NOVEL SYNTHETIC ROUTES TO 8-VINYL CHLOROPHYLL DERIVATIVES

New methodology was developed toward the synthesis of 8-de-ethyl-8-vin ylchlorophyll-a 1. Such 8-de-ethyl-8-vinyl derivatives of the green pl ant pigment chlorophyll-a 2 have been proposed to be intermediates dur ing biosynthesis of chlorophylls and bacteriochlorophylls. Transformat ion of the 8-ethyl group to an 8-vinyl group was studied on derivative s (e.g. 5) of chlorin-e(6) 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, usi ng different protective groups for the 3-vinyl group. Several 8-de-eth yl-8-vinyl derivatives of 9 (e.g. 8 and its C-13-labeled analogue 22) have been synthesized. A new, mild recyclization method for fabricatio n of the isocyclic ring-E in chlorophylls was discovered which permits conversion of 8-de-ethyl-8-vinylchlorin-e(6) analogues 6 and 8 into t he corresponding 8-de-ethyl-8-vinylpheophorbides 10 and 11. A change f rom vinyl to ethyl at the 3-position in chlorophylls causes a hypsochr omic shift of 10 nm or more in the optical spectrum, whereas the ident ity of the 8-substituent (ethyl or vinyl) appears not to affect the wa velength of the band at about 660 nm. Hence, transformation from 8-vin yl to 8-ethyl during chloroplast biogenesis is a step which does not a ffect the light absorption/harvesting properties of the final isolated chlorophyll chromophores.