Complete 1H NMR assignment for poorly functionalized molecules: the case of copalic acid

Diterpenes(DT) are an important class of natural products and attract attention mainly by their wide range of activities(1).Among the different DT, copalic acid(1) is associated to some antimicrobial, antitubercular and cytotoxic activities(2–5). However, it’s poorly functionalized chemical structure, provides complex 1H-NMR spectrum multiplets that cannot be easily understood by simple inspection. Firstly, this work provided all 23 hydrogen chemical shifts for this molecule, of which only 5 were assigned before(5). All multiplicities of 1H-NMR signals from 1 were determined, including the clarification of some multiplicities doubtfully assigned before.The confirmation was achieved by comparing the complex experimental signals of H14,H17a,H17b,(H12b+H7b),(H1a+H6a),(2a+H9a) and (H11b+H2b) with the calculated spectra for these hydrogens, using FOMSC-3 and SimEsp_NMR programs(6).This work provides a complete set of simulation of complex splittings of 1H NMR for 1, which can be very useful to confirm all assignments and all J values of poorly functionalized natural and synthetic substances.