NMR and molecular mechanics study of pyrethrins I and II

Bioassay-directed fractionation of the organic extract of the Kenyan pyreth rum flowers (Chrysanthemum cinerariaefolium Vissiani) resulted in the isola tion of two natural pyrethrin eaters, pyrethrin I (PI) and pyrethrin II (PI I) as the major constituents. These esters elicited inhibition of the multi ple drug resistant (MDR) Mycobacterium tuberculosis. The high-field H-1 and C-13 nuclear magnetic resonance (NMR) chemical shifts of PI and PII were u nequivocally assigned using modern two-dimensional (2D) proton-detected het eronuclear multiple-quantum coherence (HMQC) and heteronuclear multiple-bon d correlation (HMBC) experiments. The conformations of both esters were ded uced from H-1-H-1 vicinal coupling constants and confirmed by 2D nuclear Ov erhauser effect spectroscopy (NOESY). Computer molecular modeling (MM) stud ies revealed that PI and PII molecules adopt a "love-seat" conformation in chloroform (CDCl3) solution.