Enzyme catalyzed oxido-reductive reactions : synthetic applications and mechanistic studies

Abstract

Chapter II describes the synthetic application of a non-heme monooxygenase from Pseudomonas oleovorans (POM) toward the preparation of a series of epoxides from their protected allylic alcohol precursors. Like other monooxygenases, this enzyme system preferred hydrophobic substrates to hydrophilic substrates. The enantiomeric excess (ee) of the epoxide products depended on the shape of the substrates. The best ee value was found when the free hydroxy group of the allylic alcohol was protected with p-substituted phenyl or p-substituted benzyl group. Chapter III describes a mechanistic study of monooxygenases and their models. Several hypersensitive probes were designed for this purpose. A radical mechanism was proposed for the hydroxylation of a hypersensitive probe, fraras-2-phenyl-lmethylcyclopropane, catalyzed by POM since the only product obtained from the reaction was the ring-opened product. It was further supported by the fact that large intermolecular and intramolecular deuterium isotope effects are observed when deuterated probes were used as the substrates. In contrast, a concerted mechanism was proposed for the epoxidation of another hypersensitive probe, mzns-2-phenyl-lvinylcyclopropane, by two non-enzymic monooxygenase models because the epoxidation only occurred at the terminal vinyl group without the detection of any ring-opened products. Chapter IV describes utilizations of chloroperoxidase from Caldariomyces fumago in the synthesis of some highly optically pure sulfoxides, alcohols, and hydroperoxides. The stability study of this enzyme in organic solvent was also carried out. K[m] and V[max] of the substrates were determined and used as guidance for synthetic scale reactions to prevent the occurrence of the undesired nonenzymatic reaction. Chapter V describes the isolation, characterization, substrate specificity study and synthetic application of a new NADH dependent alcohol dehydrogenase from Pseudomonas sp. This unique enzyme catalyzes the reduction of ketones in an anti-Prelog's rule fashion giving R alcohols. The enzyme was relatively stable even in organic solvent at high concentration. In a synthetic scale reaction catalyzed by PED alcohol dehydrogenase, 2-propanol can be used for two purposes, the regeneration of NADH and as organic cosolvent.