1.3.4- Thiadiazolines, ring-chain tautomerism

Ring-chain tautomerism was studied for 1,3,4-thiadiazolines 296 and 1,3,4-thiadiazolidines 297 (Scheme 109) [78MI2, p. 159 84CHEC-I(6)545, p. 557 88KGS3 95ZOB705 96AHC(66)1, pp. 44, 46, 52]. In all these cases the equilibria are shifted to the cyclic structures. Quantitative aspects of these equilibria have been discussed elsewhere [96AHC(66)1, pp. 44 6]. 

Proton, C, and N NMR spectroscopy has been employed to demonstrate the existence of ring-chain tautomerism between protonated forms of thiosemicarbazones and protonated 2-aryl-5-amino-2,3-dihydro-1,3,4-thiadiazoles <92KGS1689>. The use of NMR in various solvents, indicates that the isoxazoline (28) contains between 24% and 34% of the thiadiazoline (29) <85KGS1001>. Tautomerism was also discussed in Sections 4.10.2 <90BCJ2991> and 4.10.3.3 <83HCA1755>. 

Synthesis and ring-chain tautomerism of 2-amino-l,3,4-thiadiazolines 93KGS991. 

The realization of this new type of ring-chain tautomerism suggested the existence of reverse changes, so that compounds of type (151), hitherto considered linear, might assume the tautomeric A -l,3,4-thiadiazoline structure (153) in acidic media ie.g. in F3CCO2D). N.m.r. measurements made with thiosemicarbazones (151 R = NHj or NHMe) and hydrazones of alkyl dithiocarbazates (151 R = SMe) fully confirmed these expectations. ... 

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