Tautomerism in Other Five-Membered Ring Lactams

Tautomerism in Other FIve-Membered Ring Lactams 

While 19 parallels 7, the introduction of C=C into 20 makes it an imidazolone. This has not been previously examined but the corresponding thione, 4,5-dimethyl substituted, was studied by Kjellin and Sandstrom [16] and possesses [4] logiCj 8.0. In most such comparisons, log/C for the thione is slightly the higher 

Perhaps the most interesting compounds in this series are those designed to explore the effect of placing another it-donor adjacent to NH. When C=C is added so that 25 and 26 become pyrazolones and isoxazolones respectively, it is well established [3] that many or even perhaps most such compounds exist in water chiefly as iminols, but no attempt so far has been made to dissect this transformation into its separate components. It appears possible [17] that, in simple cases such as 23 and 24, the effect of contiguous NH or Af-alkyl, and of O, could be as high as AlogAj -2.5 and -5 respectively, comparable in fact to the added effect in 25 and 26 of aromaticity. If these compounds can be successfully investigated this will result in the first true quantification of both phenomena. (Note that to use NMe in place of NH in 25 is necessary to avoid a competitive annular tautomerism.) 

Tautomeric Ratios Requiring Computation Alicyclic p-Diketones 

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