TETRAHEDRON LETTERS Tetrahedron Letters 43 (2002) 2573–2575 Pergamon A new dimeric 9,10-dihydrophenanthrenoid from the rhizome of Juncus acutus Marina DellaGreca, a, * Antonio Fiorentino, b Pietro Monaco, b Lucio Previtera a and Armando Zarrelli a a Dipartimento di Chimica Organica e Biochimica, Universita ` Federico II, Complesso Universitario Monte Sant’Angelo, Via Cynthia 4, I -80126 Napoli, Italy b Dipartimento di Scienze della Vita, II Universita ` di Napoli, Via Vivaldi 43, I -81100 Caserta, Italy Received 30 January 2002; revised 11 February 2002; accepted 13 February 2002 Abstract—In a study of the allelochemical interaction between the wetland plant Juncus acutus and microalgae an unusual dimeric 9,10-dihydrophenanthrenoid has been isolated. The structure has been determined on the basis of its spectroscopic properties. © 2002 Elsevier Science Ltd. All rights reserved. 9,10-Dihydrophenanthrenoids form a rather uncom- mon class of metabolites originated or from stilbenic precursors 1 or by coupling of two phenylethane units. 2 Many of such compounds have been isolated from the aerial part of two Juncaceae, Juncus effusus 3 and Jun - cus acutus, 4 and in vitro assays have evidenced their antialgal activity. 5 This property might justify the allelopathic interaction of these plants with microalgae. In pursuing the study of J. acutus we have isolated from the rhizome of the plant an unusual dimeric 9,10-dihy- drophenanthrenoid, which was assigned the heptacyclic structure 1. The elemental analysis † and the presence of 36 carbon signals in the 13 C NMR spectrum (Table 1) justified the molecular formula C 36 H 36 O 4 . The EIMS spectrum lacked the molecular peak and showed the fragments at m /z 250 (65) and 282 (15%) attributable to a retro Diels–Alder of the cyclohexene ring. In the IR spectrum were present hydroxyl absorptions at 3806 and 3691, and one carbonyl absorption at 1730 cm -1 . The UV spectrum exhibited a maximum at 280 nm. The 1 H NMR spectrum showed in the aromatic region two coupled doublets at  6.77 and 6.43, two coupled doublets at  7.23 and 6.70, and two singlets at  6.99 and 6.80. Besides three olefinic double doublets at  5.74, 4.86 and 4.41 belonging to a vinyl group, a double doublet at  4.22, two multiplets of a methylene group at  3.16 and 1.83, and four methyls as singlets at  2.22, 2.18, 1.91, and 1.43 respectively were detectable in the aliphatic region. All the remaining protons were present as overlapped signals in the 3.1–2.3 ppm range. The 13 C NMR spectrum and the DEPT experiment indicated the presence of a carbonyl carbon, sixteen quaternary carbons, nine methines, six methylenes and four methyls. The rough structure was defined on the basis of COSY, HMQC and HMBC experiments (Table 1). Both the A * Corresponding author. Tel.: +39-81-674162; Fax: +39-81-674393; e-mail: dellagre@unina.it † Elemental analysis: Found: C, 81.08; H, 6.77; Calcd: C, 81.17; H, 6.81%. 0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII:S0040-4039(02)00308-8