1H chemical shift differences of Prelog–Djerassi lactone derivatives: DFT and NMR conformational studies

This work reports an experimental and theoretical study of the conformational preferences of several Prelog–Djerassi lactone derivatives, to elucidate the 1H NMR chemical shift differences in the lactonic core that are associated with the relative stereochemistry of these derivatives. The boat-like conformation of 2 explains the anomalous 1H chemical shift between H-5a and H-5b, in which the two methyl groups (C-8 and C-9) face H-5b, leading to its higher shielding effect.