1.4- Thiazine ring, condense

Pyrimido[4,5-6][l, 4]thiazine can be obtained by a versatile and convenient method reported by Sako, Maki et al. <9iJCS(Pl)2675> treatment of 5-hydroxyuracil (253 X = 0) and 5-hydroxy-isocytosin (253 X = NH) with TV-bromosuccinimide in ethanol followed by the thermal condensation with (i- and y-aminothiols such as 2-aminothiophenol, cysteamine, L-cysteine, and d,l-homocysteine resulted in the formation of the pyrimido[4,5-6][l,4]thiazines (256). This new method for the construction of pyrimido[4,5-6][l, 4]thiazine ring systems was shown to involve the condensation of 5,6-diethoxy-5-hydroxy-5,6-dihydropyrimidin-4(3//)-one intermediates (255) with / -and y-aminothiols which is accelerated in the presence of an acid catalyst (Scheme 42). 

Very little is known about the chemistry of later steps of in vitro phaeomelanogenesis beyond the benzothiazine stage. Radiotracer studies (160) and model experiments (161) suggest that the alanyl side chain of the postulated intermediates does not take part in the polymerization process, which probably proceeds via an enamine-imine type condensation of the 1,4-thiazine ring system rather than by oxidative coupling at positions 2 and 8 and subsequent ring closure of the alanyl side chain as previously suggested by Minale et al. (162,163). 

The boron trifluoride-promoted condensation of cyclic thioureas with enones gives tricyclic 1,3-thiazines. Thus, in the reaction between 2-benzylidenecycloheptanone (194) and imidazolidine-2-thione (195) the product is the octahydrocyclohept[d]imidazo[2,l-(>][l,3]thiazine (196) (Equation (22)), whereas 2-benzylidenecyclohexanone (197) and the perhydropyrimidinethione (198) yield the adduct (199), in which both double bonds are exocyclic to the thiazine ring (Equation (23)) <87CB1449>. 

The formation of a pyrimido[5,4-f][l,2]thiazine by annulation of a pyrimidine ring onto a benzoH[l,2]thiazin-4-one has been reported, involving condensation with dimethylformamide dimethyl acetal followed by Bredereck s reagent (Scheme 90), and reaction of the resultant vinylogous amide with guanidine <2005W02005/037843>. Essentially, the same approach has been used to prepare annulated forms of the isomeric pyrimido[4,5-< ][l,2]thiazines, as outlined in Scheme 91 <1998W098/28281>. 

Volume 70 of Advances in Heterocyclic Chemistry consists of three chapters together with the Subject Index for Volumes 61 through 70. The first contribution by Dr. Istvan Hermecz (Chinoin, Hungary) continues with Part II of his set of three chapters on condensed pyridines. Part I, in Volume 69, covered pyrido[l,2-ft][l,2]oxazines, -thiazines, and -pyridazines, and the present chapter deals with [l,2-c]-fused 1,3-oxanes, 1,2-thiazines, and -pyrimidines. The third part will appear in a subsequent volume of our series and will deal with 1,4-oxazines and 1,4-thiazines, fused pyridine rings. 

The past year has seen the publication of Comprehensive Organic Chemistry, one volume of which contains much information on the six-membered ring systems to be reviewed in this article a monograph on the chemistry of condensed pyrazines has also appeared. Reviews on 1,4-thiazines, l,3-benzothiazines," pyridazines, benzo[c]cinnolines, quinazolines, purines, pyrrolo[3,2-c]quino-lines, 1,10-phenanthroline and its complexes, polyaza-phenanthrenes, and 1,9- and 1,10-diaza-anthracenes have been published. Other specialist reviews are devoted to catalytic methods of obtaining pyridine bases pyridine N-oxides the stereochemistry of quinolizines, indolizines, and pyrrolizines benzothiazinone dioxides 2-quinazolones and their cyclic homologues (e.g. 

The chemical structures of synthetic dyes show considerable variety. They generally contain more than one aromatic group, condensed aromatic substructures or heterocyclic rings (pyrazolone, thiazole, acridine, thiazine, oxazine) which are mainly hydrophobic, and, frequently, a polar basic or cationic group which is strongly hydrophilic. Due to these structural characteristics, they readily bind both to polar adsorptive and apolar reversed-phase (RP) chromatographic supports, making their successful separation difficult. As the synthetic dyes are not volatile... 

Benzo-l,3-thiazines.—Little work has been reported in this area. A new route to substituted 4/f-l,3-benzothiazines, e.g. (27), has been found in the acid-catalysed condensation of a nitrile with the carbinol (26), arising from the addition of Grignard reagents to 2-mercaptobenzoic esters. In view of the limitations of its applicability, and the low yields obtained, the synthetic value of this reaction is rather circumscribed. Condensation of cyanogen with 2-mercaptobenz-hydrazides results in ring-closure and formation of 3-amino-2-imino-l,3-benzo-thiazin-4-ones, but a similar reaction with 2-mercaptobenzoic acids gives predominantly the bis-l,3-benzothiazinones (28). 

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