www.nature.com/scientificreports OPEN Absolute confguration by vibrational circular dichroism of anti‑infammatory macrolide briarane diterpenoids from the Gorgonian Briareum asbestinum Dawrin Pech‑Puch1,4, Pedro Joseph‑Nathan2, Eleuterio Burgueño‑Tapia3, Carlos González‑Salas4, Diana Martínez‑Matamoros1, David M. Pereira5, Renato B. Pereira5, Carlos Jiménez1* & Jaime Rodríguez1* The four new briarane diterpenoids 2‑butyryloxybriarane B‑3 (2), 9‑acetylbriarenolide S (3), briarenolide W (4), and 12‑isobriarenolide P (5), along with briarane B‑3 (1) and the fve known diterpenes 6–10 were isolated from the gorgonian coral Briareum asbestinum collected from the Mexican Caribbean Sea. The structures were elucidated by 1D and 2D NMR and MS measurements. Since the structure of briarane B‑3 (1) was only suggested and published without any spectroscopic support, it was herein confrmed, and the supporting data are now provided. In addition, 1 provided the opportunity to explore the sensitivity of vibrational circular dichroism (VCD) to determine the confguration of a single stereogenic center in the presence of eight other stereogenic centers in a molecule possessing a highly fexible ten‑member ring. A single‑crystal X‑ray difraction study, in which the Flack and Hooft parameters of 1 were determined, further confrmed that briarane B‑3 is (1S,2S,6S,7R,8R,9S,10S,11R,17R)‑1. This paper reports for frst time the use of VCD in briarane diterpenes and with the presence of chlorine atoms. Biological evaluation of seven isolated compounds evidenced a moderate anti‑infammatory activity for compounds 6 and 9 but it did not show any cytotoxic, antiviral, antibacterial, and topoisomerase inhibitory activity. Te prodigious creativity of Nature is evidenced when contemplating the diverse biosynthetic processes of dit- erpenoids, the largest class of natural products, comprised by more than 80 thousand representatives, which are generated from simple acyclic achiral precursors1. Among them, briaranes represent one of the more common families of compounds isolated from marine sources2,3. Particularly, the gorgonian Briareum asbestinum is a prolifc source of this type of diterpenoids4–7, whose molecular architecture shows a trans-fused bicyclo[8.4.0] tetradecane ring system with hydroxyls, esters, epoxides, and chlorine atoms in their skeletons. Very distinctive is a γ-lactone moiety at C-7–C-8, and in many cases congested four-contiguous stereogenic carbon atoms at C-1, C-2, C-10, and C-14 are found. 1Centro de Investigacións Científcas Avanzadas (CICA) e Departamento de Química, Facultade de Ciencias, Agrupación Estratéxica CICA-INIBIC, Universidade da Coruña, 15071 A Coruña, Spain. 2Departamento de Química, Centro de Investigación Y de Estudios Avanzados del Instituto Politécnico Nacional, Apartado 14-740, 07000 Mexico City, Mexico. 3Departamento de Química Orgánica, Escuela Nacional de Ciencias Biológicas, Instituto Politécnico Nacional, Prolongación de Carpio Y Plan de Ayala, Col. Santo Tomás, 11340 Mexico City, Mexico. 4Departamento de Biología Marina, Universidad Autónoma de Yucatán, Km. 15.5, Carretera Mérida-Xmatkuil, A.P. 4-116 Itzimná, C.P. 97100 Mérida, Yucatán, México. 5REQUIMTE/LAQV, Laboratório de Farmacognosia, Departamento de Química, Faculdade de Farmácia, Universidade Do Porto, R. Jorge Viterbo Ferreira 228, 4050-313 Porto, Portugal. *email: [email protected]; [email protected] Scientifc Reports | (2021) 11:496 | https://doi.org/10.1038/s41598-020-79774-1 1 Vol.:(0123456789) www.nature.com/scientificreports/ 22 OR AcO OH 15 4 16 O 13 1 R Cl 6 12 10 OH Cl OH OH O O HO H 8 H H 20 17 O O O AcO 19 AcO AcO 21 18 O O O R = Ac 1 3 R = Cl 5 2 R = COCH2CH2CH3 4 R = OH OR3 AcO OAc AcO 19 O H H 20 7 H O O OH Cl 1 OH R O H 5 1 H H O H 3 O R2O 17 AcO O 16 O O 6 R1 = R2 = Ac, R3 = H 9 10 7 R1 = R2 = Ac, R3 = COCH2CH2CH3 8 R1 = R2 = R3 = Ac Figure 1. Briarane diterpenes isolated from Briareum asbestinum. Since the frst structural elucidation of briarein A isolated from B. asbestinum in 19778, more than 500 briarane-type secondary metabolites have been reported from Octocorallia Subclass, including Gorgonacea, Pennatulacea, Alcyonacea and Stolonifera Orders. Some of these compounds displayed biological activities such as cytotoxic, antiviral, anti-infammatory, immunomodulatory, antifouling