Nitrones are important compounds and are highly useful in many aspects. The first part describes the methods for synthesis of nitrones, which are useful and environmentally friendly. Catalytic oxidations, condensations, and other useful reactions are described. The nitrones thus obtained are key intermediates for the synthesis of biologically important nitrogen compounds. The second part describes the fundamental transformations of nitrones, which will provide the strategies and means for the construction of nitrogen compounds. The reactions with nucleophiles or radicals, C–H functionalization, and various addition reactions are described. The last reactions are particularly important for highly selective carboncarbon bond formations. 1,3-Dipolar cycloaddition reactions are excluded because the size of the review is limited and excellent reviews have been published in Chemical Reviews.

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Novel biomimetic, aerobic oxidation with an organocatalyst was performed. The oxidations of organic substrates such as sulfides, secondary amines, N-hydroxylamines, and tertiary amines with molecular oxygen (1 atm) or even in air in the presence of 5-ethyl-3-methyllumiflavinium perchlorate catalyst and hydrazine monohydrate in 2,2,2-trifluoroethanol occur highly efficiently to give the corresponding oxidized compounds in excellent yields along with water and molecular nitrogen, which are environmentally benign. The TON of the oxidation of sulfides amounts to 19 000.

Metal-free organocatalytic reactions, especially enantioselective ones, have attracted increasing attention as a complement to metal-catalyzed and enzyme-catalyzed reactions. [1] Organocatalytic reactions have several advantages, for exam-ple, the availability of structural diversity of the catalysts in optically pure form and their stability under aerobic and aqueous conditions, and the catalysts are often more stable than enzymes.In 1989 we demonstrated that 5-alkylated flavins can be used as organocatalysts for oxidations based on the precise kinetic study on the recycling step of flavoenzymes. [2] Thus, the flavin-catalyzed biomimetic oxidations of sulfides and amines with hydrogen peroxide occurs highly efficiently to give the corresponding sulfoxides and nitrones, respectively. A novel method for enantioselective oxidation with organocatalysts can be developed if one can design suitable chiral flavin catalysts. We report herein that a flavin-catalyzed asymmetric Baeyer ± Villiger reaction of cyclobutanones can be performed with up to 74 % ee [Eq.(Much attention has been focused on the asymmetric Baeyer ± Villiger reaction, because this is the direct route to obtain optically active lactones from cyclic ketones. [3] Transition-metal catalysts in the asymmetric Baeyer ± Villiger reactions of cyclic ketones have been studied extensively: copper with a combination of molecular oxygen and aldehyde, [4] platinum with H 2 O 2 , [5] titanium with tert-butyl hydroperoxide, [6] cobalt with urea ¥ H 2 O 2 , [7] and magnesium [8] and aluminum [9] with cumene hydroperoxide; selectivities of up to 77 % ee were observed. Enantioselective Baeyer ± Villiger reactions have been also performed by using microbial whole cell cultures [10] as well as purified enzymes [11] with stoichiometric amounts of NADPH as a cofactor.Catalytic Baeyer ± Villiger reactions have been shown to occur in the presence of flavin catalyst, [12] which is similar to our catalyst. [2] Therefore, we wanted to design chiral flavin catalysts for enantioselective oxidation reactions. Planarchiral flavins have been prepared and used for the asymmetric oxidation of sulfides. [13,14] However, the synthesis of the catalysts is very tedious because of the need for optical resolution with preparative HPLC. To prepare chiral flavin catalysts simply without optical resolution, we designed planar-chiral C 2 -symmetric bisflavinium perchlorate 1 (Scheme 1), in which each of the flavin moieties blocks one plane of the other flavin moiety. The bisflavin catalyst 1 was prepared in three steps without resolution. Treatment of (S,S)-1,2-diaminocyclohexane (4) with o-fluoronitrobenzene (5) gave (S,S)-1,2-bis[(2-nitrophenyl)amino]cyclohexane (6) in 77 % yield. Catalytic hydrogenation of 6 over palladium on charcoal and subsequent treatment with 3-methylalloxan [15] gave C 2 -symmetric bisflavin 7 in 90 % yield [m.p. 223.5 ± 224.6 8C; [a] 25 D 401 (c 0.20 in CHCl 3 )] in diastereomerically pure form. The stereochemistry was determined by difference NOE experiments of 7; [1...

Highly chemoselective Baeyer–Villiger oxidations can be performed in the presence of other reactive functionalities such as alcohols, olefins, and sulfides, which would undergo electrophilic oxidation under conventional conditions (see scheme). [DMRFlEt]+[ClO4]− (depicted blue) is a new class of flavin compound that catalyzes aerobic Baeyer–Villiger oxidations in the presence of Zn dust as the electron source.

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Flavin-catalyzed green oxidation of heteroatom compounds such as sulfides and amines with molecular oxygen and even air in the presence of hydrazine monohydrate in a fluorous solvent such as 2,2,2-trifluoroethanol at room temperature gives the corresponding oxidation products highly efficiently and selectively along with water and molecular nitrogen, which are environmentally benign by-products. The proposed reaction mechanism is based on the kinetics, solvent effect, and redox properties of flavin catalysts.

The first green and practical method for "aerobic hydrogenation" involving the use of hydrazine and an organocatalyst is described. Olefins can be hydrogenated by treatment with hydrazine in the presence of a 5-ethyl-3-methyllumiflavinium perchlorate (FlEt+.ClO4-) catalyst under O2 atmosphere to give the corresponding hydrogenated products in excellent yields along with environmentally benign water and molecular nitrogen as the only waste products.