Natural Products Chemistry & Research

The genus Laurencia, commonly known as red seaweeds, is a marine algae belonging to family Rhodomelaceae [1]. Laurencia has been established by JV Lamouroux in 1813, with only 8 species originally recognized. At present ~430 described species are identified, of which 134 are, taxonomically, accepted [2]. Seaweeds of the Laurencia genus have a wide geographical distribution and occur in all oceans and seas at all attitudes particularly in temperate to tropical shores constituting a considerable part of the flora [3]. It is estimated that over 60 species throughout the world have been investigated and more than 700 compounds possessing unique structural features have already been isolated from red algae of genus Laurencia [4]. In particular, species of Laurencia are known to be the richest producers of halogenated secondary metabolites with diverse and unique structural features depending on species, localities and season [5]. Laurencia species produce bromine-containing compounds in much larger numbers than either chlorine- or iodine-containing ones whereas, the Clcontaining compounds usually also possess Br atom(s) [6]. A large number of these compounds have been obtained from Laurencia species having an intracellular, membrane-bound vesicles known as "corps en cerise" (cherry bodies) in the outer cell layer (the cortical layer) where, these inclusions are considered as a synthesis and/or storage sites of halogenated secondary metabolites [7]. On the other hand, Laurencia species without "corps en cerise" does not produce any halometabolites [8].

Laurencia chemistry is dominated by the presence of sesquiterpenes which are the most abundant members in the terpenoid groups isolated from this genus, with relatively fewer reports of diterpenoids, triterpenoids, steroids, alkaloids and C15 acetogenins [9]. Many of these compounds which exhibit significant ecological role as anti-epibiosis [10] are also reported to possess a variety of biological effects, such as cytotoxic activity against various cancer cell lines [11,12], antiviral, antibacterial, antifouling, antifungal, antioxidant, antimalarial, anthelmintic, antiasthmatic, antifeedant and other activities [13-19].

More than 200 sesquiterpenoidal metabolites, isolated from Laurencia, will be discussed, in term of their occurrence, structural type and reported pharmacological activity and presented in an order based on their general structural type. Table 1 summarizes the taxonomical position of genus Laurencia. On the other hand, alphabetical listing of all cited species with the corresponding compounds isolated from each and the related references is presented in Table 2.

Table 1: Taxonomical Position of Laurencia.

Table 2: Sesquiterpene constituents isolated from the species of genus Laurencia.

The genus Laurencia is the most attractive source of sesquiterpenes among all marine macroalgae. It has a remarkable capacity to biosynthesize a huge variety of structurally diverse sesquiterpenes, with varied skeletons including chamigrane, bisabolane, laurane, snyderane brasilane along with some unique rearranged derivatives occurring prevalently [20,21] (Table 1).

