epoc ABSTRACT: 1 H NMR spectroscopy was used to investigate hydrogen bonding in the structurally related (Z)-and (E)-5-substituted-2-alkylidene-4-oxothiazolidines in polar and apolar solvents. The equilibrated mixtures of these typical push-pull alkenes consist of the intramolecularly H-bonded E-isomer and intermolecularly H-bonded Z-isomer in varying proportions which depend on the solvent polarity. For a representative of the series, (E)-(5-ethoxycarbonylmethyl-4-oxothiazolidin-2-ylidene)-1-phenylethanone (1), the lack of a concentration and temperature dependence of the large chemical NH shift ( 12.06 ppm) in CDCl 3 indicates strong intramolecular resonance-assisted hydrogen-bond formation (RAHB). The upfield chemical shifts of the NH proton of the (Z)-1 isomer as a function of temperature increase and the large 1 H NMR Á/ÁT value (À11.82 ppb C À1 , Z/E
The title compounds containing an exocyclic double bond of exclusively the Zconfiguration were prepared in anhydrous ethanol by the regioselective base catalyzed reaction of diethyl 2-mercaptobutanedioate with nitrile precursors possessing an acidic a-hydrogen. The 'H NMR data indicating the presence of both geometrical isomers in primarily nonpolar media favoring the Eform are presented in terms of the solvent influence on intra-and intermolecular H-bonding and stereochemical outcome of the reaction. Various 4-oxothiazolidines, possessing a trisubstituted exocyclic double bond at C-2, have attracted considerable interest due to their structural diversity and wide variety of their biological activities.' Likewise these derivatives can he useful synthetic intermediates that offer access for preparing different heterocyclic systems.''2 Although examination of the literature data indicates that thiazolidine derivatives 3 of this type exist nearly exclusively as conjugated enamines, there appears to be no direct and unambiguous evidence concerning the stereochemistry of the exocyclic double h~n d .~'~ Thus, the purpose of this paper is to report the regioselective synthesis and spectral characterization of as yet unreported (4and (E)-5-ethoxycarbonylmethyl-4-oxothiazolidin-2-alkylidene derivatives (3a-c) (Scheme I), resembling known pharmacologically active agents,' from the 0x0 nitriles (1) and diethyl 2mercaptobutanedioate. A number of pure 2-isomers of this class have been characteri~ed,~.' hut the separation and the spectral data of the Ecounterparts are either very sparse, or absent.'"
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