Efficient Chemoenzymatic Synthesis of Pelitrexol via Enzymic Differentiation of a Remote Stereocenter

An efficient chemoenzymatic process is described for the synthesis of pelitrexol, a novel GARFT inhibitor. The remoteness of this molecule's stereocenter in the tetrahydropterin moiety from the terminal carbonyl group provided a significant challenge in synthesis. The introduction of an oxalamic ester adjacent to the stereocenter dramatically enhanced an enzyme's enantioselectivity for hydrolysis at the terminal ester, producing the desired S-acid with high optical purity and yield. The recycling of the “wrong” enantiomer is achieved via a dehydrogenation/hydrogenation strategy.