Spirane type sesquiterpenes –chamigrenes– are the most widespread sesquiterpenes from the genus Laurencia. Over the last twenty years, large number of structurally novel chamigrane metabolites has been isolated from Laurencia [21]. In 1970, a brominated ketone spirolaurenone (1), was the first reported chamigrane sesquiterpene; obtained from the neutral essential oil of Laurencia glandulifera (Japan) [22]. The Chinese L. okamurai, collected from the coast of Rongcheng, China, was the source of laurokamin A (2), laurokamin B (3), laurokamin C (4), 10-bromo-α-chamigrene (5), 10-bromo- β-chamigrene (6), 2,10-dibromo-3-chloro-β-chamigrene (7), and obtusane (8) [23]. Compounds 6, 7 (named as nidificene) and 8 were earlier described from L. mariannensis, in addition to (-)-(10R)-bromo-a-chamigrene (9) [24]. Furthermore, 10-bromo-7α,8α- expoxychamigr-1-en-3-ol (10) and 10-bromo-β-chamigren-8-ol (11) were isolated from another collection of the Chinese L. okamurai [25]. L. okamurai, collected along Weihai coastline in Shandong Province, China yielded okamurene E (12) [26]. On the other hand, compounds (13) and (14) were isolated as a 1:1 diastereoisomeric mixture from L. okamurai (Qingdao, China) [27]. The same species, L. okamurai, was also the source of the rearranged chamigrane laurenokamurin (15) [28]. L. composita collected from Pingtan Island, China yielded laurecomins A-D (16-19), 2,10-dibromo-3-chloro-7-chamigren-9-ol acetate (20) and the known compound, deoxyprepacifenol (21) [29]. Compounds 16 and 17 displayed potent brine shrimp toxicity with LC50 values of 51.1 and 37.0, μg/mL, respectively. Additionally, compound 17 was active against the plant-pathogenic fungus Colletotrichum lagenarium with an inhibitory diameter of 10 mm [30]. Another Chinese L. composita sample (Nanji Is.) afforded 2-bromospironippol (22) and laurencomposidiene (23) (named as laurencomposene elsewhere in the same paper). The authors suggested that the occurrence of rearranged chamigranes in L. composita but not in L. okamurai could provide a useful chemotaxonomic marker to distinguish these two similar species [31]. However, several rearranged chamigranes were reported, later, from L. okamurai [26]. In 2010, a L. composita sample collected from the same area contained (8β)-10-bromo-3-chloro-2,7- epoxychamigr-9-en-8-ol (24) [32]. Elatol (25), isolated for the first time from L. elata in 1974 [33], was obtained in high yield of ca. 10% (w/w) from the ethanolic extract of L. microcladia [34]. Isoobtusol (26), described originally from L. obtusa [35], was isolated together with elatol (25) from L. majuscula in waters of Sabah, Malaysia and both compounds 25 and 26 were found to be active against some marine bacteria [8]. Vairappan et al. reported significant antibacterial activity for compound 25 against Staphylococcus epidermis, Klebsiella pneumonia and Salmonella sp, whereas, isoobtusol was significantly active against K. pneumonia and Salmonella sp. Further tests indicated that both compounds were bacteriostatic rather than bactericidal [36]. Elatol showed potent antiproliferative activity against promastigote and intracellular amastigote forms of Leishmania amazonensis, with an IC₅₀ of 4.0 μM and 0.45 μM, respectively [37]. Recently, Desoti et al. reported the effective trypanocidal activity of (-)-elatol, extracted from L. dendroidea, against Trypanosoma cruzi. The mechanism of action was also investigated and discussed thoroughly [38]. Moreover, in-vitro and in-vivo experiments suggested that elatol acted as antitumor agent, against HeLa and Hep-2 human carcinoma cell lines, by activating the apoptotic process [39]. The Australian L. elata yeilded two C16 chamigrenes, named cycloelatanene A (27) and B (28) [40]. L. majuscula collected from the South China Sea was the source of 8-bromochamigren-1-en (29) [41], while L. majuscula from Okinawa was the source of (6R,9R,10S)-10-bromo-9-hydroxychamigra-2,7(14)- diene (30) in a first report of this compound from a natural source. Compound 30 showed activity against Alcaligenes aqua-marinus, Azomonas agilis, Erwinia amylovora, and Escherichia coli with MIC values in the range of 20–30μg/disk [8]. A biogenetically interesting halogen-devoid metabolite chamigrenelactone (31), with a high oxygencontent, has been isolated from L. obtusa from Isla Grande (Caribbean Panama) [42]. Two sesquiterpenes with an oxacyclic chamigrene skeleton, oxachamigrene (32) and 5-acetoxyoxachamigrene (33), were isolated from a Cuban L. obtusa sample [43]. In 2007, Ji et al. reported the isolation of 9-deoxyelatol (34), deschloroelatol (35); and compounds 36 and 37 from L. mariannensis [24]. Compounds 34 and 35 were obtained, previously, from L. rigida (Hainan and Weizhou Islands, China) [13], while compounds 36 and 37 were reported in 1982 as intermediates in a biomimetic synthetic study of rhodolaureol and rhodolauradiol [44].

