A synthetic approach to poly(gamma-glutamyl) congugates of methotrexate

Methotrexate poly(gamma-L-glutamate)s bearing two and three glutamate units above that present in methotrexate have been synthesized by extension of a previously described route used to synthesize the lower conjugate bearing one added glutamate unit. Key steps in the sequence are the peptide coupling of N-[4-[[(benzyloxy)carbonyl]-methylamino]benzoyl]-L-glutamic acid alpha-benzyl ester (5) with oligo(gamma-L-glutamate) benzyl esters, removal of blocking groups by catalytic hydrogenolysis, and introduction of the (2,4-diamino-6-pteridinyl)methyl grouping by alkylation with 6-(bromomethyl)-2,4-pteridinediamine hydrobromide. Elaboration of the required oligo(gamma-L-glutamate) chain was achieved one unit at a time, beginning with the coupling of L-glutamic acid dibenzyl ester with [(tert-butyloxy)carbonyl]-L-glutamic acid alpha-benzyl ester (7), followed by selective removal of the tert-butyloxycarbonyl grouping and another coupling step with 5 or 7 as required. Diphenylphosphoryl azide was used as the coupling reagent in each conversion producing a peptide linkage.