A process for preparing a compound of formula, or a salt thereof, preferably wherein the compound of formula is of formula (2-a) or salt thereof comprising the steps (a) reacting a compound of formula, or salt thereof wherein X is halogen, preferably chloro, or -O-R5, wherein R5 is C 1 -C 6 -alkyl, preferably tert -butyl; preferably wherein the compound of formula is of formula (3-a) or salt thereof wherein X is halogen, preferably chloro, or -O-R5, wherein R5 is C 1 -C 6 -alkyl, preferably tert -butyl; with an imide or azide nitrogen nucleophile under Mitsunobu conditions; (b) (i) conversion of the resulting imide intermediate compound of formula or salt thereof, wherein X is halogen, preferably chloro, or -O-R5, wherein R5 is C 1 -C 6 -alkyl, preferably tert -butyl, and R3 is an imide, preferably wherein the compound of formula is of formula (3-1-a) or salt thereof, wherein X is halogen, preferably chloro, or -O-R5, wherein R5 is C 1 -C 6 -alkyl, preferably tert -butyl, and R3 is imide, by hydrolysis or by treatment with hydrazine to obtain a compound of formula or salt thereof, preferably of formula (2-a) or salt thereof, or (ii) reduction of the resulting intermediate azide compound of formula or salt thereof, wherein X is halogen, preferably chloro, or -O-R5, wherein R5 is C 1 -C 6 -alkyl, preferably tert -butyl, and R3 is azide, preferably wherein the compound of formula is of formula (3-1-a) or salt thereof, wherein X is halogen, preferably chloro, or -O-R5, wherein R5 is C 1 -C 6 -alkyl, preferably tert -butyl, and R3 is azide, to obtain a compound of formula or salt thereof, preferably of formula (2-a) or salt thereof. (a) reacting a compound of formula, or salt thereof wherein X is halogen, preferably chloro, or -O-R5, wherein R5 is C 1 -C 6 -alkyl, preferably tert -butyl; preferably wherein the compound of formula is of formula (3-a) or salt thereof wherein X is halogen, preferably chloro, or -O-R5, wherein R5 is C 1 -C 6 -alkyl, preferably tert -butyl; with an imide or azide nitrogen nucleophile under Mitsunobu conditions; (b) (i) conversion of the resulting imide intermediate compound of formula or salt thereof, wherein X is halogen, preferably chloro, or -O-R5, wherein R5 is C 1 -C 6 -alkyl, preferably tert -butyl, and R3 is an imide, preferably wherein the compound of formula is of formula (3-1-a) or salt thereof, wherein X is halogen, preferably chloro, or -O-R5, wherein R5 is C 1 -C 6 -alkyl, preferably tert -butyl, and R3 is imide, by hydrolysis or by treatment with hydrazine to obtain a compound of formula or salt thereof, preferably of formula (2-a) or salt thereof, or (ii) reduction of the resulting intermediate azide compound of formula or salt thereof, wherein X is halogen, preferably chloro, or -O-R5, wherein R5 is C 1 -C 6 -alkyl, preferably tert -butyl, and R3 is azide, preferably wherein the compound of formula is of formula (3-1-a) or salt thereof, wherein X is halogen, preferably chloro, or -O-R5, wherein R5 is C 1 -C 6 -alkyl, preferably tert -butyl, and R3 is azide, to obtain a compound of formula or salt thereof, preferably of formula (2-a) or salt thereof. (i) conversion of the resulting imide intermediate compound of formula or salt thereof, wherein X is halogen, preferably chloro, or -O-R5, wherein R5 is C 1 -C 6 -alkyl, preferably tert -butyl, and R3 is an imide, preferably wherein the compound of formula is of formula (3-1-a) or salt thereof, wherein X is halogen, preferably chloro, or -O-R5, wherein R5 is C 1 -C 6 -alkyl, preferably tert -butyl, and R3 is imide, by hydrolysis or by treatment with hydrazine to obtain a compound of formula or salt thereof, preferably of formula (2-a) or salt thereof, or (ii) reduction of the resulting intermediate azide compound of formula or salt thereof, wherein X is halogen, preferably chloro, or -O-R5, wherein R5 is C 1 -C 6 -alkyl, preferably tert -butyl, and R3 is azide, preferably wherein the compound of formula is of formula (3-1-a) or salt thereof, wherein X is halogen, preferably chloro, or -O-R5, wherein R5 is C 1 -C 6 -alkyl, preferably tert -butyl, and R3 is azide, to obtain a compound of formula or salt thereof, preferably of formula (2-a) or salt thereof.