Guaiane sesquiterpenes from seaweed Ulva fasciata Delile and their antibacterial properties

Two new guaiane sesquiterpene derivatives, guai-2-en-10 alpha-ol (1) and guai-2-en-10 alpha-methanol (2), were chromatographically purified as major constituents of the CHCl(3)/CH(3)OH (1.1, v/v) soluble fraction of Ulva fasciata. Acetylation of 2 furnished guai-2-en-10 alpha-methyl methanoate (3) with acetyl group at CH position. The structures of the compounds were elucidated using one and two-dimensional NMR and mass spectrometric analysis. Compounds 2 and 3 exhibited significant inhibition to the growth of Vibrio parahaemolyticus with minimum inhibitory concentrations of 25 and 35 mu g/mL, respectively. The electronegative C10 acetyl group with high polarisability (7.02 x 10(-24) cm(3)) in 3 appeared to withdraw electron cloud from substituted cycloheptyl ring and (R)-3-methylcyclohept-1-ene moiety, thus acting as the nucleophilic center of the molecule resulting in high bioactivity.