In 2009, L. saitoi (Shandong Province, China) yielded 10-bromo-3-chloro-2,7-epoxychamigr-9-en-8α-ol (38) and 2,10β-dibromochamigra-2,7-dien-9α-ol (39), in addition to the known compound 2,10-dibromo-3-chlorochamigr-7-en-9α-ol (40) [45,46] . Prepacifenol (41), originally described from a Laurencia sp. [47], and its acetate derivative, 5-acetoxy-2,10-dibromo-3-chloro-7,8-epoxy- α-chamigrene (42), were isolated From L. filiformis collected from Taroona, Tasmania. An X-ray analysis was reported for Prepacifenol and its NMR spectra were fully assigned for the first time. Both compounds exhibited moderate activity in the brine shrimp bioassay [48]. L. pannosa from Malaysia was the source of two halogenated chamigranes with unusual rearranged framework named pannosanol (43) and pannosane (44). Both compounds showed antibacterial activities [49]. The rearranged chamigrane Scopariol (45), the β-chamigrene isorigidol (46) and the geometric isomers (+)-3-(Z)- bromomethylidene-10 β -bromo- β -chamigrene and (-)-3-(E)- bromomethylidene-10 β -bromo- β -chamigrene (47, 48) were reported from L. scoparia collected in Brazilian waters. Compound 46 and ma’ilione (49), previously isolated from L.cartilaginea [50] exhibited moderate in vitro anthelmintic activity against the parasitant stage of Nippostrongylus brasiliensis [17]. From L. scoparia and in a separate report, Suescun et al. isolated and established the absolute stereochemistry of isorigidol (46) and ma’ilione (49) by an X-ray crystal as (3R,6S,9S,10S) and (6S,9R,10S) respectively [51]. Ma’iliohydrin (50), a cytotoxic tribrominated chamigrene with dibromohydrin functionality from a Laurencia sp. (Philippines) exhibited cytotoxicity in the NCI 60-cell line human tumour screen and potent activity against the NCI/ADR-RES breast cancer cell line [52]. Recently, dendroidone (51) and dendroidiol (52) were isolated from the Brazilian species L. dendroidea collected at Biscaia Inlet, Rio de Janeiro [53] (Figure 1).

Figure 1: Chamigrane sesquiterpenes from Laurencia.

The aristolane sesquiterpenes, derivatives of 6,11-cycloeremophilanes, are mainly reported from the species L. similis. A Chinese sample of L. similis (Hainan Is., China) yielded aristolane (53), formerly known as a synthetic compound [54], from a natural source for the first time. In addition, aristolan-10-ol-9- one (54), aristolan-8-en-1-one (55), aristol-1(10)-en-9-one (56), 9 β -aristol-1(10)-en-9-ol (57), aristol-1(10),8-diene (58) and aristol- 1,9-diene (59) were also isolated and identified [55]. In 2010, Li et al. obtained aristolan-1α-bromo-9β, 10β–epoxide (60) and aristol- 9(10)-en-8-one (aristolone) (61) from the same species [56]. A former report published in the same year, 2010, established the isolation of 61 from Laurencia for the first time [57]. Recently, a Bornean L. similis population yielded ent-1(10)-aristolen-9 β -ol (62) as a new optical isomer of compound 57, in addition to (+)-aristolone (63), axinysone B (64) and 9-aristolen-1 β -ol (65) [58] (Figure 2).

