Quinazoline antifolates inhibiting thymidylate synthase: synthesis of four oligo(L-.gamma.-glutamyl) conjugates of N10-propargyl-5,8-dideazafolic acid and their enzyme inhibition

The synthesis is described of four oligo(gamma-glutamyl) conjugates of N10-propargyl-5,8-dideazafolic acid containing a total of two, three, four, and five L-glutamic acid residues. The tert-butyl group was chosen as the carboxyl protecting group in order to obviate the use of alkali and thus the possibility of gamma---alpha transpeptidation. The starting material, di-tert-butyl glutamate, was coupled to N-(benzyloxycarbonyl)-L-glutamic acid alpha-tert-butyl ester via a mixed anhydride with isobutyl chloroformate. Hydrogenolysis of the benzyloxycarbonyl group in the product gave a carboxyl-protected diglutamate, which either was acylated with 4-[(benzyloxycarbonyl)amino] benzoyl chloride to give a protected aminobenzamide or was cycled further by using the above mixed anhydride/hydrogenolysis sequence into tri-, tetra-, and pentaglutamates. Each of the last named was also acylated, as above, to give a benzamide. The benzyloxycarbonyl group in the benzamides was removed by hydrogenolysis and the amino groups thus exposed were N-alkylated with propargyl bromide. The resulting proparglyamines were further alkylated with 2-amino-6-(bromomethyl)-4-hydroxyquinazoline hydrobromide to give the antifolate poly(t-Bu) esters. Deprotection with trifluoroacetic acid in the final step delivered the desired antifolates as their trifluoroacetate salts. The di- to pentaglutamates were, respectively, 31-, 97-, 171-, and 167-fold more inhibitory to WI-L2 human thymidylate synthase than the parent compound.