| Marine fouling organisms often cause technical and economic problems by settling on artificial surfaces submerged in seawaters.Although organotin compounds and booster biocides have been widely used for controlling these fouling organisms, they may also pollute the aquatic environments.Along with the increasing concern about environment,effective and environmentally friendly antifoulants are urgently needed.Therefore,an important source of such effective and environmentally friendly antifoulants is based on the research of natural product antifoulants.Ceriops tagal Perr.(Rhizophoraceae) is an important mangrove species.This species is rich in terpenoids(including a series of new terpenoids),while marine new terpenoids are an important source of marine antifoulants.The chemical constituents and antifouling activities of the mangrove plant C.tagal were studied in this paper.The EtOH extracts from the roots of C.tagal were chromatographed over Silica gel,Sephadex LH-20 and HPLC respectively to yield compounds.The structure and relative stereochemistry were elucidated by means of extensive NMR(including 1H-NMR, 13C-NMR,DEPT,HMQC,HMBC,1H-1H COSY,NOESY),IR and MS(including ESIMS and HRESIMS) analyses.By using the settlement inhibition assay with cyprids larvae of barnacle Balanus albicostatus Pilsbry,the roots of C.tagal was investigated for testing antifouling activity.The main results were shown as followed:(1) Four novel backbone compounds were isolated from the roots of C.tagal. These compounds are all dimeric diterpenes,namely,tagalsin L(1),tagalsin M(2), tagalsin N(3) and tagalsin P(4).In addition,one new diterpene was isolated from the the roots of C.tagal and determined as tagalsin O(5).(2) Thirteen known compounds were isolated from the roots of C.tagal.Among these compounds,three diterpenes are ent-8(14)-pimarene- 15R,16-diol(6),ent-8(14) -pimarene-15,16-O- isopropylidene(7) and tagalsin C(8);one dimeric diterpene is tagalsinâ… (9);five triterpenes are betulin(10),lup-20(29)-en-3-oxo-oic acid(11), betulin acid(12),lupane-20(29)-en-3-oxo-28-diol(13) and lup-20(29)-en-3β-24-diol (14);two phytosterins are stigmasterol(15) andβ-sitosterol;two aromatic compounds are coffeic acid methyl ester(17) and coffeic acid(18).Among these compounds, compounds 6,7,14 and 17 were isolated from the roots ofC.tagal for the first time.(3) Using the settlement inhibition assay with cyprids larvae of bamacle Balanus albicostatus,the compounds from the roots of C.tagal was investigated for testing antifouling activity.Capsaicin,as a widely used antifoulant,was chosen as the reference.According to the antifouling activity,five grades were set:very strong (EC50＜0.1μg/cm2),strong(0.1μg/cm2＜EC50＜1μg/cm2),medium(1μg/cm2＜EC50＜10μg/cm2),feeble(10μg/cm2＜EC50＜50μg/cm2) and non-antifouling(EC50＞50μg/cm2).Comparing with the antifouling activity of capsaicin(EC50= 1.32±0.02μg/cm2),thirteen compounds were approved to be active.Among these compounds,one natural product antifoulant with very strong antifouling activity is ent-8(14)-pimarene-15R,16-diol(EC50 = 0.04±0.00μg/cm2);two natural product antifoulants with strong antifouling activity are tagalsin O(EC50 = 0.32±0.01μg/cm2) and tagalsin C(EC50 = 0.65±0.02μg/cm2);six natural product antifoulants with medium antifouling activity are tagalsin N(EC50 = 8.79±0.36μg/cm2),betulin (EC50 = 9.27±0.30μg/cm2),lupane-20(29)-en-3-oxo-28-diol(EC50 = 8.73±0.43μg/cm2),ent-8(14) -pimarene-15,16-O-isopropylidene(EC50= 4.02±0.06μg/cm2), stigmasterol(EC50 = 4.05±0.15μg/cm2) and lup-20(29)-en-3-oxo-oic acid(EC50 = 3.20±0.17μg/cm2);four natural product antifoulants with feeble antifouling activity are tagalsin L(EC50 = 26.05±0.26μg/cm2),tagalsin M(EC50 = 22.19±0.21μg/cm2),tagalsinâ… (EC50 = 11.67±0.47μg/cm2) andβ-sitosterol(EC50 = 18.47±0.40μg/cm2).On the other hand,their toxicities to cyprids were quite low and they all inhibited cyprid settlement in a non-toxic way.These compounds are great potential marine environmentally friendly antifoulants.(4) The structure of ent-8(14)-pimarene-15R,16-diol was modified and five compounds was synthesized.Among these compounds,one compound with very strong antifouling activity is MOT-3-4-3(EC50= 0.05±0.00μg/cm2);three compounds with strong antifouling activity are MOT-1-2-2(EC50 = 0.14±0.01 μg/cm2),MOT-1-1(EC50 = 0.30±0.01μg/cm2) and MOT-2(EC50 = 0.57±0.01μg/cm2);one compound with medium antifouling activity is MOT-3-3-4(EC50 = 7.47±0.13μg/cm2).5.Our structure-activity relationship analysis of ent-pimarane diterpenoid is summarized as follows:a.The antifouling activity might be due to the number of free hydroxyl group in side chain of pimarane diterpenoid.However,there was no great effect of the number of free hydroxyl group on the toxicity of pimarane diterpenoids.b.Effect of substituent group in side chain on the activity of ent-pimarane diterpenoids:hydroxyl group＞epoxy＞acetyl group＞methoxyl group＞mesyl group.
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