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.