From Other Heterocycles

Shvaika and Fomenko demonstrated the formation of 4,5-dihydro-1,2,4-triazines (149) from recyclization reactions of 1,3-oxazolium salts (148a),236 

3-thiazolium (148b),237,238 or 1,3-selenazolium salts (148c)239 with hydrazine [Eq. (67)]. 

An interesting ring contraction to form dihydro-1,2,4-triazine (151) was reported by Halladin and Hassner240 by the reaction of l,4-oxazepin-7-ones (150) with hydrazine [Eq. (68)]. 

The N—H stretching vibrations of 4,5-dihydro-1,2,4-triazines usually appear at 3250-3260 cm 1,233,239 with the C=N absorptions being found in the 1620-1680 cm-1 region.239 The N—H stretching band of the 1,2-dihydro compound (144) displays two peaks at 3180 and 3380 cm-1.233 The C=N absorption bands in the 2,5-dihydro compounds were found at 1645-1650 cm-1. 

Shvaika and V. I. Fomenko, Dokl. Akad. Nauk SSSR 200,134 (1971). 

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VIII. THIAZOLES FROM OTHER HETEROCYCLIC COMPOUNDS... 

Closing this chapter on thiazoie synthetic methods, I would like to point out that it is possible to obtain thiazoles from other heterocyclic compounds. 

VIII. Thiazoles from Other Heterocyclic Compounds... 

There are several examples of the formation of pyridazines from other heterocycles, such as azirines, furans, pyrroles, isoxazoles, pyrazoles or pyrans and by ring contraction of 1,2-diazepines. Their formation is mentioned in Section 2.12.6.3.2. 

Syntheses of the Pyrimidine Ring from Other Heterocycles... 

Transformations from Other Heterocyclic Ring Systems... 

SYNTHESIS FROM OTHER HETEROCYCLIC COMPOUNDS 4.17.10.1 From Six-membered Heterocycles... 

Formation from other heterocyclic systems 5.05.6.4 Conclusions... 

Transition metal complexes have been used in a number of reactions leading to the direct synthesis of pyridine derivatives from acyclic compounds and from other heterocycles. It is pertinent also to describe two methods that have been employed to prepare difficultly accessible 3-alkyl-, 3-formyl-, and 3-acylpyridines. By elaborating on reported194,195 procedures used in aromatic reactions, it is possible to convert 3-bromopyridines to products containing a 3-oxoalkyl function196 (Scheme 129). A minor problem in this simple catalytic process is caused by the formation in some cases of 2-substituted pyridines but this is minimized by using dimethyl-formamide as the solvent.196... 

Ring syntheses of 1,2,4-oxadiazoles from other heterocycles 282... 

The first 3-azapyrylium salts 48 and 50 were obtained from other heterocycles by oxidative dehydrogenation (Section III,C) and by 5-alkylation of 4-thiono-l,3-oxazine (49) (62CB937 72S333). Until recently only two... 

Examples of the Dimroth rearrangement (Section IV, F) include several s3mtheses of monocyclic triazoles from other heterocyclic systems (cf. Scheme 25). Triazole-5-thiols can be prepared by treatment of 5-amino-l,2,3-thiadiazoles with bases.A similar base-induced rearrangement of sydnoneimines provides a synthesis of 4-hydroxy-triazoles. ... 

From Other Heterocyclic Systems 4-Arylazoisoxazoles can rearrange to 2-aryltriazoles, and 4-amino-2//-triazoles can be prepared by reaction of 4-nitrosoimidazoles with hydrazines (Scheme 34). The hydrazones of 3-benzoyloxadiazoles are intermediates in the latter reaction.The generality of rearrangements of this type has been discussed, and a further example, involving the rearrangement of a 1,2,5-oxadiazole to a triazole, has been described. 

The various possibilities for the first two types of syntheses are listed schematically in Table 4, and the possibilities for synthesis from other heterocycles are listed in Table 5. In each case the method is discussed in more detail in the following sections Tables 4 and 5 indicate the section(s) in which a particular reaction is described. In the case of sulfur-containing heterocycles, 5-oxides and 5,5-dioxides are treated alongside the basic parent structures, since for 1,3,2-dioxathiolane, 1,2,3-oxadithiolane, and 1,2,5-oxadithiolane the unoxidized rings are unknown. 

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