Polar Metabolites of the Tropical Green Seaweed Caulerpa taxifolia Which Is Spreading in the Mediterranean Sea: Glycoglycerolipids and Stable Enols (α-Keto Esters)

Examination of the polar components of the green seaweed Caulerpa taxifolia (VAHL) C. AGARDH, which is heavily spreading in the northeastern Mediterranean, led to two families of compounds. The new (2R)-3-O-β-D-galactopyranosyl-1-O-[(7Z,10Z,13Z)-hexadeca-7,10,13-trienoyl]- 2-O-[(9Z,12Z,15Z)-octadeca-1,12,15-trienoyl]-sn-glycerol (2) was isolated in low abundance, like the analogues 1 and 3 already known from freshwater cyanobacteria. The acyl positions in 1-3 were determined by enzymatic methods and the absolute configuration from the O-galactosylglycerol obtained upon alcaline methanolysis. More abundant were the (4-hydroxyphenyl)- and (3,4-dihydroxyphenyl)pyruvic acid methyl esters, occurring in the enol (Z) forms 13a and 14a accompanied by very minor (E) forms 13b and 14b. The latter became predominant on UV irradiation of 13a or 14a, allowing the determination of the C=C configuration of these isolatable, stable enols from 1H,13C NMR couplings (larger H-C(3)/C(1) coupling constant in the (E) than in the (Z) isomer). Contrary to literature implications, the O-galactosylglycerolipids 1-3 lack any cholinergic or histaminergic activity; similarly, enols (= α-keto esters) 13 and 14 or terpenoids of this seaweed were also devoid of such biological activities (see Table).