Effects of Hydroxypropyl Cyclodextrins on Photo-Reactions of Coumarins

The effects of hydroxypropyl cyclodextrins (HPCDs) on the photo-dimerization of coumarin, 7-hydroxycoumarin, and 7-acetoxycoumarin were investigated in an aqueous phase. 7-acetoxycoumarin was derivatized from 7-hydroxycoumarin using sodium acetate, confirmed by H-1 NMR spectrum. The effectiveness to solubilize coumarin was in the order of HP gamma CD>HP beta CD>HP alpha CD. The benzene ring seemed to interact with HPCDs. The effectiveness to solubilize 7-hydroxycoumarin and 7-acetoxycoumarin was in the order of HP beta CD>HP gamma CD>HP alpha CD. Due to the hydroxyl groups and acetoxyl ones, they seemed to interact with HPCDs through their hydrophobic parts opposing to the polar groups. HP gamma CD promoted the photo-dimerization of coumarin, but HP alpha CD and HP beta CD had little effect. Due to the large cavity, HP gamma CD may include more than one molecule of coumarin, leading to a close contact of the double bonds. HP gamma CD promoted the photo-dimerization of 7-hydroxycoumarin and 7-acetoxycoumarin, but HP beta CD suppressed the photo-dimerization. The cavity of HP beta CD may stably sequester the hydrophobic part including double bond of coumarin possibly due to their comparable sizes.