Patricia A. Rose

8[prime]-Methylene Abscisic Acid (An Effective and Persistent Analog of Abscisic Acid)

We report here the synthesis and biological activity of a new persistent abscisic acid (ABA) analog, 8[prime]-methylene ABA. This ABA analog has one additional carbon atom attached through a double bond to the 8[prime]-carbon of the ABA molecule. (+)-8[prime]-Methylene ABA is more active than the natural hormone (+)-ABA in inhibiting germination of cress seed and excised wheat embryos, in reducing growth of suspension-cultured corn cells, and in reducing transpiration in wheat seedlings. The (+)-8[prime]-methylene analog is slightly weaker than (+)-ABA in increasing expression of ABA-inducible genes in transgenic tobacco, but is equally active in stimulating a transient elevation of the pH of the medium of corn cell cultures. In corn cells, both (+)-ABA and (+)-8[prime]-methylene ABA are oxidized at the 8[prime] position. ABA is oxidized to phaseic acid and (+)-8[prime]-methylene ABA is converted more slowly to two isomeric epoxides. The alteration in the ABA structure causes the analog to be metabolized more slowly than ABA, resulting in longer-lasting and more effective biological activity relative to ABA.

The 7[prime]-Methyl Group of Abscisic Acid Is Critical for Biological Activity in Wheat Embryo Germination.

Wheat (Triticum aestivum L.) embryo germination is inhibited by natural (S)-(+)-abscisic acid (ABA). In this report we have determined critical structural features of the ABA molecule, particularly the methyl and ketone groups of the ABA ring, required for inhibitory activity. To examine the ring residues a series of new optically active ABA analogs have been synthesized in which the 4[prime]-keto, 7[prime]-, 8[prime]-, or 8[prime]- and 9[prime]-carbons have been replaced with hydrogen atoms. Each of the analogs was tested over a range of concentrations as a germination inhibitor. Enantiomers of the analogs altered at the 4[prime]-keto or 8[prime]- and 9[prime]-methyl groups were active, but less so than ABA. Both enantiomers of 7[prime]-demethylABA were inactive as germination inhibitors. The results show that the 7[prime]-methyl group is absolutely required for activity, but that the other residues are less critical for hormone recognition.

Synthesis, metabolism and biological activity of a deuterated analogue of the plant hormone S-(+)-abscisic acid

Abstract The synthesis of a deuterated analogue of the plant hormone S-(+)-abscisic acid (ABA), S-(+)-[7′, 7′, 7′, 8′, 8′, 8′, 9′, 9′, 9′]-nonadeuteroabscisic acid [(+)-d9-ABA], is described. The biological activity of (+)-d9-ABA was compared to that of (+)-ABA in two assays. Analogous to the metabolism of (+)-ABA to (−)-phaseic acid (PA) by maize (Zea mays L. cv Black Mexican Sweet) cell suspension cultures, (+)-d9-ABA was oxidized but at a slower rate to d8-PA at 25°. At low concentrations the deuterated analogue was a more effective inhibitor of cress seed (Lepidium sativum L.) germination than the natural hormone.

Synthesis and biological activity of tetralone abscisic acid analogues

Bicyclic analogues of the plant hormone abscisic acid (ABA) were designed to incorporate the structural elements and functional groups of the parent molecule that are required for biological activity. The resulting tetralone analogues were predicted to have enhanced biological activity in plants, in part because oxidized products would not cyclize to forms corresponding to the inactive catabolite phaseic acid. The tetralone analogues were synthesized in seven steps from 1-tetralone and a range of analogues were accessible through a second route starting with 2-methyl-1-naphthol. Tetralone ABA 8 was found to have greater activity than ABA in two bioassays. The absolute configuration of (+)-8 was established by X-ray crystallography of a RAMP hydrazone derivative. The hydroxymethyl compounds 10 and 11, analogues for studying the roles of 8- and 9-hydroxy ABA 3 and 6, were also synthesized and found to be active.

