Synthesis of regioselectively 18O-labelled chlorophyll derivatives at the 31- and/or 131-positions through one-pot exchange of carbonyl oxygen atoms

31- and/or 131-18O-oxo-labelled methyl pyropheophorbides (84–90% 18O atoms at each position) were prepared by exchanging carbonyl oxygen atoms under biphasic conditions of acidic H218O (ca. 95% 18O) and dichloromethane. The (un)labelling occurred more rapidly at less sterically hindered (13-Cdouble bond; length as m-dashO possessing 132-COOCH3>13-Cdouble bond; length as m-dashO lacking it) or more reactive carbonyl groups (formyl>keto group), and not at any hydroxy groups. Reduction of a carbonyl to carbinol group was useful for preparation of regioselectively 18O-labelled chlorophyll derivatives. Following the labelling procedure, 131-18O-pheophytin-a and 31-18O-pheophytin-d were obtained. All the synthetic 18O-labelled compounds were characterized by their FAB-mass, 13C NMR and IR spectra. Especially, 18O-labelling induced 0.02 (13C–O) and 0.04–0.05 ppm high field shifts (13Cdouble bond; length as m-dashO) in 18O-attached carbon resonances and about 30 cm−1 down-shifts in 18O-labelled carbonyl stretching vibrational bands.