Oxiranes with acids

In cationic initiation, true copolymerization is difficult due to the wide range of reactivity of oxiranes with acid or cationic attack (Table 1). Generally, the copolymer formed is lower in molecular weight, with a significant amount of cyclic product formed, than is the case in homopolymerization. Except at very low temperatures, around -78°C, linear copolymers tend to terminate by proton transfer before significant molecular weights are achieved when cationic initiation is used. 

The cycloahphatic products are generally Hquids of lower viscosity than the standard glycidyl ether resins. The peroxidized resins contain no chlorine and low ash content and their ring-contained oxirane group (cyclohexene oxide type) reacts more readily with acidic curing agents than the bisphenol A-derived epoxy resins. 

The reactions of oxiranes with thiocyanate ion or with thiourea are usually done in homogeneous solution in water, alcohols or alcohol-acetic acid. The use of silica gel as a support for potassium thiocyanate in toluene solvent is advantageous for the simple work-up (filtration and evaporation of solvent) (80JOC4254). A crown ether has been used to catalyze reactions of potassium thiocyanate. 

Group of plastics composed of resins produced by reactions of epoxides or oxiranes with compounds such as amines, phenols, alcohols, carboxylic acids, acid anhydrides and unsaturated compounds. 

Polymerization of oxiranes with succinic anhydride proceeded in the presence of PPL at 60 °C or 80 °C [63]. The ring-opening of the oxirane might proceed by a carboxylic acid catalyst, which is formed by the reaction of succinic anhydride with serine residue of the lipase catalyst. 

We owe to Kato and his colleagues a considerable advance in furan copper reagents. They have demonstrated the formation of the lithium di(3-furyl) cuprate species 87 which is highly reactive and possesses hard properties that suit it to reaction at hard centers, mainly carbonyl carbon.223 The reagent is easily prepared in situ from 3-furyllithium and Cu2I2. Simple copper derivatives do not react with ketones, but this cuprate reacts well and quantitatively with acid chlorides. It also reacts well with some epoxides (oxirans). Moreover, there is another form prepared in the presence of... 

The reactivity of oxiranes with lithium enoiates and related compounds has been widely explored and reviewed . Dianions derived from carboxylic acids react readily with oxiranes, but the reaction can be slowed by steric hindrance . The reaction of oxiranes with dianions of acetoacetates is greatly accelerated by the addition of BF3 Et20 " . Oxiranes react readily with lithium salts derived from nitriles , malonates and analogues , lithiated oxazolines and lithio enamines . 

The most important use of oxiranes for the preparation of six-membered heterocycles is in 1,4-dioxane formation. 1,4-Dioxane, for example, is obtained in excellent yield by treatment of oxirane with dilute sulfuric acid (equation 157), and substituted dioxanes can be prepared in a similar manner. 1,4-Dioxanes can also be conveniently obtained by acid-catalyzed condensation of oxiranes with glycols (equation 158), while use of... 

The relative stability of the oxiranes to bases at room temperature allows the selective ammonolysis of carboxylic ester groups with ammonia in methanol,49 and, conversely, free hydroxyl groups may be esterified with acid chlorides or anhydrides in pyridine without affecting the oxirane ring. Hydroxyl groups may also be converted into alkyl ethers by using either the Haworth50 or the Purdie-Irvine procedure.5152... 

The asymmetric epoxidation of several chalcones (39) and other electron-poor olefins in a triphase system (water/organjc solvent/chiral polyamino acid) afford optically active oxirans with optical yields of up to 96%. The influence of the molecular structure of the catalysts and substrates, the solvent, and the temperature on the stereochemistry was investigated by a group of chemists from Italy and Spain 77). 

Dioxanes (337) are also conveniently obtained by acid-catalyzed condensation of oxiranes with glycols, while use of ethanolamine gives morpholine (338). Base-catalyzed reaction of oxiranes with a-amino acids and esters gives tetrahydro-l,4-oxazin-2-ones, e.g. propene oxide + RNHCH2C02H — (339). 1,4-Dithianes have been prepared by the dimerization of thiiranes either in the vapor phase or in the presence of acid catalysts. 

Few solid-phase syntheses of oxazoles have been reported (Table 15.17). The most general strategy is the dehydration of a-(acylamino) ketones (Entry 2, Table 15.17) or 2-(acylamino)phenols (Entry 1, Table 15.17). Oxazolidin-2-ones have been prepared by intramolecular nucleophilic cleavage of carbamates from insoluble supports (Entries 5 and 6, Table 15.17). Resin-bound 2-aminoethanols, which are accessible by nucleophilic ring-opening of oxiranes with amines, undergo cyclocondensation with aldehydes to yield oxazolidines [220,221]. These compounds are unstable towards acids, and can be released from the support only under neutral or basic reaction conditions. 

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