Alternative procedures to obtain compounds of formula V consists in alkylating the appropriate monosubstituted piperazine derivatives with a compound with the formula X--CH(R1)(CH2)n-1 CH2 --OPrG or X--(CH2)n CH(R1)--X where X is a leaving group and n has the same meaning as above, and PrG is a protecting group (e.g. O-tetrahydropyranyl), which can be removed after alkylation of the piperazine.