Halogenated Meroditerpenoids from a South Pacific Collection of the Red Alga Callophycus serratus Victoria H. Woolner, †,§ Rose M. A. Gordon, ‡,§ John H. Miller, ‡,§ Matthias Lein, † Peter T. Northcote,* ,†,§,⊥ and Robert A. Keyzers* ,†,§ † School of Chemical and Physical Sciences, ‡ School of Biological Sciences, and § The Center for Biodiscovery, Victoria University of Wellington, P.O. Box 600, Wellington 6012, New Zealand * S Supporting Information ABSTRACT: A detailed examination of the red alga Callophycus serratus collected in Tonga led to the isolation of six new halogenated meroditerpenoids: callophycol C (1), callophycoic acid I (2), iodocallophycols E (3) and F (4), iodocallophycoic acid B (5), and callophycoic acid J (6). Of these, compounds 3-5 are new iodinated additions to the growing family of Callophycus meroditerpenoids. The relative configurations of compounds 1-6 were deduced by analyses of 1D NOE data and 1 H- 1 H scalar coupling constants, and 3-6 are proposed to differ from the closely related compounds reported in the literature, iodocallophycoic acid A and iodocallophycols A-D. Iodocallophycol E (3) exhibited moderate cytotoxicity against the promyelocytic leukemia cell line HL-60 with an IC 50 value of 6.0 μM. R ed algae of the genus Callophycus have been found by the Kubanek group to be prolific producers of structurally diverse meroditerpenoids. 1-8 The majority of those reported are halogenated, predominantly by bromine, but recently, a report featuring the first Callophycus-derived di-, tri-, and tetraiodinated meroditerpenoids was published. 8 The osten- sibly low number of iodine-containing marine natural products present in the literature has led to the perception that iodinated compounds are rarer than those that contain bromine and chlorine. 9-12 However, when the relative halide concentrations in seawater are considered (0.5, 0.8 × 10 -3 , 0.4 × 10 -6 mol L -1 , 13 giving an approximate ratio of 1 million:2000:1 for chloride, bromide, and iodide ions, respectively), the ratio of iodinated to brominated and chlorinated metabolites reported is significantly higher on a relative scale. 8 Biohalogenation processes that incorporate halides into organic substrates and the higher reactivity of the iodide ion due to its enhanced oxidation potential are believed to be responsible. 8 Specimens of C. serratus were collected from ′Eua, Tonga, and examined in this study. Purification was guided using HRESIMS in addition to 1 H NMR spectroscopy, which resulted in the isolation of six new compounds, callophycol C (1), callophycoic acid I (2), iodocallophycols E (3) and F (4), iodocallophycoic acid B (5) and callophycoic acid J (6), 14 in addition to the known macrolides bromophycolides A (7) and T(8). As the name implies, iodocallophycols E (3) and F (4) and iodocallophycoic acid B (5) incorporate iodine within their structure, bringing the total number of iodinated Callophycus metabolites reported to eight. ■ RESULTS AND DISCUSSION Methanolic extracts of C. serratus (181.5 g, wet weight) were semipurified using polystyrene(divinylbenzene) (PSDVB), and the resulting fractions analyzed by HMBC NMR spectroscopy. In a search for structurally interesting, potentially new natural products featuring unusual functional/structural elements, our attention was drawn to a series of aromatic proton resonances and a pair of deshielded methyl singlets that were found through HMBC correlations to form a substituted dimethy- lallyl substructure. Further fractionation by silica and diol benchtop column chromatography, followed by purification using HILIC or C 18 HPLC, yielded compounds 1-8. Callophycol C (1) was isolated as a white, amorphous solid. Negative-ion-mode HRESIMS data provided a deprotonated molecule at m/z 632.9787, consistent with the parent molecular formula C 26 H 34 Br 3 ClO, which required eight hydrogen deficiencies. The isotopic distribution pattern confirmed the presence of bromine and chlorine. The 13 C NMR spectrum revealed 26 carbons, and a multiplicity-edited HSQC experiment accounted for 33 protons connected to 17 carbons. The remaining nine nonprotonated carbon centers comprised seven olefinic/aromatic carbons and two quaternary Received: June 14, 2018 Article pubs.acs.org/jnp Cite This: J. Nat. Prod. XXXX, XXX, XXX-XXX © XXXX American Chemical Society and American Society of Pharmacognosy A DOI: 10.1021/acs.jnatprod.8b00487 J. Nat. Prod. XXXX, XXX, XXX-XXX J. Nat. Prod. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 11/08/18. For personal use only.