Cyclic analogs with imino group

The amatoxins are cyclic octapeptides of L-amino acids the backbone is divided into two rings by a sulfur bridge analogously to the phallotoxins, but with an (jR)-sulfoxide. The amanitins (a-, j8-etc.) are derived from 6-hydroxy tryptophan, amanin lacks the phenolic OH-group. As in collagen, 4-frans-hydroxyproline is the imino acid the occurrence of y, -dihydroxyisoleucine is uncommon. 

Monoaminomonocarboxylic a-amino acids with a primary amino group produce sensory active aldehydes called Strecker aldehydes. Strecker degradation of P-amino acids yields alkan-2-ones known as methylketones (see Section 8.2.4.1.2). By analogy, alkane-3-ones (ethylketones) are formed from y-amino acids. The general reaction is schematically indicated in Figure 2.43. The reaction mechanism, however, varies considerably depending on the type of oxidant and amino acid. 2-Imino acids and 2-oxoacids can in some cases apparently form as intermediates, analogous to enzymatically catalysed transamination and oxidative deamination of amino acids (see Section 2.5.1.3.2). Some Strecker aldehydes readily decompose, such as methional, or yield cyclic products, such as 5-aminopentanal, which dehydrates to 2,3,4,5-tetrahydropyridine. 

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