Thiopyrones properties

Whereas zwitterionic forms may be invoked to explain the properties of 2- and 4-thiopyrones, the 3-thiopyrone can only exist as the betaine (42) and is therefore to be compared with the pyridinium system for a bonding and structural likeness. A theoretical treatment (73JPR690) has been published using CNDO/2 methods, and the calculated parameters were found to agree well with experimentally determined spectral properties. 

Compound 139 and the other derivatives of 3,5-epidithio-2,4-pentadien-1 -one (132), which are obtained in good yields in accordance with Scheme 8, exhibit only weak ketonic properties. This has been interpreted as resulting from polarization towards the betaine structure (140). Similar effects and explanations are already familiar from the cases of tropone102 and 1,4-thiopyrone.103... 

During the last years remarkable advances have been made in the chemistry of thiopyrones, the main types of which are represented by formulas 3-8. The present review reports on the monocyclic sulfur-containing 4//-pyran-4-ones (3, 4, 5), types A, and 2 -pyran-2-ones (6, 7, 8), types B, which derive from the pyrones (1 and 2), dealing with their synthesis, and their chemical and physical properties. 

As can be seen from the foregoing, it does not seem possible, in the light of present knowledge, to make valid generalizations concerning the reactivity of the individual thiopyrones, their aromatic character, or other properties. In special cases reactivity and stability suggest a certain aromatic behavior of thiopyrones (see Ref. 128), but other facts seem to contradict this. It is debatable whether one can deduce from the physical data an important contribution from the zwitter-ionic state and a pronounced aromaticity from the salt like character which is a result of this. 

Metal chelates of thio-analogues of -diketones are still the subject of current research, and a variety of papers, dealing with preparation, reactions, and physical properties of such compounds, have appeared during the period under review/ " A recent paper, describing the preparation and properties of a series of metal chelates of 2-methyl-3-hy-droxy-4-thiopyrone (81), should also be studied/ ... 

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