Transformation of carbonyl to vinylidene groups in the π-conjugated peripheral substituent of chlorophyll derivatives by Tebbe reagent

Abstract

The 131-oxo-moiety of chlorophyll-a derivatives including methyl (pyro)pheophorbides-a, mesopyropheophorbide-a, and bacteriopheophorbide-d possessing vinyl, ethyl, and 1-hydroxyethyl groups, respectively, at the 3-position was transformed into the corresponding exo-methylene group by treatment of Tebbe reagent. The synthetic procedures were useful for the regioselective conversion of the carbonyl to vinylidene group at the 3-position and naturally occurring chlorophyll-d (3-CHO) was successfully modified to chlorophyll-a (3-CHCH2). Under the methylenation conditions, ester–carbonyl, hydroxy, and allyl groups in the peripheral substituents were tolerant as well as an acid-labile magnesium at the central position of chlorophylls.