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The geometric isomers of caespitate : a computational study in vacuo and in solution

Caespitate is an acylphloroglucinol isolated from a plant utilized in traditional medicine in South Africa and exhibiting antituberculosis, antibacterial and antifungal activities. The molecule is characterized by a COCH(CH3)2 acyl chain and by a prenyl chain ending with an acetic-acid ester group, in meta to the acyl chain. The double bond of the prenyl chain gives origin to Z and E isomers, with the former being the biologically active one. An extensive computational study of the conformational preferences of the two isomers was carried out in four media (vacuum, chloroform, acetonitrile and water) to try and identify significant differences. The main difference concerns the ability of the prenyl/ester chain to form an intramolecular hydrogen bond with one of the neighbouring OH: the ability is greater for the Z isomer and the corresponding hydrogen bond has greater conformation-stabilizing effect. The difference is also analysed in terms of the competition between intramolecular hydrogen bond engaging the O atoms of the ester function and the O H interaction between phenol OH and the double bond in the prenyl/ester chain. Keywords—Acylphloroglucinols, Caespitate, Geometric isomers, Intramolecular hydrogen bonding, Polyphenols.