Partial syntheses of [2α-2H]- and [2α-3H]-gibberellin A29 and [2α-2H,-15,17-3H4]gibberellin A51 from gibberellin A3

Abstract

In the partial syntheses of the title compounds gibberellin A₃(GA₃) is ozonised to the norketone and the methyl ester (12) is chlorinated with lithium chloride and toluene-p-sulphonyl chloride to give the 3α-chloro-13-alcohol (15) and its 13-toluene-p-sulphonate (14). The 13-alcohol (15) is converted, by tri-n-butylstannane reduction, then reaction with acetyl hypobromite, and reduction with tri-n-butylstannane, into the 2α-acetoxy-16-ketone (23). This intermediate (23) is converted successively into the 16-ene (24), the 2α-alcohol (25), the 2-ketone (26), [2α-²H]- and [2α-²H]-GA₂₉ methyl esters (29) and (31), and finally into [2α-²H]- and [2α-³H]GA₂₉(30) and (32).

An attempt to convert the 3α-chloro-13-toluene-p-sulphonate (14) into GA₂₉ by analogous reactions was abandoned since tri-n-butylstannane reduction of the 2α-acetoxy-13-toluene-p-sulphonates (18) and (19) resulted in extensive loss of the 13-toluene-p-sulphonyloxy-group. However the resultant 13-deoxy-compound (22) was converted into [2α-²H, 15,17-³H₂]GA₅₁(37) as for the 13-alcohol (15).