Gibberellin derivatives

The u.v. irradiation of the gibberellin derivative (16a) in CeHe-T O gave the tritiated product (16b). 

The gibberellin derivative (55) is photochemically reactive and on irradiation affords the lactone (56). The reaction presumably involves a photoreaction akin to the Norrish Type II hydrogen abstraction process, in thii case a 1,8-hydrogen transfer, from the allylic site to yield the biradical (57). Bond... 

This method was also used to construct the strained oxetane 22 from the gibberellin derivative 20. Acid-catalyzed ring opening of 22 yields the C-ll hydroxylated gibberellin 23, in which nucleophilic attack has occurred stereospecifically with inversion at C-867. 

Some microbiological reactions were attempted either on natural kaurane diterpenoids isolated from Sideritis or on their hemisynthetic derivatives, by incubating them with Gibberella fujikuroi. The purpose was the transformation of the products into gibberelline derivatives. However, epicandicandiol (7) yielded no gibbane compounds, only oxidation of the 19-Me occurred, with the formation of the CH2OH and COOH derivatives (A) and (B) [112]. 

The alternative fate, i.e. other than elimination of the 1,4-biradical formed by a Norrish Type II process, is the formation of cyclobutanols. Such is the outcome of the irradiation of the gibberellin derivative (64a) which yields the cyclobutanol (65). Treatment of this derivative with a tritium or deuterium donor affords the labelled derivative (64b).Azetidinols (66) are formed on irradiation of the ketones (67) in ethereal solution. The azetidinols (66) arise by way of the Norrish... 

Indeed, the reproducibility of this method was questioned by Lombardo in 1982 [14]. He attempted to apply Takai and Oshima s procedure to the methylenation of a gibberellin derivative with Zn (pure, without lead)-CH2Br2-TiCl4, but only decomposition of the substrate was observed. Lombardo later demonstrated an improved procedure. According to his report, the requisite ageing period for the preparation of the reagent was three days (Scheme 5.7). 

Scheme S.7. Methylenation of a gibberellin derivative by Lombardo s method.

Abscisin II is a plant hormone which accelerates (in interaction with other factors) the abscission of young fruit of cotton. It can accelerate leaf senescence and abscission, inhibit flowering, and induce dormancy. It has no activity as an auxin or a gibberellin but counteracts the action of these hormones. Abscisin II was isolated from the acid fraction of an acetone extract by chromatographic procedures guided by an abscission bioassay. Its structure was determined from elemental analysis, mass spectrum, and infrared, ultraviolet, and nuclear magnetic resonance spectra. Comparisons of these with relevant spectra of isophorone and sorbic acid derivatives confirmed that abscisin II is 3-methyl-5-(1-hydroxy-4-oxo-2, 6, 6-trimethyl-2-cyclohexen-l-yl)-c s, trans-2, 4-pen-tadienoic acid. This carbon skeleton is shown to be unique among the known sesquiterpenes. 

Since carotenoids are isoprenoids, they share a common early pathway with other biologically important isoprenoids such as sterols, gibberellins, phytol and the terpenoid quinones (Fig. 13.3). In all cases, these compounds are derived from the C5 isoprenoid, isopentenyl diphosphate (IPP). Until a few years ago it was believed that a single pathway from the Cg precursor mevalonic acid (MVA) formed IPP, which itself was synthesised from hydroxymethylglutaryl coenzyme A (HMG CoA) by the action of HMG... 

Since carotenoids are derived for the central isoprenoid pathway (Fig. 13.3), the regulation of their formation must involve a co-ordinated flux of isoprenoid imits into this branch of the pathway as well as into others such as the biosynthesis of sterols, gibberellins, phytol and terpenoid quinones. An imderstanding of the complexities of regulation of the pathway is necessary in order to target the regulatory steps for genetic manipulation. 

A number of papers on the biological activity of gibberellins and their derivatives have appeared. The stimulation of plant mRNA synthesis by gibberellic acid has been noted. 

Gibberellins which have a saturated ring A undergo epimerization at C-2 with dilute alkali at room temperature, presumably by a reversed aldol mechanism via intermediate (XXXIII) as first suggested by J. W. Comforth (3a). This epimerization has been shown to occur with gibberellin At and its methyl ester (XXXIV), with both C-8 epimeric tetrahydro derivatives of gibberellic acid and... 

Table IV shows the Rf values for all the gibberellins except A2, on paper and in four different solvent systems. The gibberellins are arranged in order of increasing mobility (in fact, in increasing number of hydroxyl groups). The relative mobilities (R.M.) in brackets are given to compare with relative mobilities derived from published Rf values for unidentified gibberellin-like materials.

Acyl. The acetyl, butyryl, benzoyl, and diacetyl derivatives of gibberellin were reported by Moffatt and Radley (14) to be as active as the add when applied to the roots or leaves of pea. 

Derivative formation is essential for analysis of gibberel-lins because they only absorb radiation below 230 nm, which is an extremely nonspecific region. Benzyl esters (49) and p-nitro-benzyl esters (60) of gibberellins have successfully been synthesized prior to injection to permit their detection as they elute from HPLC columns. Unfortunately, these derivatives have added little selectivity to the analytical procedure. The derivatives are monitored at 254 or 265 nm which, as previously mentioned, is a nonspecific region. 

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