Novel Synthetic Routes to 8-Vinyl Chlorophyll Derivatives

Abstract

New methodology was developed toward the synthesis of 8-de-ethyl-8-vinylchlorophyll-a 1. Such 8-de-ethyl-8-vinyl derivatives of the green plant pigment chlorophyll-a 2 have been proposed to be intermediates during biosynthesis of chlorophylls and bacteriochlorophylls. Transformation of the 8-ethyl group to an 8-vinyl group was studied on derivatives (e.g. 5) of chlorin-e6 trimethyl ester 9. The reported methodology involves regioselective osmylation on ring-B, followed by dehydration of the resulting 7,8-diol (e.g. 7). Based on a model synthesis, three partial synthetic approaches starting from 2 have been developed, using different protective groups for the 3-vinyl group. Several 8-de-ethyl-8-vinyl derivatives of 9 (e.g. 8 and its 13C-labeled analogue 22) have been synthesized. A new, mild recyclization method for fabrication of the isocyclic ring-E in chlorophylls was discovered which permits conversion of 8-de-ethyl-8-vinylchlorin-e6 analogues 6 and 8 into the corresponding 8-de-ethyl-8-vinylpheophorbides 10 and 11. A change from vinyl to ethyl at the 3-position in chlorophylls causes a hypsochromic shift of 10 nm or more in the optical spectrum, whereas the identity of the 8-substituent (ethyl or vinyl) appears not to affect the wavelength of the band at about 660 nm. Hence, transformation from 8-vinyl to 8-ethyl during chloroplast biogenesis is a step which does not affect the light absorption/harvesting properties of the final isolated chlorophyll chromophores.