Conformational analysis. Part 30. The conformational analysis of some lactones by the lanthanide induced shift (LIS) technique

Abstract

An improved LIS technique, using Yb(fod)₃to obtain the paramagnetic induced shifts of all the spin 1/2 nuclei in the molecule, together with complexation shifts obtained by the use of Lu(fod)₃, has been used to investigate the conformations of several lactones. The appropriate complexation model was obtained by investigations on the planar well-defined structures of isocoumarin (1) and coumarin (2). This complexation model was then used to investigate the conformations and conformational equilibria in 3,4-dihydrocoumarin (3), β-butyrolactone (4), γ-valerolactone (5), δ-valerolactone (6) and ε-caprolactone (7).3,4-Dihydrocoumarin is puckered with both C2 and C3 displaced from the benzene ring plane. β-Butyrolactone is planar. γ-Valerolactone interconverts between the two envelope conformations with C4 out of the plane of the other ring atoms with 70% of the conformer with a pseudo-equatorial methyl group. For δ-valerolactone the two interconverting conformations are the half-chair and the boat form and analysis of the data suggests that there is ca. 20% of the boat form. In ε-caprolactone the LIS data gives a well-defined minimum for 100% of the chair form with no other significantly populated conformer. The LIS results agree with both the ab initio and MM optimised geometries and the observed and calculated conformer energies are in reasonable agreement to give ΔG (ax–eq) 0.6 kcal mol^-1 for γ-valerolactone, ΔG (boat–half-chair) 0.9 kcal mol^-1 for δ-valerolactone and ΔG (boat–chair) > 3.5 kcal mol^-1 for ε-caprolactone.