Synthesis of Cyclic Pyranopterin Monophosphate, a Biosynthetic Intermediate in the Molybdenum Cofactor Pathway

Cyclic pyranopterin rnonophosphate (1), isolated from bacterial culture, has previously been shown to be effective in restoring normal function of molybdenum enzymes in molybdenum cofactor (MoCo)-deficient mice and human patients. Described here is a synthesis of 1 hydrobromide (1 center dot HBr) employing in the key step a Viscontini reaction between 2,5,6-triamino-3,4-dihydropyrimidin-4-one dihydrochloride and D-galactose phenylhydrazone to give the pyranopterin (5aS,6R,7R,8R,9aR)-2-amino-6,7-dihydroxy-8-(hydroxymethyl)-3H,4H,5H,5aH,6H,7H,8H,9aH,10H-pyrano-[3,2-g]pteridin-4-one (10) and establishing all four stereocenters found in 1 Compound 10, characterized spectroscopically and by X-ray crystallography, was transformed through a selectively protected tri-tert-butoxycarbonylamino intermediate into a highly crystalline tetracydic phosphate ester (15). The latter underwent a Swern oxidation and then deprotection to give 1 center dot HBr. Synthesized 1 center dot HBr had in vitro efficacy comparable to that of 1 of bacterial origin as demonstrated by its enzymatic conversion into mature MoCo and subsequent reconstitution of MoCo-free human sulfite oxidase-molybdenum domain yielding a fully active enzyme. The described synthesis has the potential for scale up.