DETERMINATION OF THE ABSOLUTE STEREOCHEMISTRY OF NEMADECTIN-ALPHA-2 AND NEMADECTIN-ALPHA

The CD exciton chirality has been applied to determine the absolute configuration of the nemadectins (formerly LL-F28249 series). The CD spectra of the 5-benzoates 6 and 7 of nemadectins alpha (1) and alpha(2) (5), respectively, show that both have R configurations at C-5. Furthermore, what appears to be the short-wavelength wing of a split CD due to an exciton-coupled allylic benzoate system has been observed for the first time at ca. 190 nm. The CD of the 5,6-bis(p-methoxycinnamate) 23-acetate (9) of nemadoctin alpha-2 also shows that the C-5 and C-6 configurations are both R. The consistent results from both approaches establish the absolute configuration of this important group of antiparasitic agents.