Figure 2: Aristolane sesquiterpenes from t

The bisabolane skeleton develops from the cyclization of the geranyl cation and results in the formation of a monocyclic ring structure [59]. In 2007, L. microcladia (Chios Is., North Aegean Sea) afforded the hydroxyl derivative of β -bisabolene, (66) [60], while, (5S)- 5-acetoxy- β -bisabolene (67), was isolated, recently, from L. okamurai Yamada [61]. L. tristicha (Naozhou Island, China) yielded ar-bisabol- 9-en-7,11-diol (68) [12]. On the other hand, L. scoparia (S˜ao Paulo, Brazil) was the source of compounds (69–71). The confirmation of the structure and the absolute configuration of all stereo centers of 69 were proved by single-crystal X-ray crystallography. Compound 69 exhibited weak in vitro anthelmintic activity against Nippostrongylus brasiliensis [62]. L. composita from Nanji Is., China contained 2-bromo-3-chlorobisabola-7(14),11-diene-6,10-diol (72) [32], while, L. saitoi (Shandong Province, China) yielded (9S)-2-bromo-3-chloro- 6,9-epoxybisabola-7(14),10-diene (73) and (9R)-2-bromo-3-chloro- 6,9-epoxybisabola-7(14),10-diene (74) as an inseparable 1 : 1 mixture [46]. Biogenetic considerations were useful to assign the novel oxacyclic bisabolene-type structures of aldingenins A–D (75-78), isolated from L. aldingensis (Castelhanos, Brazil) [63,64]. The okamurenes A–D (79- 82) were isolated from L. okamurai, whereas, okamurenes A (79) and B (80) were the first examples of bromobisabolanes possessing a phenyl moiety among sesquiterpenes derived from Laurencia as claimed by the authors. The isolated compounds displayed potent cytotoxicity when evaluated in the brine shrimp lethal assay [26] (Figure 3).

Figure 3: Bisabolane sesquiterpenes from Laurencia.

Cuparane skeleton is formed by cyclisation between carbons 6 and 11 of the bisabolane skeleton. In cuparane-type compounds, the three methyls in the aliphatic portion are located at positions 1, 2 and 2 [65]. In addition to marine organisms, cuparanes are found in liverworts and higher plants [66] (Figure 4).

Figure 4: Structure of cuprane.

L. okamurai, collected from the coast of Rongcheng, China, yielded 10-bromo-3-chlorocupar-5-en-2-ol (83) [25]. It is notable that most of the cuparane sesquiterpenes isolated from seaweeds have an aromatic ring such as compounds 84–86, isolated from the Greek L. microcladia, collected at Chios Island in the North Aegean Sea. A strong cytotoxic activity was recognized for compounds 84–86 against NSCLC-N6 and A-549 lung cancer cell lines. Compound 85 showed a unique (for the cuparene class of sesquiterpenes) migration of the C-1 methyl group. The authors speculated that the aromatic hydroxyl group increased the cytotoxicity observed in compounds 85 and 86 [67]. Later on, laurokamurenes A (87) and B (88), were isolated from L. okamurai (Nanji Is., China) [68]. In 2007, total synthesis of the (±)-laurokamurene B (88) was completed, employing a combination of the Ireland–Claisen rearrangement and ring-closing metathesis reactions [69]. Lately, laurokamurene D (89) was isolated from L. okamurai Yamada [61] (Figure 5).

Figure 5: Cuparane sesquiterpenes from Laurencia.

In Laurane-type compounds, contrary to the cuparanes, the three methyls in the aliphatic portion are located at positions 1, 2 and 3. Laurencia is considered the main producer of laurane-type sesquiterpenes among marine organisms in general [65]. The Chinese L. okamurai collected from the coast of Rongcheng, yielded laurene (90), originally isolated from L. glandulifera and L. nipponica [70], and 7-hydroxylaurene acetate (91). Compound 90 exhibited potent antibacterial activity and lethal toxicity to brine shrimp [25]. However, 7-hydroxylaurene (92) was reported in 1977 as novel compound from L. subopposita and L. okamurai [71]. The compounds 12-hydroxy isolaurene (93), isolauraldehyde (94) and 8,11-dihydro-12-hydroxy isolaurene (95), isolated from L. obtusa, exhibited potent activity against the gram-positive Bacillus subtilis and Staphylococcus aureus, with 94 being the most active (MIC of 35 and 27 μg/mL, respectively). Compound 94 showed, also, significant activity against Candida albicans (MIC of 70 μg/mL) and promising in vitro activity against Ehrlich ascites carcinoma [72].

