Antialgal ent-labdane diterpenes from Ruppia maritima Marina DellaGreca a , Antonio Fiorentino b , Marina Isidori b , Pietro Monaco a, *, Armando Zarrelli a a Dipartimento di Chimica Organica and Biologica, Universita Á Federico II, Via Mezzocannone 16, I-80134 Naples, Italy b Dipartimento di Scienze della Vita, II Universita Á di Napoli, Via Vivaldi 43, I-81100 Caserta, Italy Received 25 April 2000; received in revised form 13 June 2000 Abstract Seven ent-labdane diterpenes have been isolated from Ruppia maritima. The structures 15,16-epoxy-ent-labda-8(17),13(16),14- trien-19-al; 15,16-epoxy-ent-labda-8(17),13(16),14-trien-19-ol acetate; methyl 15,16-epoxy-12-oxo-ent-labda-8(17),13(16),14-trien- 19-oate; 15,16-epoxy-ent-labd-8(17),13E-dien-15-ol and 13-oxo-15,16-bis-nor-ent-labd-8(17)-ene have been assigned to the ®ve new compounds by spectroscopic means and chemical correlations. The phytotoxicity of the diterpenes has been assessed using the alga Selenastrum capricornutum as organism test. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Ruppia maritima L.; Potamogetonaceae; Selenastrum capricornutum; Diterpenes; ent-Labdanes; Toxicity; Microbiotests 1. Introduction Our studies on metabolites of aquatic plants have shown that many of them have a strong in vitro antialgal eect (DellaGreca et al., 1998), which could justify the reduction of phytoplankton in natural ecosystems (Rice, 1984). In pursuing our chemical investigation of aquatic plants distributed in Italy, as well as the assessment of the antialgal properties of their components, we are now examining two species of Potamogetonaceae, which grow in the river Volturno near Naples. The ®rst one Potamogeton natans is a fresh water species while Rup- pia maritima, commonly known as sea hay, lives at the mouth of the river in brackish waters. In this paper we report the chemical and phytotoxicological investigation of R. maritima. This plant has been already studied and the presence of ¯avonoids (Boutard et al., 1973), sterols (Attaway et al., 1971) and phenolic compounds (Char- riere et al., 1991) has been reported. The plant, collected in the Summer, was air dried and extracted with solvents with increasing polarity. Chro- matographic processes of the light petrol extract led to the isolation of seven diterpenes with the ent-labdane skeleton, ®ve of them isolated for the ®rst time. 2. Results and discussion The known compounds have been identi®ed as 15,16- epoxy-ent-labda-8(17),13(16),14-trien-19-ol (1) and methyl 15,16-epoxy-ent-labda-8(17),13(16),14-trien-19-oate (2) by comparison of their physical data with those repor- ted by Canonica et al. (1969) and Heauser and Lombard (1961) respectively. Compound 3,[a] D 10.0  , was assigned structure 15,16-epoxy-ent-labda-8(17),13(16),14-trien-19-al. The molecular peak at m/z 300 in the EI mass spectrum and the elemental analysis de®ned the molecular formula C 20 H 28 O 2 . The 1 H±NMR spectrum (Table 1) showed the aromatic protons H-14± H-16 at  6.24, 7.20 and 7.38, the H-20 and H-18 methyl singlets at  0.60 and 1.01, the H-17 methylene protons as two singlets at d 4.60 and 4.95 and the H-19 formyl proton at  9.78. In the 13 C-NMR spectrum (Table 2) 20 carbon signals were present, which were de®ned by a DEPT experi- ment. The signals at  13.5 and 24.3 were attributed to the C-20 and C-18 methyl carbons, the signal at  205.7 was attributed to the C-19 formyl carbon, while the signals at  125.3, 110.8, 142.7 and 138.7 corresponded to the C-13±C-16 furan carbons. The NOE interaction of the H-19 with the H-20 methyl justi®ed the a-orien- tation of the formyl group. According to the assigned structure, NaBH 4 reduction of 3 gave 15,16-epoxy-ent- labda-8(17),13(16),14-trien-19-ol (1). 0031-9422/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0031-9422(00)00253-3 Phytochemistry 55 (2000) 909±913 www.elsevier.com/locate/phytochem * Corresponding author. Tel.: +39-81-7041261; fax: +39-81-552 1217. E-mail address: monaco@unina.it (P. Monaco).