Absolute Configuration Assignment of 3',4'-di-O-acylkhellactones Using Vibrational Circular Dichroism Exciton Chirality.

The 3'R,4'R absolute configuration (AC) of the angular-type pyranocoumarins (-)-3',4'-di-O-acetylkhellactone (2), (-)-4'-O-acetyl-3'-O-angeloylkhellactone (3), (+)-3'-O-acetyl-4'-O-isobutyroylkhellactone (4), and (-)-3'-O-angeloyl-4'-O-senecioylkhellactone (5), isolated from the aerial parts of Prionosciadium thapsoides, was assigned by vibrational circular dichroism exciton chirality (VCDEC), and confirmed by comparison of their VCD frequencies with those calculated using DFT at the B3LYP/DGDZVP level. This again reveals that AC assignments based on optical rotation data are not very confident. Evaluation of Flack and Hooft parameters obtained after single-crystal X-ray diffraction analysis of 3, independently confirmed this AC.