Gibberellins—LXXXIX : Synthesis of gibberellin A 55 and A 57 as well as 1-oxygenated gibberellin A 5 and A 20 analogues—a new principle for the regioselective

Abstract

The synthesis of a series of 1-oxygenated gibberellins starting from GA 3 ( 1) is described. Nucleophilic addition of hydrazoic acid to 3-dehydro GA 3 ( 2) was followed by NaBH 4 reduction of the resulting 1 -azido-3-ketones 4 and 5 to the corresponding azido alcohols 8–10, and photolysis of the latter compounds to instable 1-imines which were smoothly hydrolysed to the 1-oxo-3-hydroxy gibberellins 13 and 14. Subsequent NaBH 4 reduction led to GA 57 ( 19) and GA 55 ( 20) and their 3-epimers, 17 and 18 respectively. In further steps 1-oxo-GA 5 ( 21), 1α- and 1β-hydroxy-GA 5 ( 23 1-oxo-GA 20( 25)as well 1α-and 1β-hydroxy-GA 20( 26 and 27) were available. The structures of the synthesized gibberellins were determined by physical data, in regard to the stereochemistry at C-1 and C-3 especially on the basis of 1H NMR and ORD measurements.