| Salacia madagascariensis (Lam.) DC, a member of the Celastraceae family, is a shrub found in East Africa and Madagascar. It is used in traditional medicine to treat fever, menorrhagia, and malaria. Previous phytochemical investigation of S. madagascariensis resulted in the isolation of isoiguesterin, a bisnortriterpene quinone methide. It showed moderate antileukemic activity but was not further developed. In initial screening at Walter Reed Army Institute of Research in 2001, isoiguesterin showed potent activity against leishmaniasis. This research focused on the reexamination of S. madagascariensis and on the structural modification of isoiguesterin.; The first phase of this research involved the extraction and isolation of isoiguesterin and other natural products, especially triterpenoid quinone methides, from S. madagascariensis. Two new bisnortriterpene quinone methides, 20-epi-isoiguesterinol and 6-oxoisoiguesterin, were isolated as well as the known triterpene quinone methides, isoiguesterin and isoiguesterinol. The structures were elucidated using nuclear magnetic resonance (NMR) spectroscopy, including two-dimensional techniques, and mass spectrometry.; The second phase of the investigation was the structural modification of isoiguesterin. The modifications occurred at either the quinone methide of the A-B ring or the double bond of the E ring. Modifications to the exocyclic double bond included epoxidation and hydrogenation. The epoxidation formed 20,29-epoxyisoiguesterin. Hydrogenation procedures resulted in the reduction of the quinone methide and a migration of the exocyclic double bond to the endocyclic forms. The quinone methide of the A-B ring was found to be very reactive, readily adding a variety of nucleophiles. Derivatives included 6-tolylthioisoiguesterin, 6-cyanoisoiguesterin, and 6-acetonylisoiguesterin. The 6-tolylthioisoiguesterin had to be converted to the diacetate in order to isolate it. The structures were elucidated using NMR spectroscopy. Celastrol, 20-epi-isoiguesterinol, and four homologues (20,29-epoxyisoiguesterin, 6-cyano-isoiguesterin, 6-acetonylisoiguesterin, and dihydroisoiguesterin diacetate) were then subjected to testing against leishmaniasis. The exocyclic double bond of the E ring was not needed for activity against leishmaniasis. With regard to the quinone methide, substituents at the C-6 position reduced the activity by an order of magnitude; likewise, the diphenol also had an order of magnitude less activity than the quinone methide form. Triterpene quinone methides had better activity against leishmaniasis than the current treatments and have excellent potential for future drug development. |
