# Stereochemical Analysis of Natural Products: Bisindole Alkaloids of the Strychnos-Strychnos Type

## TL;DR

This paper uses computational methods to analyze the stereochemistry of bisindole alkaloids from Strychnos plants using NMR data.

## Contribution

The study introduces a novel methodology combining conformational analysis and high-level DFT NMR calculations for stereochemical elucidation.

## Key findings

- A good agreement between calculated and experimental NMR chemical shifts was achieved with low error margins.
- The methodology effectively identifies conformational states in complex bisindole alkaloids.
- Statistical descriptors were successfully used to correlate experimental and calculated NMR data.

## Abstract

Results of the high-level computational study of 12 bisindole alkaloids of the Strychnos-Strychnos type are reported in addition of that of five retuline-like monomers frequently encountered in moieties constituting dimeric alkaloids. Based on the comparison of theoretical and experimental NMR chemical shifts, a detailed conformational survey of these stereochemically rich natural products containing multiple asymmetric centers was performed. Our original methodology is based on searching multiple conformational states, geometry optimization, and high-level NMR calculations at the DFT level. Taking into account all known experimental chemical shifts together with their calculated values in the natural products under study, a correlation estimate of NMR chemical shifts was performed by using statistical descriptors. In general, a good agreement of the performed calculations with experiment was achieved, which is manifested by the Corrected Mean Absolute Error of about 0.2 ppm for 1H and 1.9 ppm for 13C NMR chemical shifts in this series.