Ring alkaloids

Cyanobacterial neurotoxins are small ringed alkaloids and have dramatic effects on various components of vertebrate neurons. They are all water soluble and are synthesized by several cyanobacterial genera (Table 5.1). The most commonly isolated neurotoxins are the paralytic shellfish toxins, although several other potent neurotoxic alkaloids are synthesized by freshwater cyanobacteria (Table 5.1). 

Thiazole-containing pyridoacridines are a group of fused ring alkaloids having a pyrido[4,3,2-w, ]thiazolo[3,2-6]acridinium skeleton related to shermilamines, varamines and diplamine, which were discussed in the sulfide section [37]. The pyridoacridines have a characteristic UV absorption pattern [A.max(MeOH) 245, 307, and 361 nm], which is highly sensitive to the pH of the medium. 

One part is the large collection of psychoactive compounds known as the phenethylamines. The first known plant psychedelic was mescaline, or 3,4,5-trimethoxyphenethylamine. This simple one-ring alkaloid was discovered in the North American dumpling cactus Peyote (Anhalonium williamsii) in the late nineteenth century, and is now known to be a component of over fifty other cacti. Over a dozen other cactus phenethylamines have been isolated and identified, and there are perhaps a hundred synthetic analogues that are now also known to be psychedelic in action. This body of information has been published by my wife Ann and me as a book entitled "PIHKAL A Chemical Love Story." PIHKAL stands for Phenethylamines I Have Known and Loved. 

A number of new 14-membered-ring alkaloids incorporating the p-hydroxy-styrylamine and j8-hydroxyphenylalanine structural features have been isolated and structurally elucidated, mainly by application of high-resolution mass spectrometry. Cre natine A (55) has been obtained from Discaria crenata while alkaloid A, (feretine) (56) and alkaloid A2 (57) have been isolated from Feretia apondanthera. In addition to four known alkaloids, the root bark of Zizyphus nummularia has yielded nummularine-D (58) and nummularine-E (59). ... 

Nummularine-F (60) and zizyphine-G (61), elaborated by Z. nummularia and Z. oenoplia respectively, represent the only new additions to the 14-membered-ring alkaloid class which exhibit the p-hydroxystyrylamine and 3-hydroxyproline units as characteristic features. The known cyclopeptide mauritine-A, together with minor amounts of amphibines-A, -E, and -F, and mauritine-C have been isolated from Z. spinachristi of Nigerian origin. ... 

Procter and coworkers also applied their connective Pummerer methodology to the total synthesis of the fused-ring alkaloid cryptotackieine (neocryptolepine). To this end, they followed the S5mthetic route shown in Scheme 20.3S. which includes a connective Pummerer cyclization from 179 followed by a p5a idine-forming reaction sequence mediated by samarium, also developed by the Procter group. 

For the most part, five-membered ring alkaloids tend to make one think of pyrrole- and imidazole-derived alkaloids. There are, however, also alkaloids that fall into this class that are, for example, derived from 1,2,4-thiadiazoles, for example, 19, the characterization of which is discussed Section 7.1.8. 

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