Synthesis of 18O-Labelled chlorophyll derivatives at carbonyl oxygen atoms by acidic hydrolysis of the ethylene ketal and acetal

Abstract

The ethylene ketal of pyropheophorbides, chlorophylls possessing the 13-keto carbonyl group and lacking the 13 2-methoxycarbonyl group, reacted with H 2 18O (ca. 95% 18O atom) by acidic hydrolysis to give efficiently and regioselectively 13 1- 18O-oxo-labelled compounds (ca. 92% 18O). The resulting 18O-labelled chlorin was modified by several chemical reactions to afford some derivatives with little loss of the 18O atom. Following the same procedures, 3 1,13 1-doubly- 18O-labelled pyrochlorophyll derivatives were also prepared. All the synthetic 18O-labelled compounds were identified by FAB-mass and vibrational spectra. Especially, in the vibrational spectroscopic results including IR and resonance Raman spectra, an about 30 cm −1 wavenumber down-shift of the 3- and/or 13-CO stretching vibrational bands was observed by exchanging 3 1- or 13 1-oxo-oxygen atom from 16O to 18O.