Abstract:
Red algae of the genus Laurencia Harvey
(Ceramiales, Rhodomelaceae) is a complex genus,
encompassing a large variety of morphologicallycomplex
algal species. In continuation of our work on
chemistry and biological activity studies of some Sri
Lankan seaweeds, we examined the chemistry of
MeOH extract of Laurencia hetroclada collected from
Tangalle coast. Above extract was subjected to column
chromatography followed by PTLC gave a new
brominated sesquiterpene 1 along with two known
compounds, algoane 2, and caulerpin 3. The 13C NMR
spectrum along with the DEPT experiments of 1
revealed only 15 carbon signals corresponding to four
methyls, five methylene, one methine and five
quaternary carbons. The 1H NMR (CD3OD, 500 MHz)
spectrum of 1 showed signals due to three methyls,
four methylenes, and two methine protons. The
singlets resonated at 1.25 (3H, s), 1.27 (3H, s), and
1.85 (3H, s) were assigned to Me-8’, Me-6’, and Me-9’,
respectively. The characteristic downfield proton at
4.83 (dd, J = 4.2,15 Hz) was assigned to H-4, geminal
to the Br atom, whereas the proton that resonated at
3.96 (d, J = 1.7 Hz) was assigned to the oxymethine H-
1. Furthermore, the olefinic singlet resonated at 5.26
was assigned to Ha-7’ and Hb-7. The down field
carbon atoms resonated at 110.2 and 165.2 were
assigned to olefinic carbon atoms. The positive CI MS
of 1 showed the molecular ion peak [M+H]+ at m/z
349 along with isotopic peak at m/z 351 (1:1),
indicating the presence of a Br atom in the molecule.
The above MS data were found to be consistent with
the molecular formula C15H25O4Br with 3 degrees
of unsaturation. A comparison of the NMR
spectroscopic data of 1 with those of 2 suggested a
similar skeleton with differences in substitution pattern
and unsaturation sites. From the above spectral data,
the structure of 1 was confirmed as a new natural
product 4-bromo-5-methyl-2-(3’-hydroxy-1’,3’-
dimethyl-2’ methylenecyclopentyl) cyclohexane-1,2,5-
triol. Caulerpin (3) showed a significant dose
suppressive effect with an IC50 5.8 ± 1.0 μg/mL on Tcell
proliferation assay.
