the use thereof for preparing optically active dihydropyrones of formula B, ##STR2##

and the use of the dihydropyrones of formula B thus prepared as starting compounds for preparing pharmaceutically active compounds, particularly Tipranavir.