Response to Heterocyclic tautomerism: reassignment of two crystal structures of 2-amino-1,3-thiazolidin-4-one derivatives by Gzella et al. (2014).

Prototropy tautomerism and E/Z-stereoisomerism of 2-iminothiazolidin-4-ones was extensively studied in the literature during the past several decades. This tautomerism was studied for both alkyl/aryl–amino/imino-substituted thiazolidinones/ thiazolinones by several authors (Comrie, 1964; Åkerblom, 1967; Chowdry et al., 2000; Mornon & Raveau, 1971; Ramsh et al., 1978) in the solid state, as well as in solution, and the results can be generalized as follows. Exo-N-unsubstituted and N-alkyl-substituted compounds exist in solution preferentially as 2-(alkyl)aminothiazolin-4-ones, whereas N-aryl-substituted compounds prefer the 2-(arylimino)thiazolidin-4-one arrangement, although some exceptions to this rule were also reported (Sedlák et al., 2002; Kammel & Hanusek, 2014). For example, in a recent article (Kammel & Hanusek, 2014) we have found that base-catalyzed cyclization of N-isopropyl-S-(2-oxo-2,3dihydro-1-benzofuran-3-yl)isothiouronium bromide gives 5-(2-hydroxyphenyl)-2-isopropylimino-1,3-thiazolidin-4-one, which during crystallization or simple heating undergoes the above-mentioned prototropy tautomerism to give 5-(2-hydroxyphenyl)-2-isopropylamino-1,3-thiazolin-4-one. From this observation it is evident that the solvent, purification method, temperature and other parameters can influence the position of tautomeric equilibrium and also the crystal packing. In our article from 2009 (Váňa et al., 2009), the single crystal of 5-(2-hydroxyethyl)-2-[(pyridin-2-yl)imino]-1,3-thiazolidin-4one was grown from dimethyl sulfoxide (DMSO), whereas the single crystal studied by Gzella et al. (2014) was grown from methanol from a sample previously prepared in refluxing ethanol. In order to support our refinement (Váňa et al., 2009), the following arguments are given: three peaks assignable to the ‘amino or imine’ H atoms were located on our Fourier difference map under the given measurement conditions (150 K). Both tautomers were refined but based on the known NMR assignment, literature data and higher electron density we decided to refine the structure as the imine tautomer. To conclude, we are surprised how Gzella et al. (2014) could suggest that our structure had an incorrectly-positioned H atom based on the measurement under different conditions on crystals grown from a different solvent, and last, but not least, from a disordered structure, when tautomerism is always a chemical equilibrium.