The conformation of t-butyl 2-methoxy-5, 6-dihydro-2 H-pyran-6-carboxylates and 6,6′- disubstituted 2-methoxy-5,6-dihydro-2H-pyran derivatives on the basis of 1H and 13 C NMR spectra

The ¹H and ¹³C NMR spectra of trans- and cis-tert-butyl 2-methoxy-5,6-dihydro-2H-pyran-6-carboxylates (1 and 2) and 6,6′-disubstituted 2-methoxy-5,6-dihydro-2H-pyrans (3-7) have been recorded. HH and CH coupling constants are discussed in terms of the ¹H₆?⁶H₁ conformational equilibrium. It has been found that 1 occurs exclusively in the ¹H₆ conformation, whereas its cis isomer, 2, exists in an equilibrium of both half-chair forms. 6,6′-Disubstituted 2-methoxy-5,6-dihydro-2H-pyrans 3-6 display spectral and conformational behaviour similar to that of 1, whereas 7 resembles 2 in this respect.