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The Challenging Complete and Detailed 1H and 13C NMR Assignment for ent-Kaurenoic Acid, a Remarkable Natural Product
Alexsandro Eurípedes Ferreira1, Ana Carolina Ferreira Soares Rocha1, Julian Carlos da Silva Pavan1
1Núcleo de Pesquisas em Ciências Exatas e Tecnológicas, Universidade de Franca, Av. Dr. Armando de Salles Oliveira, 201 - Pq. Universitário, 14404-600, Franca, SP, Brazil.
Abstract:
In this work, we report a comprehensive structural assignment of the 1H and 13C nuclear magnetic resonance (NMR) data for kaurenoic acid (KA, ent-kaur-16-en-19-oic acid), a natural diterpene with diverse biological activities. To support this analysis, the methyl ester derivative (ent-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.
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