Two successful and two unsuccessful synthetic routes are described for the conversion of commercially available fungal gibberellins to the antheridiogen GA73 methyl ester (5), originally isolated from gametophytes of the fern Lygodium japonicum. The more efficient route began with the Birch reduction of either the GA7 derivative (31) or methyl gibberellate 3,13-dimesylate (32) to give the ∆1(10),16-diene acid (33), which was converted into the ∆1,9-diene acid (40) by means of a four step procedure. This intermediate could be selectively hydrogenated to the ∆9-ene derivative which underwent iodolactonization and then elimination of HI to afford enone (29), Wittig methylenation of which furnished the target antheridiogen (5). In addition, (29) was functionalized at C12 by free radical bromination to give access to the 12β-hydroxy derivative of (5), i.e. (6), a new antheridiogen isolated from Lygodium circinnatum.
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).
Synthesis and Structure Determination of Gibberellin-Derived Antheridiogens from Fern Gametophytes of the Lygodium Genus.-Two successful and two unsuccessful synthetic routes are described for the conversion of commercially available fungal gibberellins into the antheridiogen GA73 methyl ester (VI) originally isolated from gametophytes of the fern Lygodium japonicum. The more efficient route is shown. In addition, the enone (IV) is converted into the 12β-hydroxy derivative (X) of (VI), a new antheridiogen isolated from Lygodium circinnatum. -(FURBER, M.; KRAFT-KLAUNZER, P.; MANDER, L. N.; POUR, M.; YAMAUCHI, T.; MUROFUSHI, N.; YAMANE, H.; SCHRAUDOLF, H.; Aust.
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