Process for the preparation of the compound of formula or R or S optical isomer: wherein R 2 is hydrogen, methyl or ethyl, comprising the following steps (a) and (b), or step (a-bis) and (b): (a) reaction of the compound of formula: wherein R 1 is C 1-4 linear or branched alkyl, with a compound of formula: wherein R 2 is hydrogen, methyl or ethyl, and wherein X is chosen between hydroxyl, halogen or any group used to activate the carboxyl function; for providing the compound of formula or salt thereof: wherein R 2 is hydrogen, methyl or ethyl, and wherein R 1 is C 1-4 linear or branched alkyl; or (a-bis) reaction of the compound of formula: wherein R 1 is C 1-4 linear or branched alkyl, with a compound of formula: wherein R 2 is hydrogen, methyl or ethyl; for providing the compound of formula or salt thereof: wherein R 2 is hydrogen, methyl or ethyl, and wherein R 1 is C 1-4 linear or branched alkyl; (b) cyclization of the compound of formula obtained in step (a) or (a-bis) to give the compound of formula: wherein R 2 is hydrogen, methyl or ethyl; comprising the previous steps for the preparation of the compound of formula: wherein R 1 is C 1-4 linear or branched alkyl, said previous steps being the following steps: a-1) reaction of the compound of formula: wherein R 1 is C 1-4 linear or branched alkyl, with a compound of formula: wherein X 1 and X 2 are independently an halogen, mesilate, tosilate, besilate or triflate; to give the compound of formula: wherein R 1 is C 1-4 linear or branched alkyl; b-1) deprotaction of the compound of formula: obtained in step a-1) to give the compound of formula: wherein R 1 is C 1-4 linear or branched alkyl. (a) reaction of the compound of formula: wherein R 1 is C 1-4 linear or branched alkyl, with a compound of formula: wherein R 2 is hydrogen, methyl or ethyl, and wherein X is chosen between hydroxyl, halogen or any group used to activate the carboxyl function; for providing the compound of formula or salt thereof: wherein R 2 is hydrogen, methyl or ethyl, and wherein R 1 is C 1-4 linear or branched alkyl; or (a-bis) reaction of the compound of formula: wherein R 1 is C 1-4 linear or branched alkyl, with a compound of formula: wherein R 2 is hydrogen, methyl or ethyl; for providing the compound of formula or salt thereof: wherein R 2 is hydrogen, methyl or ethyl, and wherein R 1 is C 1-4 linear or branched alkyl; (b) cyclization of the compound of formula obtained in step (a) or (a-bis) to give the compound of formula: wherein R 2 is hydrogen, methyl or ethyl; comprising the previous steps for the preparation of the compound of formula: wherein R 1 is C 1-4 linear or branched alkyl, said previous steps being the following steps: a-1) reaction of the compound of formula: wherein R 1 is C 1-4 linear or branched alkyl, with a compound of formula: wherein X 1 and X 2 are independently an halogen, mesilate, tosilate, besilate or triflate; to give the compound of formula: wherein R 1 is C 1-4 linear or branched alkyl; b-1) deprotaction of the compound of formula: obtained in step a-1) to give the compound of formula: wherein R 1 is C 1-4 linear or branched alkyl. a-1) reaction of the compound of formula: wherein R 1 is C 1-4 linear or branched alkyl, with a compound of formula: wherein X 1 and X 2 are independently an halogen, mesilate, tosilate, besilate or triflate; to give the compound of formula: wherein R 1 is C 1-4 linear or branched alkyl; b-1) deprotaction of the compound of formula: obtained in step a-1) to give the compound of formula: wherein R 1 is C 1-4 linear or branched alkyl.