Synthesis of New 9,15-Cyclogibberellins from Developing Apple Seeds: Confirmation of Structure for GA105 and GA108

Abstract

The structures of two new gibberellins (GA₁₀₅ and GA₁₀₈) from developing apple seeds were confirmed as 2β- and 11β-hydroxy-9,15-cyclo-GA₉, respectively, by synthesis of their methyl esters (7) and (8); the analogous 1α-, 2α- and 11α-isomers were also prepared, the last two being identified as metabolites of 9,15-cyclo-GA₉ (2) formed in feeding experiments with fern prothallia of Lygodium circinnatum. The route to (7) began with the Birch reduction of gibberellic acid (GA₃) methyl ester 3,13-dimesylate, thereby removing the oxygen substituents from C3 and C13. A series of iodolactonizations and elimination processes followed by ozonolysis of the 17-methylene group furnished (19) which, on treatment with potassium hydride, underwent intramolecular alkylation to afford the cyclogibberellin (20). Further functional group manipulation then led to (7) via its 2α-epimer (23). The previously prepared 11β-bromocyclogibberellin (28) was also subjected to a series of functional group manipulations and deletions to afford the 11α-hydroxy analogue (33), and thence the 11β-epimer (8) by means of an oxidation/reduction cycle; 1α-hydroxycyclo-GA₉ methyl ester (26) was similarly prepared from the methyl ester of the known 1β-epimer (3).