Highly effective mechanism-based inactivation of dopamine .beta.-hydroxylase by a novel ketene thioacetal

Electrophile-Directed Diastereoselective Oxonitrile Alkylations

Diastereoselective alkylation of prochiral oxonitrile dianions with secondary alkyl halides efficiently installs two contiguous stereogenic centers. The confluence of nucleophilic trajectory and the electrophile chirality causes distinct steric differences that allow efficient discrimination for one of the six possible conformers. Numerous oxonitrile-derived dianions efficiently displace secondary alkyl halides propagating the electrophile chirality to efficiently install two contiguous tertiary centers. The prevalence of chiral, secondary electrophiles makes the interdigitated alkylation of chiral electrophiles a particularly attractive route because the resulting oxonitriles are readily transformed into bioactive heterocycles.

The involvement of free radicals in the mechanisms of monooxygenases

The occurrence of free radicals in the mechanisms of monooxygenases reflects the chemistry of dioxygen and the inertness of typical substrates. Thus, oxidation of such substrates requires attack by reduced dioxygen-derived free radicals. Consequently, a molecule of NAD(P)H must be invested for each substrate molecule oxidized. Furthermore, since free radicals are difficult to control, deviations from the intended reaction course are frequent. These considerations are illustrated by examination of the generation and fate of enzyme- and substrate-derived free radicals at various stages in the catalytic cycles of two monooxygenases important in xenobiotic biotransformation, dopamine beta-hydroxylase and cytochrome P-450.