Oxazines, Dimroth rearrangement

Benzoyl isocyanate trimerizes to form 7-benzoylimino-2,5-diphenyloxazolo[5,4-rf]-[l,3]oxazine (243). The latter in the presence of sodium methoxide undergoes a Dimroth-type rearrangement to yield the fused pyrimidine (244) after debenzoylation (72JOC2583). 

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). 

Oxazines have also been shown to undergo rearrangement as illustrated by the treatment of 152 with base. In this case, elimination of the corresponding Dimroth product occurred spontaneously to give the more stable compound 153. 

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