2H-1,4-Thiazines

H- 1,4-Thiazines and benzothiazines undergo oxidation to dehydro dimers (83) and (84) which are of considerable interest as they are the parent chromophores of the trichosiderin (trichochrome) pigments which occur in mammalian red hair and in the feathers of some birds (74T2781). In the case of monocyclic thiazines, reagents such as nitrobenzene or picric acid are required, but air oxidation is sufficient in the bicyclic series. It is curious that whereas aerial oxidation of the ester (85) gives both the dehydro dimers (86) and (87), ethyl azodicarboxylate only yields the tautomer (87) <70AC(R)35l). 

H- 1,4-Thiazine itself has been claimed to be produced by the pyrolysis of the salt (234 Scheme 107) and 3,5-disubstituted analogues are formed when the sulfides (235) are reacted with ammonia (Scheme 108). Unfortunately, attempts to extend this last synthetic reaction by using amines rather than ammonia were unsuccessful (67JMC501). 

H-1,4-Thiazine 6-(4-Chloro-phenyl)-3-formyl- E9a, 433 ( — R2NH -> Imin)... 

H-1,4-Thiazines are weak bases (Section 11,D,5), and the parent compound is reported to form salts with picric acid, chloroplatinic acid, and hydrogen chloride. Alkylation, to give the thiazinium iodide 25, occurred when the thiazine 15 was treated with methyl iodide the derivative 25 is the only known example of a 4//-l,4-thiazinium salt. 

In contrast with the parent compounds, 3,4-dihydro-6-methoxycarbonyl-2H-1,4-thiazine 1 -oxides show two absorption maxima at ca. 210 and 280 nm the former band is usually less intense (s ca. 7000) than the latter band (s ca. 14,000). ° The long-wavelength absorption is undoubtedly the result of n -> it transitions involving the N—C=C(C02Me)—SO chromophore. It appears that the sulfinyl group does not play an important role in the excitation, since the tetrahydropyridine 238 is reported to absorb at 286 nm(s 21,000). ... 

In principle, three tautomeric structures are possible for 1,4-thiazine these tautomers are Iff-1,4-thiazine (5), 2H- 1,4-thiazine (6), and 4H- 1,4-thiazine (7). 

Attempts to utilize aldehydes in place of ketones in the foregoing reactions have met with limited success.51 Thus propanal, in the presence of N,N-dimethylformamide or potassium carbonate, reacted with sulfur and aziridine to give a 1.3 1 mixture of 3,4-dihydro-6-methyl-2H-1,4-thiazine and 2-ethylthiazolidine in 23% yield. Although somewhat better yields were achieved by using butanal and pentanal, the thiazolidines were then the predominant products. 

An example of process 92 is provided by the reaction of thiazolium salts with dialkyl acylphosphonates it provides a valuable synthetic entry to 5,6-dihydro-2H-1,4-thiazin-3-ones. 

There is very little information on the NMR spectral properties of 5,6-dihydro-2H-1,4-thiazines. The vinylic proton of 87 absorbs at 8 8.04 and it displays long-range coupling (J = 2.3 Hz) with the 4-proton.65... 

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