Complexation of hydroxylated ent-kaurane diterpenoids with boric acid as an aid to the assignment of the 13C NMR spectra

The ¹³C NMR spectra of some hydroxylated ent-kaurane diterpenoids were measured in CDCl₃-Py-d₅ (1:1) solution and also after addition of boric acid. Complexation of ent-3β,18-, ent-7α,15β-, ent-7α,15α-, ent-15β,16β- and ent-16β,17-dihydroxy derivatives with boric acid produced considerable chemical shift changes and marked broadening of the signals of the hydroxy-bearing and neighbouring carbon atoms. This behaviour provides a useful and reliable method for assigning the ¹³C NMR spectra of these compounds.