BIOSYNTHETIC-STUDIES ON ANSATRIENIN-A - FORMATION OF THE CYCLOHEXANECARBOXYLIC ACID MOIETY

The formation of the cyclohexanecarboxylic acid moiety in the biosynthesis of ansatrienin (mycotrienin) has been studied. C-13- and H-2-labeled samples of shikimic acid were used to probe the stereochemistry of processing the cyclohexane ring of shikimic acid and to establish the fate of all the precursor hydrogens in this transformation. A sample of [2-C-13]shikimic acid was fed to Streptomyces collinus Tu 1892, and C-13 in the resulting ansatrienin was found to reside exclusively at C-36. The 1-cyclohexenecarboxylic acid accompanying the cyclohexanecarboxylic acid in the hydrolysis of the biosynthetic sample of ansatrienin carried the C-13 label not at C-2 but at C-6. Samples of [2-H-2]-, [3-H-2]-, [4-H-2]-, [2,5-H-2(2)]-, [2,3,4,5-H-2(4)]-, and [6-H-2(1)]shikimic acid were fed to S. collinus. Deuterium from C-2, C-3, C-4, and C-5 was effectively incorporated and occupied the 36R (axial), 35R (equatorial), 34E (equatorial), and 33R (axial) positions, respectively, in the resulting ansatrienin A. However, absolutely no deuterium from C-6 of shikimic acid was incorporated. Potential intermediates specifically labeled with C-13 and H-2 were used to further delineate the pathway. Combined, the results from these studies define the pathway by which shikimic acid is converted into cyclohexanecarboxylic acid. 1,4-Conjugate elimination of the hydroxy group at C-3 and a proton from C-6 of shikimic acid gives rise to a cross-conjugated dihydroxy diene, which undergoes reduction of the double bond conjugated to the carbonyl group. Another 1,4-elimination involving the C-4 hydroxy group and the proton at C-1 gives a 5-hydroxy 1,3-diene. Reduction to 5-hydroxycyclohex-1-enecarboxylic acid proceeds either directly or via the DELTA2 isomer. Another reduction gives the hydroxy acid, which undergoes dehydration involving a nonacidic proton. Isomerization of the double bond into conjugation and a final reduction completes the sequence.