Chemoenzymatic synthesis of enantiomerically enriched diprophylline and xanthinol nicotinate

A concise chemoenzymatic route toward enantiomerically enriched active pharmaceutical ingredients (API) - diprophylline and xanthinol nicotinate - is reported for the first time. The decisive step is an enantioselective lipase-mediated methanolysis of racemic chlorohydrin-synthon acetate, namely 1-chloro-3-(1,3-dimethyl-2,6-dioxo-1,2,3,6-tetrahydro-7H-purin-7-yl)propan-2-yl acetate, performed under kinetically-controlled conditions on a preparative 500 mg-scale. The best results in terms of reaction enantioselectivity (E = 14) were obtained for the enantiomers resolution performed with lipase type B from Candida antarctica immobilized on acrylic resin (CAL-B, Novozym 435) suspended in homophasic acetonitrile-methanol mixture. The elaborated biocatalytic system furnished the key chlorohydrin intermediate (in 71% ee and 38% yield), which was then smoothly converted into enantioenriched active agents: (R)-(-)-diprophylline (57% ee) and (S)-(+)-xanthinol nicotinate (65% ee). To support the assignment of absolute configurations of EKR-products as well as to confirm the stereochemical outcome of the remaining reaction steps, docking studies toward the prediction of enantiomers binding selectivity in CAL-B active site as well as the respective chemical correlations with enantiomerically enriched analytical standards obtained from commercially available (R)-(-)-epichlorohydrin, were applied. In addition, single-crystal X-ray diffraction (XRD) analyses were performed for the synthesized optically active APIs furnishing by this manner a first crystal structures of nicotinic acid salt of xanthinol.