AN NMR AND LC-MS BASED MIXTURE ANALYSIS APPROACH FOR STRUCTURAL ELUCIDATION OF ANTIMICROBIAL DERIVATIVES OF CALOPHYLLOLIDE

Abstract

This account details the structural modification of Calophyllolide 1, using Ritter reaction with various nitriles to study the antimicrobial properties. Ritter reaction of calophyllolide results in a 1:1 mixture of two structurally related compounds 2 and 3. A stereogenic center was generated in the reaction products resulting in a racemic mixture. NMR mixture analysis strategy was employed for the chemical structural analysis. 2D NMR experiments such as gDQCOSY, gHSQC, gHMBC at 500 MHz, and LC-MS analysis allowed the complete assignment of ¹H and ¹³C NMR spectra of mixture components 2 and 3. These results suggested an unusual carbon-carbon bond cleavage in the reaction product 3. Our effort highlights the NMR mixture analysis approach for characterization of mixture of compounds. An efficient and operationally simple chiral method by HPLC was developed to evaluate the enantiomer compositions. Based on the aforementioned spectral data, the compounds 2 and 3 were characterized as (E)-N-(5-methoxy-2,2-dimethyl-6-(2-methylbut-2-enoyl)-8-oxo-10-phenyl-2,8-dihydropyrano[2,3-f]chromen-4-yl)acetamide and N-(5-methoxy-2,2-dimethyl-8-oxo-10-phenyl-2,8-dihydropyrano[2,3-f]chromen-4-yl)acetamide, respectively. The possible mechanisms for these compounds formation were also proposed.

Keywords:

• 2D NMR
• calophyllolide
• characterization
• LC-MS
• mixture analysis
• ritter reaction
• structural modification

ACKNOWLEDGMENT

We would like to thank Dr. Pratap Reddy for the chemistry support. We would also like to thank R. Vasudev and all the co-workers, who significantly contributed to this work.

Notes

¹Refer the atom numbering in Figure 3. The correlations shown in the ¹H and ¹³C NMR columns are in agreement with HSQC and HMBC spectra.