Four dimeric prenylindoles occurring in diastereomeric pairs, the caulindole A-D, 5-(3-methyl-2-butenyl)-1H-indole, and (E)-5-(3-methylbuta-1,3-dienyl)-1H-indole were isolated from the stem and root barks of Isolona cauliflora, an ecologically endangered Annonaceae species. Structural determination was achieved based on interpretation of spectroscopic data. Biogenetically, the caulindoles are considered as Diels-Alder-type cycloaddition products of mono- and/or bis-prenylindoles [e.g. 5-(3-methyl-2-butenyl)-1H-indole and (E)-5-(3-methylbuta-1,3-dienyl)-1H-indole] as the dienes and dienophiles. (C) 2003 Elsevier Ltd. All rights reserved.