Heterocyclic compounds Dimroth rearrangement

Much work has been devoted to [2 + 3] cycloadditions both in the synthesis of and in reactions of this kind of heterocycle. Dimroth rearrangement is another feature which has been explored widely. X-ray structural analysis was extensively used alongside physico-chemical methods to elucidate the structure of novel compounds. 

A new aromaticity index was calculated for heterocyclic compounds (87T4725). Comparison of the index of 2-ribosyl-5,7-dimethyl TP with that of the isomeric [4,3-a] compound suggests that the driving force for the Dimroth rearrangement of the latter substance cf. Section n,C,2) is the increase in aromatic character. 

Georges and co-workers used the Dimroth rearrangement in their synthesis of substituted verdazyl compounds as templates for the diversity-oriented synthesis of heterocyclic compounds. Verdazyl derivative 71 did not undergo rearrangement to form 72 when refluxed in ethyl acetate (not shown). However, treatment of 71 with either two equivalents of sodium hydride at room temperature or lithium diisopropyl amine in THF at 0 °C gave the corresponding Dimroth product 72 in 82% yield. 

Within the huge field of heterocyclic rearrangements [53], the Dimroth rearrangement gives 5-mercapto-l,2,3-triazoles from 5-amino-l,2,3-thiadiazoles and allows interconversion of 5-amino-l,2,3-triazoles into a mixture of isomers under basic condition [54]. Thus, this reaction, discovered in 1909, involves a linear intermediate that can cyclize into two isomeric compounds (Scheme 15) [55]. This ring-chain tautomerization is reversible, but the equilibrium can be pushed toward an end if one of the two isomers is stabilized. 

---