1.4- Thiazines tautomerism

Tile tautomeric equilibrium of pyrimido[4,5-h][4, 5 -e]thiazine 157 was studied in DMSO-dg by NMR spectroscopy (92CHE1219). Based on and NMR chemical shifts, fast proton exchange was concluded to occur between 157b and 157c. Monoprotonation of 157 has been assumed to form... 

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... 

Representatives of all three types of isomeric 1,3-oxazines 82-84 (X = O) and 1,3-thiazines 82-84 (X = S) and of the corresponding partially hydrogenated derivatives are known. However, no tautomeric interconversions between these structures have been observed. 

The potentially tautomeric 127/-quinoxalino[2,3-6]benzo-1,4-thiazines are alkylated at either the 11- or 12-positions [100] in a ca. 3 7 ratio (Scheme 5.24) under liquiddiquid two-phase conditions, analogous to those employed for the alkylation of amidic heterocycles. 

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>. 

Infrared (IR) spectra of the different 1,3-thiazine stmctural types have been well documented <1996CHEC-II(6)383>. IR spectroscopy provides valuable information on structure and tautomerism in the solid state and in solution. 

It is well known that l,3-thiazine-4,6-diones can exist in three tautomeric forms <1960CB671, 1976KGS1042>. Cycloaddition of chlorocarbonyl ketenes with thioamides has been reported to produce only the 4-hydroxy-l,3-thiazin-6-ones, whereas the same reaction with amides gives either 4-hydroxy-1,3-oxazin-6-ones or a mixture of tautomers depending on the substituents on the starting materials <2005ARK(xv)88>. 

Oxazines and thiazines are unstable with respect to ring-opened isomers (cf. 30—>31). l,2,3-Benzotriazin-4-ones on protonation undergo ring-chain tautomerism to yield diazonium ions (Scheme 8 see CHEC 2.18). 

At present no systematic study of the factors which influence tautomeric behaviour in 1,3-oxazines and thiazines has been undertaken. All three types of 1,3-oxazines are known and both 2-amino-4-thioxo- and 2-imino-4-mercapto-l,3-thiazines exist in equilibrium with each other (74MI22700). When, however, the 4-position is disubstituted the amino isomer predominates (71KGS946). 

Not too much is known about tetrahydro-l,3-thiazines. However, 2-imino derivatives are well established compounds which exist in tautomeric equilibrium with 2-aminodihydro-1,3-oxazines (Scheme 34) (1890CB87,70TL2167). 

Oxazines, 1,4-Thiazines and their Benzo Derivatives 2.27.2.3.1 Parent ring systems (i) Structure and tautomerism... 

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. 

In the case of thiazine-4,6-diones, in contrast to the spectrum of the unsubstituted compound 92 (C=0,1640 cm ), there are three absorption bands. One is at 1640 cm-1, corresponding to the enol form, and two are at 1690 and 1730 cm"1, corresponding to the diketo form. In this example, the H-NMR spectrum suggests the presence of three tautomeric forms 93,94 and 95. In conclusion, we record a few absorptions bands for two thiazine-2-thiones (96) [82JCS(P1)2149]. 

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