Synthesis and evaluation of a putative acyl tetramic acid intermediate in tenellin biosynthesis in Beauveria bassiana.: A new role for tyrosine

The acyltetramic acid 6 previously proposed as a putative intermediate in tenellin biosynthesis in Beauveria barsiana has been synthesised in two isotopically labelled forms. This compound was not incorporated into tenellin and was not identifiable in extracts of B. bassiana and is unlikely to be involved in tenellin biosynthesis. On the other hand a re-evaluation of the role of tyrosine reveals that it is a good precursor to tenellin and is probably generated in vivo by the action of a phenylalanine hydroxylase directly from L-phenylalanine. Thus the long held contention that acyltetramic acid 6 undergoes oxidative ring expansion to a pyridone no longer appears a valid hypothesis for tenellin biosynthesis. (C) 1998 Elsevier Science Ltd. All rights reserved.