Gibberellic acid reduction

Alkylation of the enolate of a carboxylic acid, formed as an intermediate in the Birch reduction of an aromatic acid, has been successfully exploited in synthesis, e.g., in the synthesis of gibberellic acid. A model compound 24 was reduced with sodium in diethyl ether-liquid ammonia and the resulting carbanion was alkylated with iodomethane to give 25 in ca. 80% yield87. 

Since the use of techniques to measure IAA, bioassays have been important to discover PGR activity of many other compounds. Several bioassays for the PGR gibberellic acid (GA3) have been developed. One bioassay was based on reduction of amaranthin levels in Amaranthus caudatus (tassel flower) seedlings.63 This method was sensitive to GA3 from 0.01 to 1 mg L"1. GA3 was also bioassayed, based on anthocyanin reduction in tomato (Lycopersicon esculentum L.).62 Reduction of anthocyanin in tomato seedlings was linear from 10"5 to 10 mg L 1, and thus, this latter plant bioassay method was more sensitive. A multitude of bioassays for nonvolatile and volatile plant growth regulator compounds have been developed and their uses and limitations have been discussed.129 Since many allelochemicals have been shown to have relatively weak phytotoxicity (especially compared to herbicides), some of these bioassays that have been developed for detecting and quantitatively measuring PGR activity may be useful in allelopathy. 

Delcosine Reduction of cambial growth, gibberellic acid (GA) — 249,252... 

Reductive cyclization. Potassium in liquid ammonia is an effective reagent for reductive cyclization of 5-alkynyl ketones. This reaction provides an efficient route to the D ring of gibberellic acid (1 —> 2). Sodium naphthalenide has been employed in a similar fashion for the preparation of 2-methylenecyclopentanols and 2-giethyl-enecyclohexanols (3 — 4). ... 

Hydroxy-(—)-kaur-16-en-19-oic acid and 6j ,7 -dihydroxy-( —)-kaur-16-en-19-oic acid have been prepared for investigation into the biosynthesis of gibberellic acid, the former by Meerwein-Ponndorf reduction of the 7-ketone and the latter by osmylation of a 6,7-olefin. 

Experiments showed that the free acid (31) could be used in this way Birch reduction quenched with halide (32) gave (33) which was hydrolysed and decarboxylated in acid solution to give (23). A new method was added to the literature of organic synthesis and was later used by Mander in his synthesis of gibberellic acid. ... 

Barton and coworkers used free radical cyclization in the synthesis of tetracyclines (Scheme 105). Photolysis of 254 (X,Y = SR or OR) gives the corresponding radical, which cyclizes to the (Cy6) compound 255 in 80% yield when X,Y = SCH2CH2O. Quite remarkably, 255 is formed only in the cis form. Another completely stereoselective reaction toward the cis compound involving intramolecular addition to an acetylenic bond has been described by Pradhan and was discussed in Section IX.2 (Scheme 70). An analogous reductive cyclization (K, NH3) of ethynyl ketones has been used by Stork in the construction of a tricyclic intermediate for the synthesis of gibberellic acid. ... 

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