As indicated by double labeling experiments with stable isotopes, the cyclization of chanoclavine-I to tetracyclic ergolines involves an intermolecular transfer of the hydrogen at C-9 into the same position of a new molecule. Also in this process, one of the methylene hydrogens at C-17 of the precursor is exchanged, the newly introduced hydrogen occupying the pro-R position at C-7. This and the fact that chanoclavine-I-aldehyde is efficiently and specifically converted into elymoclavine by the ergot fungus suggest that the cyclization involves aldehyde intermediates. Experiments with (37?,4R)-mevalonic-2-14C-4-r acid indicate that an isotope effect in the further metabolism of chanoclavine-I leads to an enrichment of tritium at C-9 of the unreacted chanoclavine-I. A mechanism involving double bond isomerization at the aldehyde stage is proposed which accounts for all the observations.
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