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

## TL;DR

This paper provides a detailed NMR analysis of kaurenoic acid and its methyl ester, offering the most complete data set for this natural compound.

## Contribution

The study presents the first comprehensive and step-by-step NMR assignment for kaurenoic acid and its derivative.

## Key findings

- Multidimensional NMR techniques and simulations enabled complete 1H and 13C NMR assignments.
- Chemical shift variations between kaurenoic acid and its methyl ester were identified and compared.
- The use of different deuterated solvents improved data clarity and reliability.

## 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.