A compound according to formula, or salt thereof, wherein R1 and R2 are, independently of each other, hydrogen or a nitrogen protecting group, which is selected from the group consisting of C 1 -C 6 -alkyl, which is mono-, di- or tri-substituted by tri-C 1 -C 6 -alkylsilylC 1 -C 7 -alkoxy; C 6 -C 10 -aryl, or heterocyclyl being a mono-, bi- or tricyclic ring system with 5 to 14 ring atoms and 1 to 4 heteroatoms independently selected from N, O, S, S or S 2, wherein the aryl ring or the heterocyclic group is unsubstituted or substituted by one, two or three residues, selected from the group consisting of C 1 -C 7 -alkyl, hydroxyl, C 1 -C 7 -alkoxy, C 2 -C 8 -alkanoyl-oxy, halogen, nitro, cyano, and CF 3; and C 6 -C 10 -aryl-C 1 -C 2 -alkoxycarbonyl; C 1 -C 10 -alkenyloxycarbonyl; C 1 -C 6 -alkylcarbonyl; C 6 -C 10 -arylcarbonyl; C 1 -C 6 -alkoxycarbonyl; C 6 -C 10 -aryl-C 1- C 6 -alkoxycarbonyl; allyl; cinnamyl; sulfonyl; sulfenyl; succinimidyl, and silyl, wherein each silyl group is a SiR11R12R13 group, wherein R11, R12 and R13 are, independently of each other, C 1 -C 6 -alkyl or C 6 -C 10 -aryl, and R3 is a carboxyl group -COOH or an ester group -COOR, wherein R is selected from the group consisting of C 1 -C 6 -alkyl, C 1- C 6 -alkoxy-C 1- C 6 -alkyl, heterocyclyl being a mono-, bi- or tricyclic ring system with 5 to 14 ring atoms and 1 to 4 heteroatoms independently selected from N, O, S, S or S 2, C 6 -C 10 -aryloxy-C 1- C 6 -alkyl, silyl, cinnamyl, allyl, C 1- C 6 -alkyl which is mono-, di- or trisubstituted by halogen, silyl, cyano or C 6 -C 10 -aryl wherein the aryl ring is unsubstituted or substituted by one, two or three residues selected from the group consisting of C 1- C 7 -alkyl, C 1- C 7 -alkoxy, halogen, nitro, cyano and CF 3; and C 1- C 2 -alkyl substituted by 9-fluorenyl, wherein each silyl group is a SiR11 R12R13 group, wherein R11, R12 and R13 are, independently of each other, C 1 -C 6 -alkyl or C 6 -C 10 -aryl. wherein R1 and R2 are, independently of each other, hydrogen or a nitrogen protecting group, which is selected from the group consisting of C 1 -C 6 -alkyl, which is mono-, di- or tri-substituted by tri-C 1 -C 6 -alkylsilylC 1 -C 7 -alkoxy; C 6 -C 10 -aryl, or heterocyclyl being a mono-, bi- or tricyclic ring system with 5 to 14 ring atoms and 1 to 4 heteroatoms independently selected from N, O, S, S or S 2, wherein the aryl ring or the heterocyclic group is unsubstituted or substituted by one, two or three residues, selected from the group consisting of C 1 -C 7 -alkyl, hydroxyl, C 1 -C 7 -alkoxy, C 2 -C 8 -alkanoyl-oxy, halogen, nitro, cyano, and CF 3; and C 6 -C 10 -aryl-C 1 -C 2 -alkoxycarbonyl; C 1 -C 10 -alkenyloxycarbonyl; C 1 -C 6 -alkylcarbonyl; C 6 -C 10 -arylcarbonyl; C 1 -C 6 -alkoxycarbonyl; C 6 -C 10 -aryl-C 1- C 6 -alkoxycarbonyl; allyl; cinnamyl; sulfonyl; sulfenyl; succinimidyl, and silyl, wherein each silyl group is a SiR11R12R13 group, wherein R11, R12 and R13 are, independently of each other, C 1 -C 6 -alkyl or C 6 -C 10 -aryl, C 1 -C 6 -alkyl, which is mono-, di- or tri-substituted by tri-C 1 -C 6 -alkylsilylC 1 -C 7 -alkoxy; C 6 -C 10 -aryl, or heterocyclyl being a mono-, bi- or tricyclic ring system with 5 to 14 ring atoms and 1 to 4 heteroatoms independently selected from N, O, S, S or S 2, wherein the aryl ring or the heterocyclic group is unsubstituted or substituted by one, two or three residues, selected from the group consisting of C 1 -C 7 -alkyl, hydroxyl, C 1 -C 7 -alkoxy, C 2 -C 8 -alkanoyl-oxy, halogen, nitro, cyano, and CF 3; and C 6 -C 10 -aryl-C 1 -C 2 -alkoxycarbonyl; C 1 -C 10 -alkenyloxycarbonyl; C 1 -C 6 -alkylcarbonyl; C 6 -C 10 -arylcarbonyl; C 1 -C 6 -alkoxycarbonyl; C 6 -C 10 -aryl-C 1- C 6 -alkoxycarbonyl; allyl; cinnamyl; sulfonyl; sulfenyl; succinimidyl, and silyl, wherein each silyl group is a SiR11R12R13 group, wherein R11, R12 and R13 are, independently of each other, C 1 -C 6 -alkyl or C 6 -C 10 -aryl, and R3 is a carboxyl group -COOH or an ester group -COOR, wherein R is selected from the group consisting of C 1 -C 6 -alkyl, C 1- C 6 -alkoxy-C 1- C 6 -alkyl, heterocyclyl being a mono-, bi- or tricyclic ring system with 5 to 14 ring atoms and 1 to 4 heteroatoms independently selected from N, O, S, S or S 2, C 6 -C 10 -aryloxy-C 1- C 6 -alkyl, silyl, cinnamyl, allyl, C 1- C 6 -alkyl which is mono-, di- or trisubstituted by halogen, silyl, cyano or C 6 -C 10 -aryl wherein the aryl ring is unsubstituted or substituted by one, two or three residues selected from the group consisting of C 1- C 7 -alkyl, C 1- C 7 -alkoxy, halogen, nitro, cyano and CF 3; and C 1- C 2 -alkyl substituted by 9-fluorenyl, wherein each silyl group is a SiR11 R12R13 group, wherein R11, R12 and R13 are, independently of each other, C 1 -C 6 -alkyl or C 6 -C 10 -aryl.