Allogibberic acid

Gibberellic acid, C19H2206 (I) is a monobasic acid of pK 4.0. With dilute mineral acid at 20° C. (4) it gives allogibberic acid (II) which with acid at 100° C. yields gibberic acid (III). Structures of these two important degradation products have been conclusively established by us. The evidence has been published in full (II, 26) and it is not dealt with here. 

Gibberellic acid is tetracarbocyclic and the ready transformation to allogibberic acid suggests that it possesses the same carbocyclic structure as allogibberic acid and differs only in ring A which can be readily aromatized. This view is supported by many facts (5,10), only one of which is mentioned here. 

The carboxyl group is in the same position in both gibberic acid and allogibberic acid, since the methyl esters of each give the methyl ester of gibberic acid on boiling with mineral acid (5, 21). 

When the dimethyl ester (XLIX) (Figure 12) of the dibasic acid, obtained by opening ring D of allogibberic acid, is treated with alkali not only does the C-9 center epimerize, but the C-8a center is also inverted. This is shown by the fact that the dimethyl ester of the product (LI) shows a negative Cotton effect in contrast to the starting keto ester (XLIX). Epimerization at C-8a presumably takes place via the 1,3-diketone (L) followed by fission as shown by the dotted line. After having epimerized at C-8a and C-9, the dibasic acid (LI) and its... 

That allogibberic acid has rings B/C trans fused can be deduced as follows. 

Similar arguments for this absolute configuration (II Figure 10) of allogibberic acid have been adduced by Stork and Newman (32). 

Left to right. Untreated gibberellic acid As mixture of gibberellins Ai and As potassium gibbereUate zinc gibberellate methyl ester of gibberellic acid and allogibberic acid... 

A1—9 = GibbereUin A1—9 El— 9 = GibbereUin-A1—9, iiiethj lesters GI = Gibberic acid GE = Gibberellenio acid -1 — AG = Allogibberic acid (10) DA = Dicarboxylic acid A... 

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