Alkaloids of Croton Species

Homoaporphine Alkaloids.—Full details of the research on the biosynthesis of the homoaporphine alkaloids of Kreysigia multiflora have been published. In essence there is no information additional to that published in preliminary form and reviewed.  

Colchicine.—It is clearly established that ring a and carbons 5, 6, and 7 of colchicine (5) arise from phenylalanine by way of cinnamic acid. Beyond cinnamic acid the next known intermediate is a phenethylisoquinoline it is not as yet known whether reduction of the double bond in cinnamic acid occurs before or after construction of the isoquinoline ring. 

The conversion of phenylalanine into cinnamic acid is mediated by phenylalanine ammonia lyase (PAL) and the elimination reaction involves loss of the 

3-pro-S hydrogen. Accordingly, incorporation of phenylalanine into colchicine might be expected to result in retention of the 3-pro-R hydrogen and results of experiments with the appropriate precursors in Colchicum autumnale have shown this to be true.  

This result obtained in a whole plant adds further weight to the conclusion reached about the stereochemistry of PAL action with an isolated enzyme and contrasts with the results obtained with Amaryllidaceae alkaloids where apparently subsequent modification of the cinnamic acid results in loss of all the tritium originally present at C-3 (see below).  

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