Application of dynamic 1H-NMR spectroscopy added to the understanding of the hydrogen bonds existing in the structurally related 5-substituted-2-alkylidene-4-oxothiazolidines in polar and apolar solvents. The equilibrated mixtures of these typical push-pull alkenes in CDCl3 consist of the intramolecularly H-bonded (E)-isomer and intermolecularly H-bonded (Z)-isomer in varying proportions which depend on the solvent polarity. For the representative of the series (Z)-2-(5-ethoxycarbonylmethyl-4-oxothiazolidin-2-ylidene)-1-phenylethanone a concentration effect on the degree of intermolecular hydrogen bonding in apolar CDCl3 has been studied.
Pyridinium hydrobromide perbromide (PHBP) is a highly efficient reagent for the conversion of 5-substituted-2-alkylidene-4-oxothiazolidine derivatives to the corresponding thiazolidines with two fully delocalized exocyclic double bonds at the C(2) and C(5) positions. This conversion as a two-step bromination-rearrangement process occurs in acetonitrile under homogeneous reaction conditions.
Astudy on the regiospecificity of the base-catalyzed reaction of activated ?-oxonitriles 1 with diethyl mercaptosuccinate affording the title compounds 3 is reported. Other competitive heterocyclic products, that is 4-oxo-1,3-thiazinanes 4, derivatives of tetrahydrothiophene 5 and/or thiacyclohexane 6 which on the grounds of mechanistic considerations could be formed, were not observed. Spectroscopic and experimental evidence together with theoretical considerations, provides a reasonable explanation for the observed regiospecificity.
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