ALKALOIDS 1 57 



subject of alkaloid biosynthesis is rarely given comprehensive formal 

 review, and then one is impressed with the incompleteness of knowl- 

 edge and the prevalence of hypotheses supported by circumstantial 

 evidence alone (Mothes and Romeike, 1958; Marion, 1958; and 

 Poisson, 1958). It should be noted that numerous alkaloids show 

 structurally a potential relationship to one or more amino acids. Con- 

 sequently, it is generally regarded that alkaloid synthesis is related 

 to amino acid synthesis. This generalization has proved helpful in 

 seeking relationships between alkaloids otherwise difficult to interpret 

 (discussed by Schhttler, 1956). Hegnauer (1958) has placed the major 

 alkaloid types into amino acid "families" for purposes of disclosing 

 useful systematic correlations. The arbitrary basis of such schemes 

 should be remembered, however. The directness of the relationship of 

 the biosynthesis of a particular alkaloid to a corresponding amino 

 acid may vary greatly in different cases. Wenkert (1959) has recently 

 suggested that we may be overemphasizing the relationship between 

 alkaloid and amino acid biosynthesis and thereby losing sight of a 

 potential relationship between alkaloid and carbohydrate metabolism, 

 particularly reaction sequences leading toward or derived from 

 aromatic synthesis. 



Some major classes of alkaloids 



protoalkaloi ds 



These comprise a group of simple alkaloids lacking a hetero- 

 cyclic nitrogen-containing ring. Their structure suggests a relationship 

 to the aromatic amino acids (for example, tyrosine), and evidence 

 exists that hordenine is formed by decarboxylation of tyrosine 

 followed by N-methylation (Marion, 1958). Mescaline is also formed 

 from tyrosine by decarboxylation followed by hydroxylation of the 

 ring and methylation (Leete, 1959). These alkaloids are found in such 

 widely separated plant families as the Gnetaceae, Gramineae, Cacta- 

 ceae and Leguminosae, and their systematic value is therefore limited 

 to considerations of intrafamihal phylogeny. 



Representative types of protoalkaloids are the following: 



CH3 



/ \cHCHNHCH3 

 ~ OH 



ephedrin 



ho/;choCH2n;;^ 

 ^=^ CH3 



hordenine 



