Probing the anomeric effect and mechanism of isomerization of oxazinane rings by DFT methods

Mechanistic studies of the thermal amine-promoted isomerization of oxazinane rings by DFT methods showed that the isomerization proceeds through abstraction of the C-3 hydrogen atom by the amine nitrogen atom followed by its re-recruitment from C-3 that helps the oxazinane ring to avoid breaking, leading to the same or an isomeric conformer. Calculations also provided evidence that steric effects are responsible for the breaking of the O-N bond in the transition state of the thermal amine-promoted transformations of oxazinane rings, leading to the transformation of the 6-membered ring to a 5-membered ring. Extensive computational studies of the origin of the anomeric effect in the di-substituted oxazinane rings, bearing the EtO substituent at C-6 and CO 2 Et at C-3, and a series of analogous tetrahydro-2 H -pyran ring conformers, revealed that the conformational preferences in both series of compounds are tuned by the balance of non-covalent (weak vDW, dipole-dipole, electrostatic forces, hydrogen bonding) steric effects and hyperconjugative interactions. Oxazinane rings can undergo isomerization or transformation to 5-membered rings tuned by the anomeric effect.