Between two heteroatoms

Syntheses of this type have been reported only recently (78H(9)1367, 79JOC3524). Unsaturated 1,4-dioximes are transformed by oxidative cyclization into pyridazine 1,2-dioxides 

On the other hand, unsaturated diazo compounds are thermally transformed by 1,1-cycloaddition into a bicyclic pyrazqle (141). Although reversibility of this cycloaddition is 

Alkylation and rearrangement occured on reductive cyclization of the phenylhydrazone of 8-nitro- 

1-naphthaldehyde with triethyl phosphite to give l-ethyl-3-phenyl-17/-benzo[ /e]cinnoline (Equation 

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No syntheses of pyridopyrimidines by formation of bonds between two heteroatoms are possible. 

CDI and the other A /V -carbonylbisazoles of sufficiently high reactivity react with alcohols ROH to produce diesters of carbonic acid RO-CO-OR, and with amines R R2NH to give diamides of carbonic acid (ureas) R N-CO-NR 2. By use of corresponding bifunctional partners, heterocyclic systems are accessible through insertion of the carbonyl group between two heteroatoms (see Chapter 7). 

There are few examples of the preparations of heterocyclic compounds containing two or more heteroatoms which involve cyclization with formation of a bond between two heteroatoms. The best known instances of this type of reaction, all of which are [6 + 0] reactions, are the preparations of benzocinnolines as outlined in equations (l)-(4). A similar type of approach to that outlined in equation (4) has been used for the direct preparation of the di-N-oxide (2) from the dioxime (1 equation 5). The naphthotriazine betaine (4) is obtained as one of the products of the thermal decomposition of the azidoazo compound (3 equation 6). 1,2-Dithiins and their dibenzo derivatives have been prepared by oxidation of appropriate dithiols and related starting materials as outlined in equation (7). All of these reactions are, however, somewhat specialized and there has been essentially no systematic study of the preparation of six-membered heterocycles via formation of a bond between two heteroatoms. 

Formation of a bond between two heteroatoms is also of considerable utility in the synthesis of sulfur-containing polyheteroatom systems. Reaction of 1,5-dinucleophiles with electrophilic sulfur reagents such as thionyl chloride or sulfur dichloride leads directly to the heterocycles (e.g. equation 55). A fuller discussion of these types of reaction is given in Chapter 2.28. 

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