1.4- Benzothiazine, tautomerism

Scheme 9 shows the tautomerism of 2-arylazosubstituted benzothiazines. Both 67 <1997H(45)1183> and 153 prefer the A form 153 can only be forced to adopt the B form by N-alkylation to give 154 <1980J(P1)2923>. 

When dihydro-1,3-benzothiazine 203 was allowed to react with KF in DMF (or NaOMe in MeOH), an open ring tautomeric isomer 204 was formed. When 2-bromoacetophenone 169 (R = H) is present in the reaction medium, a mixture of two diastereoisomers of 206 was obtained (75%) after enolate/imine addition of intermediate 205 with a slight excess of the m-isomer 206a over the trans-isomer 206b (Scheme 35). The same result was obtained by a synthesis from [6+1] fragments, reported in the next section <1995TL753>. 

Another example of bromotropone contraction is derived from Nozoe s benzotropazine chemistry [Scheme 146 89H(29)1459]. Condensation of tropone 552 with further o-aminothiophenol (117b) gave benzenoid compound 554 instead of the expected tris(benzazino)tropylidene. Presumably, this condensation takes place via initial cyclization (to benzothiazine), then cyclocondensation (to spirobenzothiazoline), followed by tautomerization (to norcaradiene 553), ring contraction, and dehydrogenation. 

Arylazo derivatives of quinoxaline, 1,4-benzothiazine and 1,4-benzoxazine 100 (X = O, S, NH R = Me, Ph, 4-MeC6H4) exist predominantly in the hydrazone tautomeric forms 100a as shown by spectral data and HMO calculations (84JHC521). 

This group represents by far the largest class of 1,2-benzothiazines. Since many of these compounds are capable of keto-enol tautomerism, the 4-hydroxy-l,2-benzothiazines (3) as well as the l,2-benzothiazin-4-ones (4) will be grouped in this section. Where the enol form has been demonstrated by spectral evidence, these compounds will be named as 4-hydroxy deriva-... 

Methylation of compound 9 occurs on the sulfonamide nitrogen.3 Although compound 9 was named as a l,2-benzothiazin-4-one, infrared spectral data obtained by these workers,3 as well as later work by others, strongly support a 4-hydroxy tautomeric form. 

This group represents by far the largest class of 1,2-benzothiazines. Since many of these compounds are capable of keto-enol tautomerism, the... 

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