Complete assignment of 1H and 13C Nuclear Magnetic Resonance data for polyalthic acid, a labdane diterpene with biological properties

In this work we performed detailed studies of 1H and 13C NMR data of polyalthic acid, a labdane diterpene with great variety of biological activities, such as fungicide, anti-inflammatory and bactericidal. For the accomplishment of this work, 1H and 13C NMR experiments were carried out in different solvents in order to avoid some overlapping on 1H NMR spectra. The study included the determination of all homonuclear hydrogen coupling constants and clarified the multiplicity for all 1H NMR signals. Thus, techniques such as 1H NMR, 13C {1H} NMR, COSY, HSQC and HMBC were applied. Moreover, the use of computer programs for the simulation of 1H NMR signals were also of great contribution for the complete assignment. Due to this work, a significant set of 1H and 13C NMR data for polyalthic acid was obtained. It also allowed to provide important data to the literature and to correct some mistaken assignments.