A 1 H and 13 C NMR conformational study of four cis-fused N-substituted hexahydrocyclopent[e][1,3]oxazin-4-ones and four cis-fused hexahydro-2H-1,3-benzoxazin-4-ones at various temperatures revealed several dynamic conformational processes to be in effect. These included a clear conformational equilibrium between the O-in (major conformer) and O-out ring forms in the hexahydro-2H-1,3-benzoxazin-4-ones, an equilibrium consisting of two intramolecularly hydrogen bonded forms (between the hydroxyl proton and the carbonyl oxygen attached to C4 or the ethereal oxygen) and a non-hydrogen bonded form in two compounds, and a restricted rotation about the amide C-N bond in the CH 2 OCONHC 6 H 4 X-p substituents (X = H, Cl) in three other compounds.
A universal scheme is proposed for the molecular design of heterocyclic recyclizations by replacing the exocyclic hydroxyl groups in e x o -t r i g-ring-chain tautomeric molecules with substituted amines or hydrazines. The practical applicability of this approach is demonstrated by the condensations of 5-hydroxy-5-methyl-3-isoxazolidinones with thioaroyl-hydrazines and 2-aminomethylaniline. The condensation products were studied by modern 1 H, 1 3 C and 1 5 N NMR spectroscopic methods using three solvents: CDCl 3 , DMSO [D 6 ] and CD 3 CN. The solvent was found to have a strong effect to the relative amounts of the tautomers.
2002 ring closure reactions ring closure reactions O 0130
-039Recyclization of 2-Aminobenzylimines and Thioaroylhydrazones of N-Substituted N-Hydroxy-3-oxobutanamides.-Based on a general scheme, a method of recyclizations is achieved by substituting amino groups of substituted amines and hydrazines to the exocyclic hydroxyl group in exo-trig-tautomeric ring-chain systems, thereby introducing another cyclic endotrig-structure in the end product. To test this idea, the condensation products of isoxazolidinones (I) with thioaroylhydrazines (II) and aminomethylaniline (IV) are examined. In the case of products (III), only one cyclic tautomer is observed, whereas products (V) and (VI) exhibit up to three accessible forms (e.g. (V), (VII), and (VIII)) in equilibrium, their ratio dependent on the nature of the solvent used. -(ZELENIN, KIRILL N.; LAGODA, IGOR V.; ALEKSEYEV, VALERIY V.; SINKKONEN, JARI; SHAIKHUTDINOV, ROUSTEM A.; PIHLAJA, KALEVI;
1,3‐Dipolar cycloaddition reactions of N‐methyl‐C‐arylnitrones with N‐phenyl‐ or N‐methylmaleimide were studied. The reaction of p‐dimethylamino‐, 4‐benzyloxy‐3‐methoxy‐, p‐nitro‐ and p‐chloro‐substituted phenylnitrones with N‐phenylmaleimide gave cis and trans cycloadducts but that of the corresponding phenylnitrones with N‐methylmaleimides only the cis adducts in the case of p‐dimethylamino and 4‐benzyloxy‐3‐methoxy substitution. All cis adducts attain a biased conformation whereas the trans forms are shown (by 1H NMR at 233 K and 13C NMR at 208 K) to be mixtures of two invertomers, namely o‐(N‐lone pair antiperiplanar to 3H; minor) and i‐conformations (3H‐C‐C‐3aH dihedral angle close to 90°; major). PM3 and DFT calculations at the B3LYP/6–31G(d) level of theory prove qualitatively that these two conformers of the trans adduct are of comparable stability and represent energy minima.
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