Dimroth bases, formation

Base-catalyzed Dimroth rearrangements of l-aryl-4,6-diamino-1,3,5-triazines or triflic acid-induced decomposition of 6-anilino-1,3,5-triazines led to the formation of the corresponding substituted 1,2-dihydro-l,3,5-triazines, whose structure was established on the basis of H NMR spectra (93JHC849). 

As indicated, many of the more highly fimctionalized building blocks did not result in 2-pyridones. However, a thorough structure elucidation of by-products and intermediates was used to propose a mechanism for the formation of the 2-pyridone core based on a Michael addition followed by a Dimroth-type rearrangement (Fig. 3). 

A classic example of this reaction is the Dimroth rearrangement, which is initiated thermally or in the presence of base catalysts and can usually occur with the retention or cleavage of the S-S bond depending on the nature of the substituents. The former pathway is typical of 3,5-diimino- and 5-imino-l,2,4-dithiazolidine-3-one derivatives and involves a rupture of one C-N bond, rotation of the N-C-N moiety around the S-C bond, and formation of a new 1,2,4-dithiazolidine derivative. The second pathway occurs when at least one N-substituent is an alkyl group and involves a rupture of the S-S bond, rotation of the N-C-S moiety around the N-C bond, and formation of the 1,2,4-thiadiazolidine derivative (Scheme 14) <1996CHEC-II(4)453>. 

It was known for long time that the 1,5- and 1,8-dinitronaphthalenes react under the action of concentrated sulphuric acid to yield naphthazarine - a valuable compound for dyeing [30], The mechanism of the formation of this compound (based on experiments of Dimroth and Ruck [18a]) probably consists in the transformation of the nitro compounds to quinone-oximes and the reduction of one of the nitro groups by hydroxylamine split off the oxime ... 

Chlorination of isothiocyanates produces iminohalomethylsulfenyl halides (269) which react further with isothiocyanates to yield dithiazolium salts of type (365). Treatment of (365) with 95% ethanol leads to the formation of salts of isothiocyanate oxides (366) whereas treatment with hydrogen sulfide produces salts of isothiocyanate sulfides (367), as indicated in Scheme 133 <65AHC(5)119>. Neutralization of salts (367) affords the free base isothiocyanate sulfides (368) which are stable with one exception. Melting or recrystallization of N,AT -dimethyliminothiono-l,2,4-dithiazolidine (368 R = Me) results in a Dimroth... 

In 2014, Odom and coworkers reported a titanium-catalyzed synthesis of 2-amino-3-cyanopyridines (Scheme 3.41) [89]. The reactions were performed in a one-pot manner with two manipulation steps. Firstly, titanium-catalyzed alkyne iminoamination and generation tautomers of 1,3-diimines. Then, 2-amino-3-cyanopyridines were formed in good to modest yields after treatment with base (DBU) and malononitrile. A Dimroth rearrangement mechanism for 2-aminopyridine formation was proposed based on the isolation of a 2-imino-l,2-dihydropyridine intermediate which undergoes rearrangement under the reaction conditions. 

---