In 2005, the Chinese L. okamurai (Nanji Island) yielded laureperoxide (96), 10-bromoisoaplysin (97), isodebromolaurinterol (98) and 10-hydroxyisolaurene (99) as new compounds together with seven previously reported and related sesquiterpenes; aplysinol (100), isoaplysin (101), debromolaurinterol (102), debromolaurinterol acetate (103), laurinterol acetate (104), debromoisolaurinterol acetate (105) and isolaurinterol acetate (106) [73-75]. In the same year, 2005, L. tristicha (Naozhou Island, China) was the source of laur-11-en-2,10-diol (107), laur-11-en-10-ol (108), laur-11-en-1,10-diol (109), 4-bromo- 1,10-epoxylaur-11-ene (110), previously reported as a synthetic racemate [76], cyclolauren-2-ol (111) and laurentristich-4-ol (112), along with the formerly known compounds aplysin (113), aplysinol (100), and the dimeric cyclolaurane sesquiterpene laurebiphenyl (114) [12,77]. Compound 112, having a novel rearranged cyclolaurane skeleton was also reported recently from L. okamurai [78]. Compound 114 exhibited moderate cytotoxicity against several human cancer cell lines, while all other compounds were inactive [12]. The Chinese species L. tristicha produced aplysin-9-ene (115), epiaplysinol (116) and debromoepiaplysinol (117). Compound 117 displayed selective cytotoxicity to the HeLa cell line [79]. Another collection of L. tristicha (Shanwei, Guangdong Province, China) yielded the stereoisomers 4-bromolaur-11-en-1,10 β -diol (118) and 4-bromolaur-11-en-1,10 β -diol (119), along with the known laur-11-en-1,10 β -diol (120), but reported for the first time as a natural product [80,81].

A Chinese collection of L. okamurai (Nanji island) yielded laurepoxyene (121), 3 β -hydroperoxyaplysin (122), 3 β -hydroperoxy- 3-epiaplysin (123) and 8,10-dibromoisoaplysin (124) [61]. The Australian L. filiformis (St. Paul’s Beach, Australia) produced cycloisoallolaurinterol (125) and isoallolaurinterol (126) as new compounds, along with the previously reported filiformin (127), filiforminol (128), allolaurinterol (129) and 10-bromolaurenisol (130) [82]. The authors suggested that both 125 and 126 are formed as artefacts from compound 129 [83]. The previously reported allolaurinterol acetate (131) [84] was isolated again in 2012 from L. okamurai [25], while, isolaurenisol, (132) previously reported from L.gracilis [85] was obtained in 2009 from L. saitoi [46]. 3 β -hydroxyaplysin (133) was isolated from L.okamurai (Nanji Is., China) [68] while, L. microcladia (Chios Is., North Aegean Sea) provided the aromatic sesquiterpenes (134-136) and the dimeric brominated sesquiterpene 137 [60]. The organic extract of L. obtusa, collected from Serifos in the Aegean Sea afforded 3,7-dihydroxydihydrolaurene (138) and (139). Both compounds showed weak cytotoxic activity against five human tumour cell lines [86]. On the other hand, a Laurencia sp. collected from South China Sea was the source of 8, 10-dibromo-3, 7-epoxy- laur-13-ol (140) [57] (Figure 6).

Figure 6: Laurane sesquiterpenes from Laurencia.

