The Structures of the Fungal Gibberellins Β. E. CROSS, J. F. G R O V E , P. McCLOSKEY, J. M a c M I L L A N , J. S. MOFFATT, a n d T. P. C. MULHOLLAND Akers Research Laboratories, Imperial Chemical Industries, Ltd., The Frythe, Welwyn, Herts, England
Investigations in Japan in the 1920's of the bak anae disease of rice, caused by the fungus Gib
Downloaded by 80.82.78.170 on January 19, 2017 | http://pubs.acs.org Publication Date: January 1, 1961 | doi: 10.1021/ba-1961-0028.ch001
berella fujikuroi, led in 1939 to the isolation of crude gibberellin, a crystalline material with plant growth-promoting properties. As a result of in tensive research in the past five years, the fungal gibberellins now comprise a group of six chemi cally closely related metabolites of Gibberella fujikuroi. CHO 19
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The structure
of
gibberellic
acid,
the most readily available of the gib
6
berellins, has been elucidated and studies of its biosynthesis have shown that it can be classed as a diterpenoid.
The other fungal gibberellins—A , 1
A , A , A , and A —have been chemically related to 2
4
7
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gibberellic acid. Evidence for the structures of the gibberellins is reviewed and the stereochemistry of gibberellic acid is discussed.
The study of the
fungal gibberellins has led to the discovery that gibberellins occur in higher plants.
•n September 1957, the American Chemical Society held its first Gibberellin Symposium. At that time only four gibberellins were known, all metabolites of the fungus Gibberella fujikuroi. Today nine gibberellins have been isolated and fully characterized. Five have only been isolated from the fungus, three from a higher plant, and one from both fungus and higher plants. The five.new gibberellins which have been isolated recently in our labora tories have been named gibberellins A to A as they were isolated and shown to be structurally related to the known gibberellins. No distinction has been made between fungal gibberellins and those of higher plants. This paper presents a brief review [for a more comprehensive review see (3) ] of the chemistry of the fungal gibberellins, gibberellic acid C H 2 0 (14), gib berellin Αχ C H O (31), gibberellin A C H O (34), gibberellin A C H O (38), gibberellin A C H 0 (6), and gibberellin A C H 0 (7). Gibberellic acid is obtained most readily pure and in quantity and knowledge of the chemistry of the gibberellins comes largely from study of the chemistry of 5
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1 9
1 9
2 4
s
2 4
e
2
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1 9
2 2
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1 9
2 6
2
a
e
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1 9
2 4
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3 GIBBERELLINS Advances in Chemistry; American Chemical Society: Washington, DC, 1961.
ADVANCES I N CHEMISTRY SERIES
gibberellic acid. I would like therefore to summarize the evidence for our struc ture of gibberellic acid and deal, at the same time, with gibberellin A We can then consider the other fungal gibberellins and their relationship to gibberellic acid. Finally, I would like to discuss the stereochemistry of gibberellic acid. v
Downloaded by 80.82.78.170 on January 19, 2017 | http://pubs.acs.org Publication Date: January 1, 1961 | doi: 10.1021/ba-1961-0028.ch001
Structure of Gibberellic Acid
Saturated 5-membered ring lactone ring Secondary hydroxyl group Double bond Gibberellic acid
Saturated 5-membered lactone Secondary hydroxyl group — Gibberellin Ai
Saturated 5-membered ring lactone Secondary hydroxyl group — Gibberellin C
Figure 1. Gibberellic acid and derivatives Gibberellic acid, C H 2 0 (I) is a monobasic acid of pK 4.0. With dilute mineral acid at 2 0 ° C . (4) it gives allogibberic acid (II) which with acid at 1 0 0 ° C . yields gibberic acid (III). Structures of these two important degrada tion products have been conclusively established by us. The evidence has been published in full (11, 26) and it is not dealt with here. Gibberellic acid is tetracarbocyclic and the ready transformation to allo gibberic acid suggests that it possesses the same carbocyclic structure as allo gibberic 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. Controlled reduction (17) of gibberellic acid (as its methyl ester) with palla dium on charcoal reduces one double bond—not the terminal methylene double bond, for in the product, gibberellin A methyl ester can be ozonized (27) to formaldehyde and an α-ketol (Figure 1). Reduction of this nonterminal meth ylene double bond blocks the ready aromatization to ring A of allogibberic acid, yet the ring C / D chemistry of gibberellin A parallels (5) that of allo gibberic acid (26). T o take only one example, Takahashi and his colleagues (34, 35) have shown that gibberellin Αχ undergoes acid
