Abstract
One new dimeric diterpenoid, 8(14)-enyl-pimar-2′(3′)-en-4′(18′)-en-15′(16′)-endolabr- 16,15,2′,3′-oxoan-16-one (1) and five known terpenoids: Tagalsin C (2), Tagalsin I (3), lup-20(29)-ene-3β,28-diol (4), 3-oxolup-20(29)-en-28-oic acid (5) and 28-hydroxylup- 20(29)-en-3-one (6) were isolated from the roots of the mangrove plant Ceriops tagal. Their structures and relative stereochemistry were elucidated by means of extensive NMR, IR and MS analysis. The antifouling activity against larval settlement of the barnacle Balanus albicostatus were evaluated using capsaicin as a positive control. All these terpenoids exhibited antifouling activity against cyprid larvae of the barnacle without significant toxicity. The structure-activity relationship results demonstrated that the order of antifouling activity was diterpenoid (Compound 2) > triterpenoid (Compounds 4, 5 and 6) > dimeric diterpenoid (Compounds 1 and 3). The functional groups on the C-28 position of lupane triterpenoid significantly affect the antifouling activity. The diterpenoid dimmer with two identical diterpenoid subunits might display more potent antifouling activity than one with two different diterpenoid subunits. The stability test showed that Compounds 2, 4, 5 and 6 remained stable over 2-month exposure under filtered seawater.
Highlights
It is well known that marine fouling organisms, settling on ship hulls and other artificial surfaces submerged in seawaters, cause technical and economic problems [1,2]
Compound 1 was obtained as yellow oil, and its molecular composition of C40H58O2 was established by HR-ESI-MS (m/z 571.4751 [M + H+]+, C40H58O2+)
Compound 1 seemed to be a dimeric diterpenoid composed of two moieties, one being a pimarane-type diterpenoid and the other a dolabradiene-type diterpenoid
Summary
It is well known that marine fouling organisms, settling on ship hulls and other artificial surfaces submerged in seawaters, cause technical and economic problems [1,2]. Antifouling paints, containing organotin compounds such as tributyltin (TBT) and tributyltin oxide (TBTO), are very effective in controlling these fouling organisms These substances have been found to be toxic to many non-target marine organisms and to pollute the marine ecosystem [2,3]. We obtained six terpenoids from C. tagal, and evaluated their antifouling activity against larval settlement of the barnacle B. albicostatus using capsaicin as a positive control. These antifouling terpenoids included one new dimeric diterpenoid, 8(14)-enyl-pimar2'(3')-en-4'(18')-en-15'(16')-en-dolabr-16,15,2',3'-oxoan-16-one (1) and five known terpenoids, designated as Tagalsin C (2), Tagalsin I (3), lup-20(29)-ene-3β,28-diol (4), 3-oxolup-20(29)-en-28-oic acid (5) and 28-hydroxylup-20(29)-en-3-one (6) (Figure 1). The isolation, structural elucidation and antifouling activity of these compounds are described in the present study
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