2α-Alkyl gibberellins A1 and A4

Abstract

An alkylcopper-magnesium complex, prepared from alkylmagnesium bromide, copper(I) iodide, and hexamethylphosphoric triamide in tetrahydrofuran, is described as a reagent for the syn-S_N2′ displacement of the allylic lactonic oxygen in gibberellin A₃ methyl ester (13). The resultant 2α-alkyl-1(10)-enes (24), (27), and (29) have been converted by known procedures into 2α-methyl (7), 2α-ethyl (9), and 2α-propyl (11) derivatives of gibberellin A₁(2). The 2α-ethyl (5) and 2β-ethyl (6) derivatives of gibberellin A₄(1) have been prepared by 13-deoxygenation of the corresponding derivatives of gibberellin A₁(2). The direct preparation of 2α-alkyl derivatives of gibberellin A₄ by syn-S_N2′ displacement of the allylic lactonic oxygen in the 3-tetrahydropyranyl ether of gibberellin A₇ methyl ester (15) with lithium dialkylcuprates works for methyl but not for higher alkyl groups. Attempts to prepare 2-methylgibberellin A₃ and 2-methylgibberellin A₇ gave the 2-methyl isomeric lactones (58) and (60).