No abstract

Asymmetric epoxidation of olefins with 30 % H2O2 in the presence of [Ru(pybox)(pydic)] 1 and [Ru(pyboxazine)(pydic)] 2 has been studied in detail (pybox = pyridine-2,6-bisoxazoline, pyboxazine = pyridine-2,6-bisoxazine, pydic = 2,6-pyridinedicarboxylate). 35 Ruthenium complexes with sterically and electronically different substituents have been tested in environmentally benign epoxidation reactions. Mono-, 1,1-di-, cis- and trans-1,2-di-, tri-, and tetra-substituted aromatic olefins with versatile functional groups can be epoxidized with this type of catalyst in good to excellent yields (up to 100 %) with moderate to good enantioselectivies (up to 84 % ee). Additive and solvent effects as well as the relative rate of reaction with different catalysts have been established. It is shown that the presence of weak organic acids or an electron-withdrawing group on the catalyst increases the reactivity. New insights on the reaction intermediates and reaction pathway of the ruthenium-catalyzed epoxidation are proposed on the basis of density functional theory calculation and experiments.

Colombian corals of the species Pseudopterogorgia elisabethae produce the title compound, colombiasin A (1). This structurally novel, biologically active tetracyclic compound has now been synthesized for the first time in racemic form. Preliminary studies toward the asymmetric total synthesis of both enantiomers indicate that the determination of the absolute stereochemistry can be expected soon.

No abstract

Novel pyboxazines and the known pybox ligands are used in the ruthenium‐catalyzed asymmetric epoxidation of olefins with H2O2 (see scheme). This new catalytic system is successful in the conversion of differently substituted aromatic olefins and gives ee values of up to 84 %.