Synthesis and characterization of chlorophyllaenol derivatives: Chlorophyllatert-butyldimethylsilyl-enol ether and 131-deoxo-131, 132-didehydro-chlorophylla

Abstract

Using the sterically hindered base, 1,8-diazabicyclo[5.4.0]undec-7-ene, for enolization and tert-butyldimethylsilyl-trifluoromethanesulfonate for silylation, chlorophyll (Chl) a produced after 15 min at 0 °C in deaerated pyridine solution under argon, after work-up and chromatographic purification on a sucrose column, tert-butyldimethylsilyl-enol ether of Chl a in a yield of 77%. The 13 1 -deoxo-13 1 ,13 2 -didehydro-chlorophyll a, was obtained in a yield of 23% through a reaction sequence, where Chl a was first reduced with sodium borohydride to 13 1 (R,S)-hydroxy-Chl a, which via demetalation yielded 13 1 (R,S)-hydroxypheophytin a. In the presence of the sterically hindered base, 1,8-bis(dimethylamino) naphthalene, trifluoroacetylimidazole dehydrated 13 1 (R,S)-hydroxypheophytin a to 13 1 -deoxo-13 1 ,13 2 -didehydro-pheophytin a, which after metalation yielded 13 1 -deoxo-13 1 ,13 2 -didehydro-Chl a. Using 1,8-bis(dimethylamino)naphthalene and trifluoroacetylimidazole, the straight conversion of 13 1 (R,S)-hydroxy-Chl a to 13 1 -deoxo-13 1 ,13 2 -didehydro-Chl a was found unsuccessful. The major products were characterized by electronic absorption spectra (UV-vis) and practically completely assigned 1 H and 13 C NMR spectra. Some intermediates of the syntheses were also characterized by ESI-TOF mass spectra. Compared with Chl a, the macrocyclic ring-current in the synthesized Chl a enol derivatives was found weakened by the expansion of the p-system to include the isocyclic ring E. Nevertheless, these enol derivatives were still considered to be diamagnetic and aromatic. The possibility of the functional role of the enol derivatives of chlorophyll in photosynthesis is discussed.