Two new guaiane sesquiterpene derivatives, guai-2-en-10a-ol (1) and guai-2-en-10a-methanol (2), were
chromatographically purified as major constituents of the CHCl3/CH3OH (1:1, v/v) soluble fraction of Ulva
fasciata. Acetylation of 2 furnished guai-2-en-10a-methyl methanoate (3) with acetyl group at C11
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 mg/mL, respectively. The electronegative
C10 acetyl group with high polarisability (7.02� 10�24 cm3) 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
