Synthesis of N-(4-aminobenzoyl)-γ-oligo (L-glutamic acid)s*

N-(4-aminobenzoyl)-γ-oligo (l-glutamic acid)s (6) containing from two to six glutamic residues have been prepared in solution using Nα-Boc-α-Bzl protections and isobutyl-chlorocarbonate activation. Key steps in the synthesis were the coupling of γ-oligo(α-benzyl l-glutamate) benzyl esters (1) with N-(4-benzyl-oxycarbonylaminobenzoyl)-l-glutamic acid α-benzyl ester (4) to blocked precursors of N-(4-aminobenzoyl)-γ-oligo (l-glutamic acid)s (5) and catalytic hydrogenolysis of 5 to 6. Elaboration of the required oligo γ-l-glutamate chains (1) was achieved step by step beginning with the coupling of glutamic acid dibenzylester with N-(t-butoxycarbonyl)-l-glutamic acid α-benzyl ester (2) to 3 followed by selective removal of the Boc from 3 with HCl-dioxane followed by coupling with 2.