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

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