Dihydro-1,4-thiazines tautomerism

Dihydro-2/7- 74 and -4//-l,2-oxazines and thiazines 75 are interrelated by prototropy, being enamines and imines, respectively. In the case of oxazines, the imine form 75 is favored, and there are several well established examples of this system, including the parent heterocycle 75 (X = O) [84MI2]. No tautomeric equilibrium between the 2H and 4H forms has been observed under normal conditions in solution or in the solid state. However, the formation of intermediate 2H isomers 77 was proposed both for the conversion of 3-phenyl-5,6-dihydro-4//-l,2-oxazine 76 (R = Ph, r = R = H) into 2-phenylpyrrole(89TL3471) under strong basic conditions and for thermal decomposition of cyclopentene-fused 1,2-oxazine 76... 

IR spectroscopy of dihydro-1,2-thiazines (cf. 65) is useful for the elucidation of the tautomeric form present, where the C=N stretch is observed at 1450 cm (Figure 18) <1999TL1505>. 

Of the two possible tautomeric structures for monocyclic dihydro-1,4-oxazines and -thiazines the 3,4-dihydro-2/f-representation is preferred in which the ring has a half-chair conformation. 

Two tautomeric structures are possible for dihydro-1,4-thiazine these tautomers are 3,4-dihydro-2/f-1,4-thiazine (50) and 5,6-dihydro-2//-1,4-thiazine (51). 

Two tautomeric structures, 206 and 207, are possible for dihydro-1,4-thiazine 1-oxide only derivatives of the former compound have been described. 

Dihydro-l,3-thiazine-6-thiones (262) are obtained from 2-aza-1,3-dienes (263) through cycloaddition reactions with carbon disulfide in the presence of a catalytic amount of boron trifluoride. It is probable that the initial adducts are 2,5-dihydro-1,3-thiazine-6-thiones (264), which are unstable and tautomerize to the observed products (Scheme 50) <86CC1179>. 

Another example of 1,3-thiazine formation is afforded by the acid-catalysed ring-opening and ring-closure of a 3,6-dihydro-2(li/)-pyrimi-dinethione to the isomeric 2-amino-4fr-l,3-thiazine thus, in hydrochloric acid solution the thione (32) is smoothly converted into (33), which appears to exist predominantly in the amino tautomeric form (u.v. and i.r. evidence). The sequence of reactions in Scheme 7 is suggested to account for the formation of (33). 

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