Identification and synthesis of trinorcadalene phytoalexins formed by Hibiscus cannabinus

Two trinorcadalene phytoalexins, hibiscanal (2,8-dihydroxy-4,7-dimethoxy-6-methyl-1-naphthaldehyde) and o-hibiscanone (3,8-dimethyl-1,2-naphthoquinone), were isolated and identified from stem stele of kenaf (Hibiscus cannabinus) inoculated with the fungal pathogen Verticillium dahliae. o-Hibiscanone also was synthesized. The ED50 values of hibiscanal and o-hibiscanone against V. dahliae were 25.83 and 1.18 mu g/ml, respectively. o-Hibiscanone killed all propagules of V. dahliae and Fusarium oxysporum rtm f. sp. vasinfectum at 8 mu g/ml and 12 mu g/ml, respectively. o-Hibiscanone is more toxic to V. dahliae than desoxyhemigossypol, the most toxic phytoalexin known in cotton (Gossypium species), or mansonone C (3,8-dimethyl-5-isopropyl-1,2-naphthoquinone) which has been isolated from other malvaceous species. The implications of these findings for the genetic engineering of cotton phytoalexins is discussed. (C) 1998 Elsevier Science Ltd. All rights reserved.