Pyridines Dimroth rearrangement

Imtdazo[4,5-c]pyridtne, 4,5,6,7-tetrahydro-synthesis, 5, 623, 640, 641 Imidazo[4,5-c]pyridine-6-carboxylic acid, 4,5,6,7-tetrahydro-synthesis, 5, 623, 641 Imidazopyridines as anthelmintic, 1, 202 synthesis, 5, 462 Imidazo[l,2-n]pyridines deuterium exchange, 5, 611 diazo coupling, 5, 614 Dimroth rearrangement, 5, 613 halogenation, 5, 611 hydrogenation, 5, 614 Mannich reaction, 5, 612 nitration, 5, 612 1-oxides... 

Triazolo[4,3-a]pyridine, 3-amino-diazonium reactions, 5, 863 Dimroth rearrangement, 5, 861 Sandmeyer reaction, 5, 862 synthesis, 5, 883... 

In all syntheses of [ 1,2,4]triazolo[4,3-a]pyridines it should be remembered that electron withdrawing substituents on the pyridine ring can cause Dimroth rearrangement of the initially formed compounds into derivatives of [l,2,4]tria-zolo[l,5-fl]pyridines (see Section B.2.c). 

Another interesting rearrangement, involving a pyrimidine-pyridine ring transformation combined with a Dimroth rearrangement, is observed when 6-nitro[l,2,4-triazolo][l,5- ]pyrimidine (74) reacts with ethyl cyanoacetate. 

Isomerization of 5-imino-l,2,4-dithiazolidine-3-thiones (34) into l,2,4-thiadiazolidine-3,5-di-thiones (51) by a Dimroth rearrangement (see Section 4.13.6.1.1) is a reversible process <91JPR107>. The equilibrium position strongly depends on the nature of substituents. When R = R = Me or Pr the equilibrium position is (34) (51) =1 2, with R = R = Et the ratio is 1 9. When R = Ar, R = Aik, the equilibrium is almost wholly shifted to the right. The forward process is usually accelerated by bases, the reverse by acids. When both substituents are Ar no isomerization occurs <91JPR107>. Isomerization of the 3,5-diimino analogue (52) to a monothione isomer (53), accelerated by pyridine, reaches an equilibrium ratio of (52) (53) =1 9 <89BSB879>. 

The Dimroth rearrangement (69ZC241) including l,2,4-triazolo[4,3-a] pyrimidines generally proceeds rather easily therefore these compounds, when prepared, are often not isolable (or only by very carefully handling). The extremely fast rearrangement, compared, for example, with that of 1,2,4-triazolo[4,3-a]pyridines, is attributed to the increase in electron de-... 

A reversible Dimroth rearrangement is observed between 1,4-dihydro-7if-w-triazolo[4,5-f>]pyridine-7-thione (655) and 4-amino[l,2,3]thiadiazolo[4,5-c]pyridine (657) on heating... 

Imidazo[l,2-a]pyridines (58) activated by a nitro group at C-6 or C-8 undergo the Dimroth rearrangement to (60) in aqueous base. Intermediates of the rearrangement (59) were detected and their structures investigated spectroscopically (73JHC755). 

In addition to those 6- and 8-substituted [4,3-n] systems mentioned previously in the Dimroth rearrangement discussion, a variety of 5-substituted derivatives has been described (78JHC439) from ring closure of 6-chloro-2-hydrazinopyridine (see synthesis section). A useful material obtained from this investigation was 5-chloro-[l,2,4]triazolo[4,3-n]pyridine (102) which was susceptible to nucleophilic substitution by strong nucleophiles (sodium... 

It is unusual for a Dimroth rearrangement (defined in Section III,D) to occur during formylation, but the following example is cautionary. 5-Aminomethyl-3-benzyl-4-methylaminotriazole and acetic formic anhydride, stirred in pyridine, gave 4-benzylamino-5-formamidomethyl-3-methyltriazole (23°C, 15 hr, 84%). Most likely, the use of pyridine determined this outcome [81JCS(P1)2344]. 

For further clarification we state that reactions such as the transformation of thiopyrylium salts 25 to pyridine or benzene derivatives 27 (71JHC301, 71T6083 84MI1 92HOU687) or the Dimroth rearrangement of thiopyrans... 

The Dimroth rearrangement of mono- and diamino-l//-l,2,3-triazolo [4,5-c]pyridines 13 to mono- and diamino-3//-l,2,3-triazolo[4,5-h]pyridines 16 took place in ethanolic ammonia and involves diazo-type intermediates 14 and 15 (72JOC3601 73JOC1095). The thermodynamic stability of 16 is greater than that of 13 in the presence of ammonia since treatment of 16 with ethanolic ammonia gave no rearrangement (Scheme 5). 

By cyclization with hydroxylamine hydrochloride at room temperature, 4- [(7V,/V-dimethy]-amino)methylene]amino pyridine-3-carbonitrile yields pyrido[4,3-rf]pyrimidin-4-amine 3-oxide.452 Due to traces of hydroxylamine hydrochloride, the attempted rccrystallization of the crude product from dimethylformamide furnishes 4-(hydroxyamino)pyrido[4,3-Dimroth rearrangement is avoided by removal of the impurities by suspending the crude product in water (see Section 7.2.2.4.1.1.1.). 

Dimroth rearrangement in ethanolic pyridine at reflux temperature converts (79) into (80) <93JHC849>. Dihydrotriazines (81) eliminate primary amine on thermal rearrangement to pyrimidines (82) (Scheme 16) <86CB3737>. 

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