From Other Heterocyclic Systems

The rearrangement of arylsubstituted 1,4-dithiadienes, obtained from the Buntes salts derived from phenacyl halides,offers a 

It now seems definitely proved that the thiothiophthene (86) rearranged reversibly to a thiophene (87) under the influence of alkali and not to a thiepin derivative, The dimethyl ether of (87) has been oxidized to 4-methylsulfonyl-2-thiophenecarboxylic acid (88) 

Thiophenes can be prepared from the sultones of a,j8-unsaturated ketones. From the 8-sultone of pulegone (89), the thiophene (91) has been obtained through reduction with LiAlHi to (90), followed by dehydration of the latter.  

4-Dichlorothiophene has become easily available through chlorination and dehydrochlorination of tetrahydrothiophened Another example of the aromatization of tetrahydrothiophene derivatives is the preparation of 3-substituted thiophenes by the reaction of 3-ketotetrahydrothiophene with Grignard reagents followed by the aromatization of the intermediate dihydrothiophene. Recent gas chromatographic analysis showed, however, that 2,3-dichlorothio-phene is the main product from the dehydrochlorination of tetra-chlorotetrahydrothiophene. 

Therefore, it may also be possible to aromatize the dihydrothiophene derivatives (93) obtained through the acid catalyzed rearrangement of a-hydroxymethylene-y-thiobutyrolactone derivatives (92) in ethanold ° 

At least 10 heterocyclic systems, other than pyridine, have been used as substrates for the synthesis of 1,6-naphthyridines (or occasionally their benzo derivatives144). However, only a few such procedures appear to have much practical potential. Some typical examples follow. 

4-Methylbenzo-1,4-thiazin-3(4//)-imine hydrochloride (132) with acetic anhydride gave the hexacyclic product (133) (neat reactants, reflux, 90 min 57%), which underwent desulfurization by Raney nickel to afford 4,5-dimethyl-2,7-bis(/V-methylanilino)-1,6-naphthyridine (134) (PhH, reflux, 12 h 20%).1032 

An easily prepared mixture of 5//-cyclopenta ft Ipyridine (135) and its 7a//-isomer underwent a one-pot ozonization and subsequent treatment with ammonium hydroxide to give a separable mixture of 1,6- (136) and 1,7-naphthyridine (137) (substrate, MeOH, —78°C, C)3 [ until blue then N2 [. NaHCOal then Me2S, 20°C, 6h NH,OH [, 20°C, 6h 24% and 48%, respectively) homologs likewise.943 

2-Amino-6-phenyl-3,5-pyrandicarbonitrile (138) underwent a multistep reaction with formaldehyde and malononitrile to give 5-oxo-2-phenyl-5,6-dihydro-l,6-naphthyridine-3,8-dicarbonitrile (139) [substrate, HCHO, H2C(CN)2, 

2 h 66% see original for proposed steps] 1378 analogs somewhat similarly.685 

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Formation from other heterocyclic systems 5.05.6.4 Conclusions... 

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. 

Hi) Preparation from other heterocyclic systems 2-Vinylaziridine (392) reacted with phenyl isothiocyanate or 4-chlorothiobenzoylthiogly-collate to give (394), presumably via the intermediate (393) (cf. the analogous preparation of 1,3-oxazepines, p. 627) (71JOC3076). 

The formation of pyrrole and its benzo analogs from other heterocyclic systems can occur in any of several general ways. In this section we will consider reactions which change the ring size and composition. In Section 3.06.6, reactions where the five-membered nitrogen-containing ring remains intact will be considered. 

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