The Unusual Structure of a C-Arylated Gibberellin Bis-[gamma]-lactone Formed from a Free Radical-Initiated Cyclization

Treatment of the thionocarbonate derivative, ent-13-acetoxy-2[beta]-hydroxy-3[alpha]- phenoxythiocarbonyloxy-20-norgibberella-1(10),16-diene-7,19-dioic acid 19,2-lactone 7-(methyl ester), (2), with Bu₃SnH and 2,2'-azobis(2-methylpropanenitrile) (AIBN) afforded in very high yield the unusual 10[beta]-phenylgibberellin bis-[gamma]-lactone, ent-13-acetoxy- 2[beta],3[alpha]-dihydroxy-1[alpha]-carboxy-10[alpha]-phenyl-20-norgibberell-16-ene-7,19-dioic acid 19,2:1',3--dilactone 7-(methyl ester), (4), instead of the expected 3-desoxy product, (3). The crystal structure of (4) as its acetonitrile solvate, C₂₉H₃₀O₈.C₂H₃N, shows that the overall shape of (4) is significantly different from that of regular gibberellins, although the C ring still retains the boat conformation that is typical of this family of natural products.

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