The Challenging Complete and Detailed ¹ H and ¹³ C NMR Assignment for <i>ent</i> -Kaurenoic Acid, a Remarkable Natural Product

In this work, we report a comprehensive structural assignment of the ¹H and ¹³C nuclear magnetic resonance (NMR) data for kaurenoic acid (KA, <i>ent</i>-kaur-16-en-19-oic acid), a natural diterpene with diverse biological activities. To support this analysis, the methyl ester derivative (<i>ent</i>-methyl-kaur-16-en-19-oate) was also investigated, allowing comparison of chemical shift variations arising from subtle structural differences. The complete assignment was achieved through the analysis of multidimensional NMR spectra and accurate determination of ¹H-¹H coupling constants and signal multiplicities. Experiments including ¹H NMR, ¹³C NMR {¹H}, g-COSY, g-HSQC, g-HMBC, and J-resolved were employed, complemented by spectral simulations using FOMSC3 and SimEsp_NMR software. Measurements in different deuterated solvents further clarified overlapping regions and enhanced data reliability. This approach resulted in the most detailed and complete NMR data set to date for KA and its methyl ester. To our knowledge, no previous studies on KA have provided this level of spectroscopic detail or a step-by-step description of the data acquisition, underscoring the novelty and relevance of this work.