Reversed Dimroth reaction

A scH alled reverse Dimroth reaction is possible using the differing electron effects produced by methyl and methoxy groups on the ease of rearrangement. " In this way a 6-methylaminopurine, after prior conversion to the 1-methoxy derivative (51), rearranges readily to the 6-methoxyimino-... 

This type of Dimroth rearrangement is strongly influenced by inductive and less by steric effects Dimroth reactions, usually reversible, almost always proceed, in the case of TPs, from the kinetically favored [4,3-a] product to the thermodynamically more stable [1,5-u] isomer (70CB3266 80PAC1611 87JHC805, 87M601). 

An interesting reaction sequence which formally represents an overall reverse Dimroth rearrangement (43) - (47) has been effected (Scheme 6). 7V-Oxidation... 

Thioxo-1,2,4-dithiazolidines can be obtained from thioxo-l,2,4-thiadiazolidines by Dimroth rearrangement (Scheme 47) or reversibly by addition and elimination of acetone (Scheme 22). 1,2,4-Dithiazole type products are also obtained from thiocarbonyl isothiocyanates (154) by reaction with CI2 <84CHEC-I(6)897>, PhMgBr, Sj, or CS2. Other routes include reactions of carbonyl isothiocyanates and thiocarbonyl isocyanates with P4S10 or Sg <84Chec-I(6)897> and sulfonyl isocyanates with dialkyl thioketenes. Phenylisothiocyanate heated with N,C-disubstituted oxaziridines affords 3,5-diimino-l,2,4-dithiazolidines (226, 227). Other miscellaneous syntheses of 1,2,4-dithiazoline derivatives include cyclization of A7-thioacyl hexafluoroacetonimine to 5,5-bis(trifluoromethyl)-... 

Dimroth rearrangement also occurs between the 5-mercapto-l,2,3-triazoles (109) and the 5-amino-l,2,3-thiadiazoles (110). If the thiadiazoles (110) are heated in basic solvents they are converted into the triazoles, whereas the reverse reaction is observed in acidic media (67LA(710)118, 69CB417, 72ACS1243). Thermal equilibria of thiadiazoles and triazoles, with both isomers present, have been reported (62Acs(B)1800, 66CB1618). 

The amino-triazole (615) undergoes a reversible photo-Dimroth rearrangement to the anilino-derivative (616) the equilibrium mixture contains more of the former isomer. In the thermal reaction, on the other hand, the second isomer is favoured. 1-Acetoxybenzotriazole (617) exists in equilibrium with the iV-oxide (618). Benzotriazolyl oxide (619), generated by the action of lead dioxide on the corresponding hydroxy-compound, reacts with aromatic compounds ArR to yield carbazolyl oxides (621) via o-nitrosophenyl radicals (620). ... 

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. 

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