Brasilane skeleton has the basic structure of octahydro-1,6,6- trimethyl-4-(1-methyl)-1H-indene [65]. The rearranged halogenated compounds, with the unique 1,6-epoxy brasilane moiety (141–143) had been isolated; along with the known compounds brasilenol (144) and epibrasilenol (145) from L. obtusa collected off Symi Island in the Aegean Sea, Greece. Relative stereochemistries of all compounds were determined by molecular modeling [87]. L. obtusa (Tekirova, Turkey) produced also new brasilane-type sesquiterpene with the systemic name 2-Chloro-4-isopropyl-1,6,6-trimethylhexahydro-1H-indene-1,3 β,7 β -triol (146) [88] (Figure 7).

Figure 7: Brasilane sesquiterpenes from Laurencia.

Eudesmane-type sesquiterpenes, formerly referred to as selinanes, have been recognized from several terrestrial and marine organisms and occasionally encountered from Laurencia [89]. Heterocladol (147) was the first example of a sesquiterpene with a selinane skeleton reported from Laurencia species and its structure was rationalized in terms of a trans-annular ring closure of a germacradiene [90]. L. microcladia, collected in the Baia di Calenzana, Elba Island, was the source of 8-cycloeudesmane (148), the first eudesmane sesquiterpene to be isolated from a marine origin [90]. In the same year, 2002, itomanol (149), was isolated from L. intricata collected in Okinawan waters [91]. In 2012, the brominated selinanes, 1-bromoselin-4(14),11- diene (150) and 9-bromoselin-4(14),11-diene (151), isolated from L. composita (Pingtan Island, China) displayed potent brine shrimp toxicity with LC50 of 15.2 and 78.7 μg/mL, respectively [30] (Figure 8).

Figure 8: Eudesmane sesquiterpenes from Laurencia.

The snyderane sesquiterpenes are bromo monocyclo-nerolidol derivatives. β -snyderol (152) and β -snyderol (153) were the first reported compounds having this skeleton from collections of L. obtusa and L. snyderae, respectively [92]. (8R*)-8-bromo-10-epi-β-snyderol (154) and (8S*)-8-bromo-β-snyderol (155) were isolated from L. obtusa collected from Bademli, Turkey. Compound 154 was active against D6 and W2 clones of the malaria parasite Plasmodium falciparum [16].

(4E)-1-bromo-5-[(1'S*,3'R*)-3'-bromo-2',2'-dimethyl-6'- methylenecyclohexyl]-3-methylpent-4-ene-2,3-diol (156) was isolated from L. similis (Sanya Bay, China) [93], while luzonensin (157) [94], luzonenone (158), luzofuran (159), 3,4-epoxypalisadin B (160), 1,2-dehydro-3,4-epoxypalisadin B (161) and 15-hydroxypalisadin (162) were obtained from L. luzonensis, from Okinawan waters [95]. The ethanolic extract of the Chinese L. saitoi (Hainan coastline) yielded 2-hydroxyluzofuranone A (163) and 2-hydroxyluzofuranone B (164) [96]. Another collection of the Okinawan L. luzonensis, a rich source of snyderane sesquiterpenes, afforded 3R*, 4S*-luzonolone (165), 3S*, 4R*-luzonolone (166), luzondiol (167), luzonenone (158) and the two isomeric compounds 168 and 169, beside the known compound, aplysistatin (170). The authors suggested that compounds 168 and 169 were the first non-halogenated compounds, from the Laurencia genus, with a rearranged snyderane skeleton, as a result of a 1,2 methyl migration [97]. Aplysistatin (170), obtained from L. snackeyi in a bioassay- guided isolation, significantly inhibited NO and prostaglandin- E2 (PGE2) production. Activity was attributed to the modulation of anti- inflammatory agents via the inhibition of nitric oxide synthase (NOS) and cyclooxygenase- 2 (COX- 2) expressions in LPS- stimulated RAW 264.7 cells [98]. On the other hand, 5β-hydroxypalisadin B (171), isolated also from L. snackeyi, exhibited profound anti-inflammatory activity in lipopolysaccharide (LPS)-induced nitric oxide (NO) production in zebrafish embryo. The protective effect was comparable to the anti-inflammatory agent dexamethasone with effective concentrations of the compound between 0.25- 1μg/mL [99]. On the other hand, L. luzonensis was the source of palisadin B (172), palisadin A (173) and its oxidized product aplysistatin (170) [94]. L. saitoi (Hainan coastline, China) was the source of 5-acetoxypalisadin B (174) [100] and 4-hydroxypalisadin C (175) [96] (Figure 9).

