Syntheses and UV Spectroscopic Study of Mono- and Dialkyloxy-4-methylcoumarins

Mono- and dialkyloxy-4-methylcoumarins were successfully synthesized starting from dihydroxy-4-methylcoumarins and an alkyl bromide, butyl bromide or octyl bromide, at 1:1 molar ratio. The alkylation of 5,7-dihydroxy-4-methylcoumarin gave 5,7-dialkyloxy-4-methylcoumarin and two monoalkyloxy-4-methylcoumarins with substitution at 5 and 7 positions. For 6,7- and 7,8-dihydroxy-4-methylcoumarins, the corresponding dialkyloxy-4-methylcoumarins and only monoalkyloxy-4-methylcoumarins at the 7 position were obtained. UV absorption studies of the dihydroxy-4-methylcoumarins and their mono- and dialkyloxy products showed a red shift of the maximum absorption wavelengths compared to 4-methylcoumarins. Additionally, substitution of a hydroxyl or alkyloxy group at the 6-position caused a further red shift. The dimerization degree from decreasing of UV absorbance revealed that it depended on the substitution positions. Among all these coumarin derivatives, 5,7-dibutyloxy-4-methylcoumarin exhibited the highest dimerization degree.