A synthetic approach to poly-gamma-glutamyl analogs of methotrexate

Methotrexate poly-gamma-glutamates bearing two and three glutamate units above that present in methotrexate have been synthesized by extension of a previously described route by which the lower congener bearing one added glutamate unit was synthesized. 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 t-butyloxycarbonyl-L-glutamic acid alpha-benzyl ester (7) followed by selective removal of the t-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. Intermediates were obtained in pure form according to thin-layer chromatography and elemental analysis results, and the final target compounds were obtained in high purity as judged by thin-layer chromatography, high-performance liquid chromatography, 1H NMR and mass spectral data, and elemental analysis results.