Synthesis of Conjugates of N-(Purin-6-yl)-6-aminohexanoic Acid with 4-Amino-1-aryl-5-oxoprolines

(2S,4S)-4-Amino-1-aryl-5-oxoprolines were synthesized by nucleophilic substitution of the bromine atom in dimethyl (2S,4RS)-4-bromo-N-phthaloylglutamate under the action of difluorosubstituted anilines followed by isolation of (2S,4S)-diastereomers and removal of protecting groups; subsequent coupling of their methyl esters with N-(purin-6-yl)-6-aminohexanoic acid and saponification of ester groups led to the formation of target conjugates. The saponification of ester groups was accompanied by minor epimerization.