A Furanoid Labdane Diterpene from Potamogeton nodosus Nazmul Qais,* ,² Mushfiqur Rahman Mandal, ² Mohammad Abdur Rashid, ² Abdul Jabbar, ² Hiroyuki Koshino, ‡ Kazuo Nagasawa, ‡ and Tadashi Nakata* ,‡ Department of Pharmacy, University of Dhaka, Dhaka-1000, Bangladesh, and The Institute of Physical and Chemical Research (RIKEN), Hirosawa 2-1, Wako-shi, Saitama 351-01, Japan Received May 2, 1997 X A new furanoid diterpene, 15,16-epoxy-12-oxo-8(17),13(16),14-labdatrien-20,19-olide (1) was isolated from an ethanolic extract of Potamogeton nodosus. Its structure was elucidated by the usual spectroscopic methods, including 2D NMR techniques. Compound 1 was found to exhibit moderate inhibitory activity against a number of both Gram-positive and Gram-negative bacteria. Potamogeton nodosus Poir. (syn. P. indicus Roxb.), belonging to the Potamogetonaceae, is a submerged aquatic herb having creeping root stocks. 1 It grows on the littoral zones of lakes and ditches in Bangladesh and is also distributed in parts of India, Sri Lanka, Myanmar, and Malaysia. 2 In the Ayurvedic system, P. nodosus has been reported to be effective in treating cancer, tuberculosis, acne, and cough. 3 Previous chemi- cal investigations on different species of Potamogeton have shown the presence of alkaloids, 4,5 flavonoids, 4,5 and a labdane diterpenoid. 6 As a part of our continuing studies on medicinal plants of Bangladesh, we have examined P. nodosus and isolated a furanoid labdane diterpene (1). In this paper, we report the structure of 1 and its antibacterial activity. Solvent-solvent partitioning 7 of the cold ethanolic extract of P. nodosus followed by purification with column chromatography afforded a new furanoid lab- dane diterpene (1) as a yellow amorphous powder, together with -stigmasterol. The structure of 1 was determined as follows. The molecular formula was suggested as C 20 H 24 O 4 by high-resolution mass spec- trometry. The IR spectrum showed bands demonstra- tive of a furan ring (1570, 1505, 878 cm -l ) and two carbonyl groups (1730 1680 cm -l ). More detailed analy- sis was performed by NMR measurements. The typical low-field signals at δ 6.81 (H-14), 7.44 (H-15), and 8.20 (H-16) in the 1 H-NMR spectrum suggested the presence of a 3-substituted furan unit, which was conjugated with a carbonyl group. 8 In the HMBC 9 spectrum, the furan protons showed long-range correlations with the ketone carbon at 194.76 ppm at C-12. Two olefinic protons appeared at 4.80 and 4.62 ppm and were assigned to an exomethylene group. On the basis of the careful analysis of its PFG-DQFCOSY, 10 HMQC, 11 and HMBC data (Table 1), the structure of 1 was suggested as 15,- 16-epoxy-12-oxo-8(17),13(16),14-labdatrien-20,19- olide. The isolated methylene protons at 4.05 and 4.21 ppm gave long-range correlations with the carbonyl carbon at 173.56 ppm (C-20); hence, the presence of a lactone ring was inferred, and these methylene protons were assigned to the δ-position of the δ-valerolactone moiety. HMBC correlations from the methyl protons (H-18) to the oxygenated methylene carbon C-19 and H-1ax, and H-5 and H-9 to the carbonyl carbon in the lactone (C-20), supported the presence of a 20,19-olide functionality. 6,12 The relative stereochemistry was es- tablished by 1D PFG-selective ROESY 13 and NOE differential experiments. Strong NOEs were observed between H-1ax and H-9 and H-5 and H-9, respectively. Thus, the configuration of the decalin system was established as trans, and the side chain at C-9 as equatorial. The other observed NOEs are summarized in Table 1. From these NOE data, together with analysis of coupling constant values, all 1 H-NMR signals could be assigned as shown in Table 1. This is the first reported isolation of 15,16-epoxy-12- oxo-8(17),13(16),14-labdatrien-20,19-olide (1) from a natural source, although this compound has previously been prepared by a semisynthetic method. 12 The 13 C- NMR data of the isolated 1 are basically identical with those of the semisynthetic substance, but the reported assignments 12 of C-1, C-3, C-7, and C-14 should be revised as shown in Table 1, according to our present investigation. 14 Compound 1 exhibited moderate antibacterial activity against Bacillus cereus, B. subtilis, Shigella boydii, S. shiga, S. sonnei, Staphylococcus aureus, and Strepto- coccus faecalis; the zones of inhibition produced were 13, 16, 8, 12, 10, 14, and 12 mm, respectively. On the other hand, the zones of inhibition produced by the standard antibiotic, doxycycline, were 31, 34, 38, 37, 30, 35, and 35 mm, respectively. Experimental Section General Experimental Procedures. Silica gel 60 PF 254 was used for TLC while silica gel (60-120 mesh) * To whom correspondence should be addressed. N. Qais: Tel.: 880- 2-504043. Fax: 880-2-865583, E-mail: email@ducc.agni.com. T. Na- kata: Tel.: 81-48-467-9373. Fax: 81-48-462-4666. E-mail: nakata@riken.go.jp. ² University of Dhaka. ‡ The Institute of Physical and Chemical Research (RIKEN). X Abstract published in Advance ACS Abstracts, December 15, 1997. 1 156 J. Nat. Prod. 1998, 61, 156-157 S0163-3864(97)00224-3 CCC: $15.00 © 1998 American Chemical Society and American Society of Pharmacognosy Published on Web 01/23/1998