Chapter 8 The Monoterpene Alkaloids

Abstract

Earlier, the biogenesis of the monoterpene alkaloids, particularly, gentianine, centered on the prephenic acid hypothesis. When theories on indole alkaloid biosynthesis from an iridoid precursor were introduced, it became clear that the other alkaloids could also be derived from the iridoids. The alkaloids, rather than condensing with tryptamine/tryptophan with subsequent reaction, would condense with ammonia and give a series of alkaloids containing a C 10 unit. From the beginning, substantial number of alkaloids formed in this manner has been isolated. As further compounds of this type were isolated, two distinct types became apparent— that is, the iridoids and those with a cleaved cyclopentane ring, the secoiridoids. The discussion of the chapter is based on this distinction as applied to the monoterpene alkaloids. Original interest in many of the plants from which these alkaloids have been isolated was in most cases based on the experimental or folkloric experience with the crude drug. This data and subsequent work on the pharmacology of the alkaloids isolated from these drugs are discussed in the chapter. One of the first applications of nuclear magnetic resonance (NMR) spectroscopy in the area of natural product structure elucidation was used successfully for skytanthine. The principal folkloric reputation of Gentiana sp. is as a tonic that may be related to the hypotensive and muscle-relaxant activity of gentianine.