The invention provides a process for preparing 3-hydroxy-1-(3-(trifluoromethyl)-5,6-dihydro-[1,2,4]triazolo[4,3-a]pyrazin-7(8H)-yl)-4-(2,4,5-trifluorophenyl)butan-1-one (Formula I), into its racemic (R/S) form or any of its optically active (S) or (R) forms or enantiomeric excess mixture of any of the forms comprising: a) reacting 4-oxo-4-[3-(trifluoromethyl)-5,6-dihydro[1,2,4]triazolo[4,3-a]pyrazin-7(8H)-yl]-1-(2,4,5-trifluorophenyl)butan-2-one of formula (III) with a suitable oxidoreductase enzymes or its suitable variants in the presence of suitable conditions and co-factor; and b) isolating 3-hydroxy-1-(3-(trifluoromethyl)-5,6-dihydro-[1,2,4]triazolo[4,3-a]pyrazin-7(8H)-yl)-4-(2,4,5-trifluorophenyl)butan-1-one, into its racemic (R/S) form or any of its optically active (S) or (R) forms or enantiomeric excess mixture of any of the forms.