Folate analogues altered in the C9-N10 bridge region. 10-Oxafolic acid and 10-oxaaminopterin.

The unambiguous synthesis of two folate analogues, in which the 10-amino group of folic acid was replaced with oxygen, is described. The synthetic sequence employed commercially available methyl p-hydroxybenzoate and n-(2,3-epoxypropyl)phthalimide as starting materials. The use of cesium bicarbonate as a coreactant in the nucleophilic displacement reaction between bromo ketone 3 and the nucleophile 4 was found to be unique in character. The aminoacetonyl oxime 7 obtained by the hydrazinolysis of 6 was used as a common intermediate for the synthesis of both compounds. The generality of the use of the TFA-HCL mixture to deprotect the carbonyl group of both 10 and 12 reductions involving sodium hydrosulfite in aqueous dmf were further substantiated by conversions of 11 and 13 to 14 and 15 quickly and efficiently without employing catalytic hydrogenations. Subsequent cyclizations, oxidations, and hydrolysis of these reduction products to the pteroate analogues 17 and 19 were carried out efficiently as described for the synthesis of the sulfur analogues. Activation of the carboxyl group of 19 by way of the mixed anhydride 22 and subsequent coupling to glutamic acid was carried out using the solid-phase coupling procedure. However, compound 17 required trifluoroacetylation to 20 prior to the coupling reaction due to solubility problems. Both 10-oxafolic acid (1) and 10-oxaaminopterin (2) showed potent antifolate activity when tested against two folate-requiring organisms. Compound 2 was a very powerful inhibitor of DCM-resistant lactobacillus casei dihydrofolate reductase. The activity was comparable to that of methotrexate while the 4-hydroxy analogue did not show inhibition. 7,8-Dihydro-10-oxafolic acid failed to show any substrate activity to this enzyme and did not inhibit the enzymatic reaction when used with an equimolar concentration of the natural substrate.