Red algae of the genus Laurencia (Rhodomelaceae, Ceramiales) are known to be prolific sources of a wide variety of halogenated secondary metabolites, such as C₁₅-acetogenins and C₁₅-, C₂₀-, and C₃₀-terpenoids [1]. In the course of our chemical and biological investigation of Laurencia species from the coastal waters of Borneo (Malaysia), we reported the chemical composition of L. snackeyi (Weber-van Bosse) Masuda [2], L. similis Nam et Saito [3], and L. majuscula (Harvey) Lucas [4,5]. As part of the chemical analysis of the undescribed Laurencia species, we examined five populations of unrecorded Laurencia sp. collected from the coastal waters of North Borneo Island, Sabah. Each specimen contained one halogenated metabolite, a total of five halogenated metabolites were isolated and identified. These specimens yielded one new 10-acetoxyangasiol (1) and four known halogenated metabolites; aplysiadiol (2), cupalaurenol (3), 1-methyl-2,3,5-tribromoindole (4), and chamigrane epoxide (5). The structure of the new compound 10-acetoxyangasiol (1) was elucidated by spectral data. The structures of the known metabolites (2–5) were determined based on the comparison of spectral data to that of the published reports of Ojika et al. [6,7], Ichiba and Higa [8] and Carter and Rinehart [9]. In this paper, we describe the isolation and structure elucidation of these compounds and their antibacterial activities against clinical bacteria.

The partially dried specimens of algae Laurencia sp. were extracted in MeOH (1:1, v/v). The concentrated extracts were partitioned between H₂O and EtOAc. The EtOAc soluble fraction was dehydrated over Na₂SO₄ anhydrous, filtered, concentrated and purified by a combination of silica gel column chromatography and High Performance Liquid Chromatography (HPLC) separation, to yield compounds 1–5 (Figure 1).

Compound 1 was obtained as white powder, [α]²⁵_D +6.4 (c 0.9, CHCl₃). The IR spectrum indicated the presence of OH group (3532 cm⁻¹), γ-lactone carbonyl group (1773 cm⁻¹) and acetoxy functionality (1734 and 1236 cm⁻¹). The positive ESI-MS exhibited a characteristic molecular-ion cluster at m/z 535/537/539 in a ratio of 1:2:1, suggesting the presence of two bromine atoms. The molecular formula was determined to be C₂₂H₃₂Br₂O₅ by HR-ESI-TOFMS, indicating six degrees of unsaturation. The ¹³C NMR spectrum (Table 1) along with the DEPT experiments showed the presence of 22 carbons including four methyls, seven methylenes, five methines and six quaternary carbon atoms. In addition, the ¹H and ¹³C NMR spectral data (Table 1) indicated the presence of a γ-lactone carbonyl group [δ_C 174.3 (s)] and an associated tertiary alkoxy group [δ_C 83.0 (s)], an acetoxy group [δ_C 169.4 (s), 20.2 (q); δ_H 1.58 (3H, s)], an acetoxymethine [δ_C 83.0 (d); δ_H 4.31 (1H, d, J = 8.9 Hz)], a tertiary alcohol [δ_C 73.7 (s)], two bromomethines [δ_C 64.4 (d); δ_H 3.52 (1H, dd, J = 13.1, 4.1 Hz) and δ_C 51.0 (d); δ_H 3.33 (1H, dd, J = 11.7, 6.2 Hz)] and three tertiary methyls [δ_H 1.25, 1.15 and 0.93 (each 3H, s)]. According to the molecular formula and the functionalities mentioned above, compound 1 is suggested to contain one γ-lactone and three carbocyclic rings. Furthermore, the ¹³C-NMR spectra of 1 closely resembled those of the known compound angasiol [10], except for the absence of one acetoxy group. It clearly suggested that 1 possesses the same skeleton and substituent.

Assignments were carried out based on ¹H-¹H COSY, HSQC and HMBC spectra data. ¹H-¹H COSY experiments revealed the sequences of the correlations depicted by the bold lines in Figure 2. Key HMBC correlations as shown in Figure 2 were consistent with the proposed structure of 1. Furthermore, the relative stereochemistry of 1 was elucidated by NOESY experiments as well as the coupling constants in the ¹H-NMR spectrum. The coupling constant (J_6,10 = 8.9 Hz) indicated the anti conformation for the H-6/H-10. The configuration at C-10 was assigned as R* on the basis of the NOESY correlations observed between H-5/H-10, H-7β/H-10, H-10/Hax-12, H-10/Hax-16 and H₃-17/H₃-Ac. In addition, the remaining stereochemistry of 1 was determined by NOESY correlations as shown in Figure 3 and assigned to be the same as for angasiol and its related compounds, irieols A–G [10–12]. In consequence, the structure of 10-acetoxyangasiol (1) must be represented by structure 1.

All five metabolites were subjected to antibacterial bioassay against five species of clinical bacteria, and their antibacterial activities at 30 mg disc⁻¹ are shown in Table 2. Compounds 1, 2 and 3 exhibited potent inhibition against three of the tested bacteria. The lowest MIC value was observed for compound 1 against Vibrio cholerae at 100 μg mL⁻¹. Compounds 4 and 5 only exhibited weak inhibition against Staphylococcus sp. with a MIC value of 300 μg mL⁻¹. Vancomycin, used as a positive control, exhibited a >23 mm inhibition zone against all the tested microbes at 30 mg disc⁻¹. Based on our findings, these halogenated metabolites could be considered as possible candidates for further investigation against clinical microbes in our endeavor to combat the rise in antibiotic resistant microbes.

This is our first report on the composition of halogenated metabolites and their activities in unrecorded Laurencia sp. from the coastal waters of North Borneo Island of Sabah, Malaysia. Our initiative pertaining to the isolation and identification of halogenated secondary metabolites from Borneon Laurencia continues to excite us, with the isolation of a wide diversity of structurally interesting metabolites. To date, we have isolated a total of 42 halogenated metabolites from L. snackeyii, L. majuscula, L. similis, L. nangii and these five species of undescribed Laurencia spp.