Heterocyclics s. a. Ring closure

N-condensed, by double ring closure 17, 445 —, 2-imino- 16, 508 0-Heterocyclics (s. a. iert Amino-O-heterocyclics)... 

Pschorr ring closure 15, 561 Pseudoacids s. Hydroxylactones Pseudoaromatic rings s. Di-thiylium salts. Metal complex compounds, ar., Tropenium salts Pseudoazulenes 15, 468 N-Pseudoazulenes 14, 791 0-Pseudoazulenes 14, 338 Pseudobase adducts and their reactions 13, 584 —, dehydrogenation 14, 769 —, p-methylation of quinolines via — 14, 769 Pseudobases 11, 323 Pseudobases, N-heterocyclic (s. a. 0-Alkylpseudobases, N-heterocyclic) 15, 304... 

Poly(arylene ether ketone)s can also be modified by introducing the functional groups using similar approaches to polysulfones. For example, poly(arylene ether ketone)s were sulfonated.189 In addition, o-dibenzoylbenzene moieties in the poly(arylene ether)s can be transformed to heterocycles by cyclization with small molecules. These polymers can react with hydrazine monohydrate in the presence of a mild acid in chlorobenzene or with benzylamine in a basic medium.190 Another example of the use of the o-benzyl cyclization strategy is the intramolecular ring closure of poly(arylene ketone)s containing 2,2/-dibenzoylbiphenyl units to form poly(arylene ether phenanthrenes).191... 

Catalysed alkylation of tosylmethylisocyanate (TOSMIC) [63, 64] has extended its versatility in the preparation of l, 4-dicarbonyl compounds and as a l, 3-dipolar precursor for the synthesis of heterocyclic compounds. The alkylation reactions should not be conducted in carbon disulphide, as nucleophilic attack by the methylene group on the carbon disulphide leads, after ring closure and S-alkylation, to a 4-alkylthio-1,3-thiazole system [65]. 

Examples of the formation of sulphur containing heterocycles by similar transformations are quite rare. The palladium catalysed ring closure of S-propargyl-2-iodophenylmercaptane in the presence of formic acid and an amine-base, analogously to phenols and anilines, led to the formation of the partially reduced benzothiophene skeleton, bearing a 3-methylydene function (3.32.),40... 

Finally, there are a mixed bag of oxidases, catalysing ethylene formation in plants and many other diverse reactions, illustrated in Figure 13.20, by isopenicillin N-synthase, IPNS, which catalyses the cyclisation of the heterocyclic P-lactam ring. The importance of penicillin- and cephalosporin-related antibiotics in clinical medicine cannot be underestimated and has stimulated the study of their biosynthetic pathways. A key step in the biosynthesis of these antibiotics involves oxidative ring closure reactions of S-(L-a-aminoadipoyl)-L-cysteinyl-D-valine (ACV) to form isopenicillin N, the precursor of penicillins and cephalosporins, catalysed by IPNS (Figure 13.20). The overall reaction utilizes the full oxidative potential of O2, reducing it to two molecules of H2O. As discussed earlier, these enzymes are technically oxidases and the four electrons required for dioxygen reduction come from the substrate. 

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