X-Ray Analysis of Volubilide, a New Decacyclic Diels±Alder C20±C30 Adduct from Hyppocratea volubilis L. Nelson L. Alvarenga, a Esteban A. Ferro, a Angel G. Ravelo, b, * Marõ Âa L. Kennedy, b Miguel A. Maestro c and Antonio G. Gonza Âlez b a Departamento de Fitoquõ Âmica, Direccio Ân de Investigacio Ân, Facultad de Ciencias Quõ Âmicas de la Universidad Nacional de Asuncio Ân, P.O. Box 1055 Asuncio Ân, Paraguay b Instituto Universitario de Bio-Orga Ânica Antonio Gonza Âlez., Universidad de La Laguna, Avenida Astrofõ Âsico Francisco Sa Ânchez 2, La Laguna. 38206, Tenerife, Spain c Servicios Xerais de Apoio a  Investigacio Ân. Universidade da Corun Äa. E-15071 A Corun Äa, Spain Received 14 January 2000; revised 21 March 2000; accepted 6 April 2000 Abstract ÐA new Diels±Alder adduct, 1, has been isolated from Hyppocratea volubilis L. and its structure determined by means of 1 H and 13 C NMR spectroscopic studies including H-C heteronuclear correlation (HETCOR), and long-range correlation with inverse detection (HMBC). The absolute con®guration was determined by X-ray analysis. q 2000 Elsevier Science Ltd. All rights reserved. The climbing plant Hyppocratea volubilis L. (Celastraceae) is distributed throughout the humid eastern region of Para- guay and investigation of the chemical composition of its roots resulted in the isolation (Fig. 1) of a new decacyclic Diels±Alder C20±C30 adduct, 1, with a structure based on two monomer units from abietane and lupane di/triterpene skeletons. From 1991 onwards, Farnsworth and co-workers have isolated several cytotoxic lupanes with a 30±21a olide 1 system and other related compounds 2 from Kokoona ochracea (Elm.) Merrill (Celastraceae). More recently, lactones 3 with farnesyl protein transferase inhibitory activity were obtained from Lophopetalum wallichii Kurz (Celastraceae). As part of our study of bioactive American Celastraceae, this paper describes the structural elucidation of a hitherto unknown metabolite, 1, a decacyclic C20±C30 Diels±Alder adduct derived from the dienophile, triterpene ochraceolide A 1 2 and a diene, diterpene 3, never isolated as a monomer, but present in the dimer maytenone 4 (4). Celastraceae species biosynthesize triterpene dimers 5,6 and trimers, 7 possibly by hetero Diels±Alder reactions 8 although the formation of the 1±4 dioxane system found in all these products may be due to a radical coupling process as postulated for other kinds of natural products. 9 Previous data 5±7 plus those reported here would seem to indicate the probable presence of enzymatic systems with homo and/or hetero Dielsalderase activity. 10 Repeated chromatography on silica gel and Sephadex LH-20 afforded a crystalline product. HREIMS, 13 C, 1 H NMR and X-ray data 11 gave the formula C 50 H 74 O 5 . The MS showed the molecular ion at m/z 754 and fragments corresponding to the two units C20 and C30. These data, together with the number of methyl groups and unsatura- tions indicated that the product consisted of two terpene units. Analysis of the 13 C NMR data and HMBC and HMQC experiments identi®ed the C-30 unit as a lupane derivative with a 30±21a lactone group having no signals for the 20(29) double bond found in ochraceolide A (2). This suggests that the 20(29) bond could be involved in the junction with the diterpene unit which contained a Tetrahedron 56 (2000) 3771±3774 Pergamon TETRAHEDRON 0040±4020/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S0040-4020(00)00301-X Figure 1. Keywords: Celastraceae; decacyclic; terpene; crystalline structure. * Corresponding author; e-mail: agravelo@ull.es