Osmium tetroxide is one of the most reliable oxidants for preparing vicinal cis-diols from olefins. In this article are reviewed some aspects of recent progress in diastereoface-selective and enantioface-selective oxidation of olefins with osmium tetroxide. The oxidation mechanism is also discussed.
Hydrogen peroxide oxidation of various organic substrates can be achieved by the use of heteropoly acids (HPA) combined with cetypyridinium chloride (CPC) as catalyst. Tris (cetylpyridinum) 12-tungstophosphate (CWP), [π-C5H5N+ (CH2) 15CH3] 3 (PW12O40) 3-, prepared from 12-tungstophosphate (WPA) and 3 equivalents of CPC, catalyzed the epoxidation of olefins and allylic alcohols in chloroform, and α, β-unsaturated carboxylic acids in water. sec-Alcohols and diols containing sec-hydroxy groups were oxidatively dehydrogenated by CWP-H2O2 system in t-butyl alcohol to give the corresponding ketones. Under the same conditions, α, ω-diols were converted to lactones in fair yields. This catalyst-oxidant system was also efficient for the oxidative cleavage of carbon-carbon bonds of vic-diols and olefins. Peroxo-heteropoly acids, derived from HPA, CPC and H2O2, oxidized stoichiometrically or catalytically a wide varaiety of substrates in a similar manner as above.
Vitamin E acts as a potent and safe, lipid-soluble antioxidant by scavenging primarily chain-carrying peroxyl radical and interrupting chain sequence. Four kinds of tocopherols and also four kinds of tocotrienols are known as vitamin E. Various kinds of vitamin E related compounds have been synthesized and their antioxidant activities have been compared. It was stressed that the antioxidant activities invivo are determined not only by their inherent reactivities toward oxygen radicals but also by their local concentrations at a specific site where the oxidations are taking place.
Effects of reaction media on photoinduced electron-transfer reactions are reviewed from synthetic and mechanistic viewpoints. This review deals with the following subjects : (1) The characteristics and the problems of photoinduced electron-transfer reactions in organic photochemistry. (2) The salt effects on organic photoreactions which proceed via exciplexes and radical ion species. In this section, examples are given that the efficiencies of the photoreactions are enhanced and suppressed by the addition of salts into the reaction systems. Examples are also given that the selectivities in the photoreactions are affected by the addition of salts. (3) The effects of other additives such as oxygen and aromatic hydrocarbons on photoreactions. These additives are shown to often act as redox sensitizers or co-sensitizers in electron-transfer mediated photoreactions. (4) The solvent effects on photoreactions. The change of solvents are shown to often affect the product distributions.
A variety of reactions of nucleophilic metal homoenolates that create new carbon-carbon bonds are reviewed. There have been reported several ways for the preparation of metal homoenolates, which employ either siloxycyclopropanes or 3-halocarbonylcompounds as starting materials. These homoenolates thus prepared react with electrophiles either by themselves or with the aid of transition metal catalysts to give 3-substituted carbonyl compounds through addition, substitution, and carbonylation reactions etc.
Electrochemical reduction of 2-pyrrolidone in DMF using tetraalkylammonium salts (R4NX) as supporting electrolytes yields the corresponding 2-pyrrolidone N-anion (PA). It is described in this article that the PA having R4N+as the counter ion is highly useful as a base to promote a variety of base-catalyzed reactions which are not always easily achievable by using usual bases. Typical examples are as follows. (1) Formation of trihalomethyl anion (CX-3, X = Cl and F) from CHX3 and their addition to aldehydes and α, β-unsaturated carbonyl compounds. (2) Esterification of carboxylic acids, and the synthesis of macrolides under very mild reaction conditions. (3) Selective C-alkylation of β-diketones. (4) Synthesis of α-alkylarylacetic acids which possess antiinflammatory and analgesic activities.