Partial synthesis of gibberellin A37 from gibberellin A13

Abstract

The partial synthesis of gibberellin A₃₇(1) from gibberellin A₁₃(5) is described. Selective reduction of the least reactive carboxy-group (that at position 10) in gibberellin A₁₃ was achieved through formation of the 20,3-lactone (24), which yielded the 3α-hydroxy-epimer (3) of gibberellin A₃₇. This epimer was converted into gibberellin A₃₇(1) by Meerwein–Ponndorf reduction of the derived 3-ketone (2). Meerwein–Ponndorf reduction of the 3-oxogibberellins (2), (12), and (31) gave 50% or greater yields of the 3β-epimers, whereas the 3-oxo-20,19-lactone (18) gave the 3α-hydroxy-epimer predominantly.

Reduction of gibberellin A₁₃ trimethyl ester with lithium aluminium hydride in ether, followed by acetylation, gave the 20,19-lactone diacetate (21) and the triacetate (23). Similar reduction of the monomethyl ester (7) unexpectedly yielded the 20,19-lactone (28) rather than a gibberellin A₃₇ derivative.