Nitrogen- and Sulphur-containing Rings

Nitrogen- and Sulphur-containing Rings.- The preparation of 1,3-thiazines as intermediates in the synthesis of cephalosporins has been published utilising a [4+2] cycloaddition of the l-thia-3-azadienes (81) with a,3-unsaturated aldehydes and ketones. The reaction proceeds thermally, except where R = CC Et when a Lewis-acid catalyst is required, and yields are generally high. 

Two research groups have published methods for the synthesis of the benzothiazepine class of molecule this year. Ishibashi et al. have prepared a 1,2,3,5-tetrahydro-4,1-benzothiazepine via intramolecular Friede1-Crafts reaction of the ortho-chloro intermediate (82) or acid-catalysed cyclization of the sulphoxide 

Thiazolidine-2-thiones can be prepared with complete control over stereochemistry from the olefins (134). Addition of iodoiso-cyanate to the olefins in MeOH generates the known B-iodocarbamates (I35) which on treatment with potassium ethyIxanthate followed by base give the thlazolidine (136). The opposite stereoisomer (138) is formed by addition of the xanthate to the azlrldine (137) formed from base treatment of (135).  

The benzothiadiazine oxides (148) are formed by an unusual [4+2] cycloaddition reaction between the sulphinyl anilines (146) and the 

Schiff bases (14 ) where the sulphinyl aniline is serving as the 125 

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In addition to benzenoid diazo components, diazotised heterocyclic amines in which the amino group is attached to a nitrogen- or sulphur-containing ring figure prominently in the preparation of disperse dyes [87,88], since these can produce marked bathochromic shifts. The most commonly used of these are the 6-substituted 2-aminobenzothiazoles, prepared by the reaction of a suitable arylamine with bromine and potassium thiocyanate (Scheme 4.31). Intermediates of this type, such as the 6-nitro derivative (4.79), are the source of red dyes, as in Cl Disperse Red 145 (4.80). It has been found that dichloroacetic acid is an effective solvent for the diazotisation of 2-amino-6-nitrobenzothiazole [89]. Subsequent coupling reactions can be carried out in the same solvent system. Monoazo disperse dyes have also been synthesised from other isomeric nitro derivatives of 2-aminobenzothiazole [90]. Various dichloronitro derivatives of this amine can be used to generate reddish blue dyes for polyester [91]. 

An exhaustive review on di- and poly-alkali-metal derivatives of heterofunc-tionally substituted organic molecules contains many useful references on aromatic ring metallation, particularly of nitrogen- and sulphur-containing heteroaromatics. Other reviews include a description of the use of chromium tricarbonyl-arene complexes in organic synthesis and the preparation and use in synthesis of highly reactive metal powders. ... 

Oxazole, imidazole and thiazole systems contain a five-membered ring and two hetero-atoms, one of which is a nitrogen atom. The hetero-atoms are separated by a carbon atom in the ring. The second hetero-atoms are oxygen, nitrogen and sulphur for oxazole, imidazole and thiazole systems, respectively. 

Three-membered Ring Systems Four-membered Ring Systems Five tnembered Ring Systems Part I Thiophens and their Selenium and Tellurium Analogues Part II Systems containing Nitrogen and Sulphur,... 

This five-membered ring, containing both nitrogen and sulphur, together with a pyrimidine ring, forms part of the structure of thiamine pyrophosphate which is an important coenzyme. 

Indole contains a benzene ring fused with a pyrrole ring at C-2/C-3, and can be described as benzopyrrole. Indole is a ten tt electron aromatic system achieved from the delocalization of the lone pair of electrons on the nitrogen atom. Benzofuran and benzothiaphene are very similar to benzopyrrole (indole), with different hetero-atoms, oxygen and sulphur respectively. 

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