Defining steric, electronic and conformational requirements of carrier-mediated uptake of abscisic acid in barley suspension culture cells

Abstract The structural and conformational requirements of a carrier responsible for the saturable component of the uptake of the plant hormone (+)-abscisic acid in barley suspension culture cells have been probed through the use of a defined series of optically pure ABA analogues. Two analogues showed a tenfold increase over (+)-ABA in inhibiting the uptake of radiolabelled (+)-ABA. Results from different studies (molecular modelling, low temperature NMR spectroscopy and assays with sterically rigid analogues) have shown the likely conformation of ABA in the binding site of the carrier is that with the sidechain in an equatorial-like orientation. Structure/activity studies show that the C-1 carboxylic acid is essential for binding, the C-4 carbonyl is moderately important while the C-1′ hydroxyl group is not important for binding to the carrier.

Probing the role of the hydroxyl group of ABA: Analogues with a methyl ether AT C-1☆

Abstract (+)-( S )- and (−)-( R )-C-1′- O -methyl abscisic acids and their methyl esters, as well as the methyl ethers of the acetylenic analogue of methyl ABA, were synthesized through an enantioselective route, giving a series of optically active, new C-1′ substituted analogues with known stereochemistry. In a wheat embryo germination inhibition assay, (−)-C-1′- O -methyl ABA shows high activity, comparable with (+)- and (−)-ABA, whereas (+)-C-1′- O -methyl ABA is less active. In a wheat seedling transpiration assay, both analogues exhibit weak activity although the (+)-ABA-like analogue is more potent than its enantiomer. The anti-transpirant response increases over time, which may indicate that the analogue is being metabolized to ABA in vivo .

Synthesis of chiral acetylenic analogs of the plant hormone abscisic acid

Abstract Syntheses of optically active acetylenic analogs of abscisic acid are described. The key step involves the diastereoselective alkylation of the (2S,3S)-butanediol ketal of oxoisophorone, which produces a 3:1 mixture of separable diastereoisomers. The absolute stereochemistry of the analogs was established by conversion to a known derivative and by correlation of ORD data.

Synthesis, resolution and biological activity of 7′,7′-difluoroabscisic acid

Abstract 7′,7′-Difluoroabscisic acid was synthesized in seven steps from the known ketoaldehyde 4,4-ethylenedioxy-2-formal-6,6-dimethylcyclohex-2-en-1-one. Fluorination with diethylaminosulphur trifluoride afforded the key intermediate 2-difluoromethyl-4,4-ethylenedioxy-6,6-dimethyleyclohex-2-en-1-one which was further transformed to 7′,7′-difluoroabscisic acid. The racemic mixture and the individual optical isomers, obtained by resolution of the methyl esters by HPLC, displayed activity similar to the optical isomers of ABA in assays for germination in cress and freezing tolerance in bromegrass cell suspension culture.

Metabolism and biological activity of (+)- and (-)-C-1′-O-methyl aba in maize suspension-cell cultures☆

Abstract (+)- C -1′- O -Methyl abscisic acid is rapidly metabolized by suspension-cultured maize cells ( Zea mays L. Black Mexican Sweet) to (+)- C -1′- O -methyl phaseic acid in an analogous process as seen for ABA. The presence of a methyl ether on the 1′-position of ABA does not interfere with enzymic oxidation at the 8′-carbon. The metabolite is demethylated to yield phaseic acid. A small amount of abscisic acid is also produced by direct demethylation. The (+)- C -1′- O -methyl ABA exhibits stronger growth inhibitory activity of the maize cells than (+)-ABA, suggesting that a free C -1′-hydroxyl group is not essential for biological activity of ABA in maize. The (−)- C -1′- O -methyl ABA is metabolized to (−)-ABA and to 7′-hydroxyABA and the corresponding C -1′- O -methyl-7′-hydroxy ABA.