L,3-Benzothiazin-4-ones

The reaction of methyl thiosalicylate with DMAD yields the Michael adduct (358), which undergoes cyclization on ammonolysis to give methyl 3-hydroxybenzo[fc]thiophene-2-carboxylate (360) through the postulated intermediate 359 (Scheme 55). On the other hand, the reaction of o-mercaptobenzamide with DMAD gives a 1 1 adduct (193) which is converted in the presence of base into the l,3-benzothiazin-4-one derivative (194) [Eq. (31)]. In the reaction of methyl... 

Reaction of 2-cyanomethyl-4H-l,3-benzothiazin-4-one with benzoylace-tonitrile at 160°C yielded 8-cyano-7-imino-9-phenyl-7//,ll//-pyrido[2,l-6][l,3]benzothiazin-l 1-one (85MI1 86MI2). Reaction of 2-ethoxy-2,3 dihydro-4//-l,3-benzothiazin-4-one with 1,2,3,4-tetramethylbutadiene and 2,4-dimethyl-l,3-pentadiene in the presence of boron trifluoride in diethyl ether gave rise to 6,7,8,9-tetramethyl- and 7,9)9-trimethyl-5a,6,9,ll-tetrahydropyrido[2,l-6][l,3]benzothiazin-ll-one, respectively (73JHC149). 

Thiazines can be oxidized at the sulfur atom to give sulfoxides and eventually sulfones. Generally speaking, however, this is not a good approach to these structures as there are inherent problems of non-selectivity and over-oxidation. Air oxidation is sufficient to convert the 1,3-benzothiazine (24 R = alkoxy) into the 1-oxide (25 n = 1), whereas potassium permanganate yields the 1,1-dioxide (25 n = 2). In the case of the unsubstituted compound (24 R = H) the sulfur atom is less reactive and oxidation affords the l,3-benzothiazin-4-one (26) (77ACH(92)317). 

Benzoxazin-4-ones have been known for many years and are made by the cyclization of O-benzoylsalicylamides or reactions between phenyl salicylates and benzamidines (l5LA(409)305>. The first of these two methods obviously has wide applicability and when 2-acyl-mercaptobenzamides are used l,3-benzothiazin-4-ones are obtained (Scheme 77) (67BSF4441). 

The tetrazolobenzo-TA 204 was obtained by a reaction of 2-chloro-l,3-benzothiazin-4-one with sodium or dibutylammonium azide (70CB413) (Scheme 73). 

O-protonated 3-azathiopyrylium forms 38 (Scheme 8). On the basis of H NMR spectra it was assumed that 2-cyanomethyl-l,3-benzothiazin-4-one exists in one of its three protonated forms (84H1677). In several papers (53AP437 57MI1 60MI1), the formation of crystalline hydrochlorides of 1,3-benzo-TA was mentioned these compounds proved to be unstable,... 

The carbonyl portion of 2-iminobenzo-TA 94 can be converted into the thioxo group with the formation of derivatives 261 (67PHA611) (Scheme 102). The analogous reactions are also known for monocyclic 1,3-thiazin-4-ones (67CJC939) and for 2,3-dihydro-l,3-benzothiazin-4-ones (55BSF-1518). 

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