Photoproducts of 7-Hydroxycoumarins in Aqueous Solution
Gerald J. Smith A E , Roderick J. Weston A , Ying Tang B , Yinrong Lu C and Jolon M. Dyer DA School of Chemical and Physical Sciences, Victoria University of Wellington, PO Box 600, Wellington 6140, New Zealand.
B Department of Biotechnology, School of Sciences, Beijing Technology and Business University, No. 11 Fucheng Road, Beijing 100048, P. R. China.
C Industrial Research Limited, PO Box 31310, Lower Hutt 5040, New Zealand.
D AgResearch Lincoln, PB 4749, Christchurch 8140, New Zealand.
E Corresponding author. Email: gerald.smith@vuw.ac.nz
Australian Journal of Chemistry 65(10) 1451-1456 https://doi.org/10.1071/CH12292
Submitted: 17 June 2012 Accepted: 3 August 2012 Published: 12 September 2012
Abstract
Near-UV irradiation of aqueous solutions of 7-hydroxy-6-methoxycoumarin (scopoletin) and 6,7-dihydroxycoumarin (esculetin) resulted in the formation of dimers. Substantially greater amounts of the 7-hydroxy-6-methoxycoumarin photodimer were produced compared with that of 6,7-dihydroxycoumarin. Fluorescence spectra indicated that the initial photolytic reaction involves the excited state of the phenolate form of the monomeric coumarin. The electro-spray ionization (ESI) mass spectrum suggested that the major product (with a sodiated quasimolecular ion at m/z 405) from the photolysis of scopoletin in water had formed by oxidative dimerisation and possibly had a 3,3′-bond. A second dimer (with a sodiated quasimolecular ion at m/z 407) was probably a 2 + 2 cyclodimer. This work concluded that photolysis of coumarins in water results in oxidative dimerisation whereas photolysis in the solid state or in organic solvents results in cyclodimerisation. Minor monomeric photoproducts were a dihydroxy-6-methoxycoumarin from 7-hydroxy-6-methoxycoumarin and a trihydroxycoumarin from 6,7-dihydroxycoumarin. During ESI-mass spectrometry (ESI-MS), sodiated quasimolecular ions of the photodimer formed more readily than protonated quasimolecular ions and the sodiated ions did not fragment readily. Protonated quasimolecular ions of the monomeric coumarins formed more readily than their sodiated counterparts and the protonated ions fragmented to create daughter ion spectra that were useful for chemical structural assignment.
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