l-Glutamic Acid and l-Lysine as Useful Building Blocks for the Preparation of Bifunctional DTPA-like Ligands

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Abstract
Bisalkylation of suitably protected L-glutamic acid and L-lysine derivatives with tert-butyl N-(2-bromoethyl)iminodiacetate 2, followed by deprotection of the omega functional group affords N, N-bis[2-[bis[2-(1, 1-dimethylethoxy)-2-oxoethyl]amino]ethyl]-L-glutamic acid 1-(1, 1-dimethylethyl) ester 4 and N2,N2-bis[2-[bis[2-(1, 1-dimethylethoxy)-2-oxoethyl]amino]ethyl]-L-lysine 1,1-dimethylethyl ester 7. Such compounds feature a carboxylic or an amino group, respectively, which are available for conjugation with a suitable partner via formation of an amide bond. The conjugates, which can be prepared in this way, contain a chelating subunit in which all five acetic residues of DTPA are available for the complexation of metal ions. Direct bisalkylation of glycine with 2 promptly gives N, N-bis[2-[bis[2-(1,1-dimethylethoxy)-2-oxoethyl]amino]ethyl]glycine 11. The latter allows to achieve conjugates in which the central acetic group of DTPA is selectively converted into an acetamide.