Figure 9: Snyderane sesquiterpenes from Laurencia.

In addition to the skeletons mentioned above, there are some other sesquiterpenes, reported from different species of Laurencia that do not fit easily into the above categories.

L. microcladia, from Elba Island, provided a novel calenzanane sesquiterpene named calenzanol (176) [101]. Later, debromoisocalenzanol (177) and an indene-type sesquiterpene (178) were, also, obtained from L. microcladia, [102]. L. obtusa collected at Milos Island in the Aegean Sea Greece yielded four undecane- 3-one sesquiterpenes (179-182) and perforenone D (183). The relative stereochemistry of the known compound perforatone (184) was revised [87]. On the other hand, perforenol B (185), which was also, isolated from L. obtusa, exhibited strong cytotoxic activity [86]. Moreover, L. obtusa from Cayo Coco, Cuba was the source of bromocyclococanol (186), possessing a novel carbon skeleton of fused cyclopropane–cyclopentane rings [103]. The Turkish L. obtusa collected from Bademli, yielded 5-bromo-3-(3’-hydroxy-3’- methylpent-4’-enylidene)-2,4,4-trimethylcyclohexanone (187), and the epoxide (188) [16]. The previously known as a synthetic compound, isoafricanol (189) [104] was reported from L. mariannensis (Hainan and Weizhou Islands, China) as a natural product, together with a chromene type sesquiterpene, isodactyloxene A (190) [24]. 4-hydroxy- 1,8-epi-isotenerone (191), 9-hydroxy-3-epi-perforenone A (192) and 3-epi-perforenone (193) were isolated from the lipophilic extract of L. perforata, collected from the Great Barrier Reef, Australia [105]. Tiomanene (194), acetylmajapolene B (195) and acetylmajapolene A (196) were isolated from an unrecorded Laurencia species collected at Pulau Tioman, Pahang (Malaysia) [106] along with the known majapolene B (197) and majapolene A (198), originally isolated from L. majuscula [107]. The use of vibrational circular dichroism (VCD) allowed the determination of the absolute configuration of 195 and 198 as 7S, 10S for both [108].

A ring-cleaved sesquiterpene, reported as having a novel skeleton, was isolated from L. okamurai Yamada and named secolaurokamurone (199) [61]. L. luzonensis from Okinawan waters yielded (3Z,6E)-1-bromo-2-hydroxy-3,7,11-trimethyldodeca-3,6,10-triene (200), isopalisol (201), luzonensol (202) and luzonensol acetate (203) [95], while, the norsesquiterpene derivative, 2-bromo-β-ionone (204) was isolated from L. saitoi (Hainan coastline, China) [97]. A triquinane derivative (205) was identified from L. dendroidea, in the Brazilian coast, and was found to be moderately active against Leishmania amazonensis (IC₅₀ 43.8μg/mL) [109] (Table 2 and Figure 10).

Figure 10: Other skeletons sesquiterpenes from Laurencia.

Among the red algae, genus Laurencia is known to be one of the most important resources to produce unique natural metabolites with novel structures. A large number of sesquiterpene compounds with unprecedented structural features have been described from different Laurencia species during the past years. In the present review, an attempt to congregate the phytochemical and biological information on Laurencia sesquiterpenes was conducted. Survey of literature revealed the presence of over 200 sesquiterpenoids isolated from different species, over the last 14 years, and reported either as novel or known compounds. The numerous chemical diversity and biological activities of genus Laurencia keep attracting the attentions of phytochemists and pharmacologists to further explore different species of this interesting red weed distributed in the oceans and seas around the world.