Reverse anomeric effect of the carbamoyl group of 2,6‐disubstituted 6‐carbamoyl‐5,6‐dihydro‐2H‐pyrans

The ¹H NMR spectra of 2,6‐disubstituted 6‐carbamoyl‐5,6‐dihydro‐2H‐pyrans and their 6‐methoxycarbonyl analogs have been recorded. With the use of coupling constants between ring protons, the conformational preference of the carbamoyl group has been evaluated. The ¹H₆⇌⁶H₁ conformational equilibria in the above compounds are discussed. It is found that the conformational tendency of the carbamoyl group to the equatorial disposition helps to determine the conformational equilibrium in the seven examples studied.