Carbonyl carbon chemical shifts in the 13c spectra of cyclic ketones

The ketonic ¹³C NMR signals of the keto lactones 1 and haplophytine (4) occur at exceptionally high field. These upfield shifts are interpreted in the context of a general consideration of the structural factors affecting the carbonyl carbon chemical shifts of cyclic ketones. It is concluded that dipole-dipole interactions are the major sources of the upfield shifts in the cases of both 1 and 4.