The present invention refers to a process for preparing a compound of formula (I) or a pharmaceutically acceptable salt thereof: wherein R is (C 3 -C 10 )alkyl, or ω-trifluoro(C 3 -C 10 )alkyl; R 1 and R 2 are, independently, hydrogen, hydroxy, (C 1 -C 8 ) alkoxy, (C 1 -C 8 ) alkylthio, halo, trifluoromethyl or 2,2,2-trifluoroethyl; or one of R 1 and R 2 is in ortho position to the R-O- group and, taken together with the same R-O-, represents a Formula (A) group where R o is (C 2 -C 9 )alkyl; R 3 and R 4 are, independently, hydrogen, (C 1 -C 4 )alkyl; or R 4 is hydrogen and R 5 is a group selected from -CH 2 -OH, -CH 2 -O-(C 1 -C 6 )alkyl, -CH(CH 3 )-OH, -(CH 2 ) 2 -S-CH 3 , benzyl and 4-hydroxybenzyl; or R 4 and R 5 , taken together with the adjacent carbon atom, form a (C 3 -C 6 )cycloalkyl residue; R 5 and R 6 are independently hydrogen or (C 1 -C 6 )alkyl; or taken together with the adjacent nitrogen atom form a 5-6 membered monocyclic saturated heterocycle, optionally containing one additional heteroatom chosen among -O-, -S- and -NR 7 - where R 7 is hydrogen or (C 1 -C 6 ) alkyl; and wherein optionally one or more hydrogen atom in the groups R, R 1 , R 2 , R 3 , R 4 , R 5 and R 6 , preferably in the R group, can be substituted by a deuterium atom.