Thiazines, Dimroth rearrangement

Glasnov TN, Vugts DJ, Koningstein MM et al (2006) Microwave-assisted dimroth rearrangement of thiazines to dihydropyrimidinethiones synthetic and mechanistic aspects. QSAR Comb Sci 25 509-518... 

The cyclocondensation of alkylidenemalononitriles (182) and thiobenzamides in acetic acid solution containing perchloric acid affords 6-imino-l,3-thiazines (183). Dimroth rearrangement of these products in the presence of a base yields pyrimidine-4(3f/)-thiones (184) (Scheme 34) <88PS55>. 

Similarly, Zigenuner and co-workers showed that thiazine 172 could undergo Dimroth rearrangement under thermal conditions to produce the corresponding pyrimidine 173 in 25% yield. ... 

Thiazines also undergo Dimroth rearrangement under basic conditions to form the corresponding thiouracils. For example, cw-2-(4-substituted phenyl)amino-5-methyl-6-phenyl-5,6-dihydro-1,3-thiazin-4-one 174 rear-ranged in the presence of lithium hydride in DMF to give a mixture... 

Fused thiazines have also been shown to undergo Dimroth rearrangement. Treatment of anilinothienothiazine 177 with sodium ethoxide in ethanol, followed by the addition of aqueous hydrochloric acid, led to the formation of the corresponding thioxopyrimidone 178 in 54% yield. ... 

Intermediate iminothiazines are not detected in the reaction mixture. Obviously, it is pyridine that acts as the base catalyzing the Dimroth rearrangement (opening the thiazine ring and subsequent cyclization into pyrimidinedithione cycle). 

Oxazines and 1,3-thiazines have been used quite extensively as substrates in pyrimidine syntheses (B-94MI 602-0l>. 1,3-Thiazines are intermediate structures in several cyclization reactions involving thioxo substrates. Pyrimidine formation from thiazines may involve a Dimroth-like rearrangement of a thiazinamine, an aminolytic displacement of the ring sulfur atom, or a combination of both. Aqueous ethanol in methylamine converts 4-phenyl-5-phenylsulfonyl-2//-l,3-thiazine-2,6(3//)-dithione (589) into the pyrimidine (590) by displacement of the ring sulfur and a nucleophilic substitution of the thioxo sulfur in the 2-position (Scheme 93) (88JHC835). 

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