Monoterpenes Cinchona

The parent compound of the secoiridoids is secologa-nin (see secoiridoids), the most important intermediate in the biogenesis of alkaloids that are not derived from an amino acid. This includes most indole alkaloids, the ipecac, the Cinchona, and the pyrroloquinoline alkaloids as well as simple monoterpene alkaloids The best known biological property of the I. is their bitter taste. However, bitter principles are mostly not used as pure substances, instead alcoholic extracts are preferred to stimulate appetite (increased secretion of gastric juice). Furthermore, bitter substances are used to modify the taste of pharmaceutical products. Some I. exert various effects on the central nervous system as a consequence of their volatility and lipophilicity, e.g., nepetalactone, iridodial, teucrium lactones, and valepotriates. 

Biosynthesis In plants the quinoline system can be formed by several different biogenetic routes (biochemical convergence). In the Cinchona alkaloids tryptophan acts as the precursor while simple quinolines, furoquinolines, and acridines originate from an-thranilic acid. The second biosynthetic parmer is often a hemiterpene (for furoquinolines) or an iridoid monoterpene (secologanin, see secoiridoids) for Cinchona alkaloids. 

The parent compound of the S. is secologanin, a key compound in the biosyntheses of most indole alkaloids, the Cinchona, ipecac, and pyrroloquinoline alkaloids as well as simple monoterpene alkaloids. Over 1000 indole alkaloids are formed from secologanin in vivo - comprising almost a quarter of this large group of natural products. 

Loganin is an iridoid glucoside which occupies a central position in the biosynthesis of Corynanthe, Aspidosperma, Iboga, Ipecacuanha, Cinchona, and structurally simpler monoterpene alkaloids. For a review of the extensive researches that led to confirmation of the key role of loganin in alkaloid biosynthesis, see A. R. Battersby, Biochem. Soc. Symp., 29, 157 (1970) A. R. Battersby, Chem. Soc. Spec. Period. Rep., 1, 31 (1971) A. I. Scott, Accts. Chem. Res., 3, 151 (1970). For a useful account of the chemistry of iridoid glucosides see, J. M. Bobbitt and K. -P. Segebarth in Cyclopentanoid terpene Derivatives, Eds., W. I. Taylor and A. R. Battersby, Marcel Dekker, New York, 1969, p 1. 

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