Dipole Moment, Dynamic NMR, and Molecular Structure of Vinylogous 4 H ‐Pyrones

The molecular structure of the vinylogous 4 H ‐pyrone 2,6‐dimethyl‐4‐(2‐oxopropylidene)‐4 H ‐pyran ( 1 ) was determined by X‐ray diffraction. The molecule is practically planar and has the s‐cis conformation. The experimental dipole moment (3.71 D) of 1 agrees with the CNDO/2 calculated value (3.54 D) for the s‐cis conformation 1 and rules out the s‐trans conformation 2 . The doubly vinylogous 4 H ‐pyrone 4 , Ar = p ‐C 6 H 4 Cl, has an experimental dipole moment of 5.65 D. Corrected 13 C chemical shifts of the ring β‐ and especially α‐carbons decrease in the series 2,6‐dimethyl‐4 H ‐pyrone ( 10 ) to 1 to 4 , indicating that the higher vinylogs have decreased contributions of the dipolar canonical structures. Dynamic 1 H NMR data, obtained using the two‐dimensional accordion technique, indicate that 1 undergoes rotation around the exocyclic C = C bond with a free energy of activation Δ G ± = 22.4 kcal/mol in [D 6 ]DMSO.