and ichthyotoxic 9–14. In the course of our ongoing chemical research on bioactive marine natural products15–18, in particular diter- penes from marine sources19–24, and more specifcally from those collected in the Yucatan peninsula (Mexico)25, we focused our attention on the sof coral B. asbestinum because it was assumed to be a rich source of briarane diterpenoids. Tus, in this paper we describe the isolation, structure elucidation, absolute confguration (AC) evaluation, and biological activity searches of the new and known diterpenoids 1–10, found as constituents of this gorgonian, whose structures are shown in Fig. 1. Results Isolation of metabolites. Specimens of B. asbestinum, collected of shore the Yucatan Peninsula, were extracted with CH2Cl2- MeOH (1:1 v/v) to give a crude extract that was partitioned using the modifed Kupchan 26,27 procedure . Te CH 2Cl2 soluble portion was subjected to SiO 2 fash column chromatography and normal phase HPLC purifcation using isocratic mixtures of hexanes-acetone as the mobile phase to aford pure samples of 1–10. All compounds could be obtained as pure samples according to their TLC and NMR evaluations. Structure and absolute confguration of 1. Briarane 1 was isolated as white crystals. Te presence of one chlorine atom became evident from the [M + Na]+ cluster ion peaks at m/z 505/507, in a 3:1 ratio, observed in the 35 + LRESIMS. Te HRESIMS ion peak at m/z 505.1584 provided the molecular composition [C 24H31O8 Cl + Na] (calculated 505.1605), accounting for an index of hydrogen defciency (IHD) of nine. Te IR spectrum showed absorption bands at 3520, 1765, and 1736 cm−1, indicating the presence of hydroxy, γ-lactone, and ester function- alities, respectively. Te 1H and 13C NMR spectra (Tables 1, 2, respectively), together with the DEPT-135 data, correlated by an edited HSQC experiment, were indicative of a briarane skeleton. Te 1H-1H COSY spectrum displays fve separate spin–spin coupling systems that map-out the four hydrogen atom sequences H-2–H-3–H- 4–H-16 (through allylic coupling), H-6–H-7, H-9–H-10–H-11–H-20, H-13–H-14, and H-17–H-18 (Fig. 1). Tese spin–spin systems were connected through the HMBC correlations, thereby confrming the briarane skel- eton, as follows: from H-2 to C-1; H-3 to C-5; H-6 to C-5 and C-8; H-9 to C-7 and C-8; H-10 to C-1; H-16 to C-4, C-5 and C-6; and the broad signal at δH 3.43, assigned to a hydroxy group, showed long-range correlations with C-7 and C-9 thereby allowing us to establish the C-1–C-10 connectivity of the ten-member ring. Te ring fusion to a methylcyclohexenone, at C-1 and C-10 was evidenced by the HMBC correlations between H-13 and Scientifc Reports | (2021) 11:496 | https://doi.org/10.1038/s41598-020-79774-1 2 Vol:.(1234567890) www.nature.com/scientificreports/ N 1 2 3 4 5 2 4.81 (d, 9.4) 4.82 (d, 9.5) 4.37 (d, 6.0) 4.40 (d, 5.4) 4.27 (d, 9.7) 3 2.41 (dt, 15.6, 9.4, 9.4) 2.40 (m) 2.78 (dt,15.4, 15.4, 5.5) 2.85 (dt, 15.5, 15.5, 4.9) 5.89 (dd, 11.0, 9.7) 1.87 (dd, 15.6, 10.9) 1.86 (m) 1.77 (m) 1.74 (m) 4 2.61 (dd, 15.0, 10.9) 2.57 (m) 2.56 (m) 2.55 (m) 6.21 (ddd, 11.0, 1.2) 1.62 (m) 1.60 (m) 2.05 (m) 2.00 (m) 6 4.83 (bs) 4.84 (d, 9.3) 5.88 (dd, 10.3, 0.6) 5.72 (d, 10.2) 5.83 (dq, 8.6, 1.2, 1.2, 1.2) 7 5.77 (bs) 5.71 (bs) 5.23 (d, 10.3) 5.28 (d, 10.2) 5.07 (d, 8.6) 9 5.11 (d, 5.1) 5.11 (d, 5.5) 5.28 (d, 4.7) 5.30 (d, 5.2) 5.14 (d, 6.7) 10 2.99 (dd, 4.7, 5.1) 2.99 (dd, 5.0, 5.5) 2.64 (dd, 4.6, 4.7) 2.77 (t, 5.2) 2.12 (d, 6.7) 11 2.61(dq, 4.7, 7.4) 2.61 (m) 2.53 (m) 2.52 (m) 1.80 (m) 12 – – – – 3.94 (dd, 5.9, 2.3) 13 5.97 (d, 10.4) 5.97 (d, 10.5) 5.84 (d, 10.5) 5.80 (d, 10.5) 3.56 (dd, 5.9, 3.6) 14 6.19 (d, 10.4) 6.20 (d, 10.5) 6.40 (d, 10.5) 6.40 (d, 10.5) 3.41 (d, 3.6) 15 1.16 (s) 1.16 (s) 1.22 (s) 1.20 (s) 1.11 (s) 16 5.69 (bs) 5.69 (q, 2.0) 4.32 (d, 12.0) 4.32 (d, 15.8) 4.24 (d, 12.8) 5.37 (s) 5.37 (bs) 4.18 (d, 12.0) 4.07 (d, 15.8) 4.18 (d, 12.8) 17 2.41 (q, 7.7) 2.45 (q, 7.3) 2.45 (q, 7.2) 2.41 (q, 7.1) 2.31 (d, 7.1) 18 1.30 (d, 7.7) 1.30 (d, 7.3) 1.19 (d, 7.2) 1.19 (d, 7.1) 1.17 (d, 7.1) 20 1.30 (d, 7.4) 1.33 (d, 7.4) 1.30 (d, 7.4) 1.29 (d, 7.4) 1.01 (d, 7.5) 21 2.14 (s) 2.15 (s) 2.25 (s) 2.24 (s) 2.18 (s) 22 2.24 (s) 2.14 (s) 2.14 (s) 1′ 2.45 (q, 7.4) 2′ 1.77 (m) 3′ 1.02 (t, 7.4) OH 3.43 (bs) 3.42 (bs) 1 Table 1.