The Challenging Complete and Detailed 1H and 13C NMR Assignment for Ent-Kaurenoic Acid, a Remarkable Natural Product

In this work, the primary objective was to achieve a complete and detailed structural assignment of the 1H and 13C nuclear magnetic resonance (NMR) data for kaurenoic acid (KA) (ent-kaur-16-en-19-oic acid), a widely studied natural diterpene with various reported biological activities. To facilitate this, the kaurenoic acid methyl ester (ent-methyl-kaur-16-en-19-oate) was also elucidated, leveraging its slight structural differences and the corresponding variations in chemical shifts. The comprehensive assignment was accomplished through the analysis of spectra obtained from each sample, enabling the measurement of all hydrogen homonuclear coupling constants and the clarification of signal multiplicity in the 1H-NMR spectra. Consequently, NMR techniques such as 1H-NMR, 13C{1H}, g-COSY, g-HSQC, g-HMBC, and J-resolved experiments were employed. Additionally, two different software programs, FOMSC3 and NMR_MultiSim, were utilized to simulate 1H-NMR signals, which further assisted in the complete assignment. Furthermore, different deuterated solvents were used to clarify overlapping regions and enrich the dataset.