3-HYDROXY-2-CYANOALK-2-ENAMIDES, AND 2-CYANO-2-(TETRAHYDROFURAN-2-YLIDENE)ACETAMIDES AND 2-CYANO-2-(TETRAHYDROPYRAN-2-YLIDENE)ACETAMIDES - SYNTHESIS, STRUCTURE, AND SOLVENT-DEPENDENT (Z) (E)-ISOMERISM/

3-Hydroxy-2-cyanoalk-2-enamides, and 2-cyano-2-(tetrahydrofuran-2-ylid ene)- and 2-cyano-2-(tetrahydropyran-2-ylidene)acetamides with N-alkyl and N-aryl substituents have been synthesized in three steps from cya noacetic acid. Their conformations were investigated by X-ray crystall ography and H-1-NMR ROESY spectroscopy at room temperature. The enolic compounds 1-3 adopt an extended conformation stabilized by a strong i ntramolecular O-H ... O=C bond both in the solid state and in (D-6)DiM SO solution. In contrast, the structure of the cyclic derivatives 5a,b -8a,b is solvent-dependent. In the solid state and in CDCl3 solution, the compounds adopt an extended conformation of type I or III, while, in (D-6)DMSO solution, their structures undergo time-dependent (Z)/(E) -isomerization structures (of type II or IV). This observation is comp atible with a dipolar transition state of rotation. The kinetics of th e isomerization are controlled by the N-substituent, the N-(t-Bu) deri vatives 7a and 7b having the highest barrier of rotation around the C =C bond. The whole body of experimental evidence together with the res ults of molecular-mechanics calculations with I-IV, indicate that, in DMSO, two (E)/(Z)-isomers with two conformations are present, and that they undergo interconversion at room temperature with four different constants. The very fast exchange rates k(I,II) and k(III,IV) in the N MR time-scale might be responsible for the detection of only two isome rs.