[Chemical studies of indole alkaloids]

Up to now, more than two thousands indole alkaloids having structural variety have been found from the three families of Gentianales: Loganiaceae, Apocynaceae, and Rubiaceae plants. These compounds generally possess characteristic biological activities and many of them are utilized for the medicinal purpose and for the lead-compounds to develop new synthetic drugs. The author of this review has been investigated the alkaloidal constituents of the above mentioned plants native to Japan and overseas countries. Especially, a cooperative work with Thai researchers concerning the plants in Thailand was developed. On the basis of biogenetic consideration of the structures of monoterpenoid indole alkaloids, chemical bond cleavage between the C3-N4 linkage in the C/D ring of indole alkaloids was discovered. It was speculated that, either in biogenesis, the flexibility of the ring-opening compounds having ten membered ring enabled the construction of indole alkaloids having highly strained polycyclic structures. Along with this conception, the syntheses of structurally complex indole alkaloids utilizing a biomimetic procedure have been investigated. In this review, recent results on the chemical synthesis of many skeletally varied Gelsemium alkaloids (Loganiaceae plant) are mainly described.