Synthesis of compounds related to gibberellic acid. Part V. Introduction of the ring B carboxy-group

Abstract

Various methods for the introduction of a carboxy-group into the benzylic position (corresponding to the ring B carboxy-group in gibberellin A₃ nor-ketone) of methyl 16,16-ethylenedioxy-3-methoxy-9αH-gibba-1(10),2,4-triene-4-carboxylate (1), of the corresponding 1(10),2,4,9(11)-tetraene (2), and of the corresponding 9βH-epimer (10b) have been explored. The best of these has been found to involve deprotonation of these acetal esters with lithium N-cyclohexyl-N-t-butylamide, followed by carboxylation, which is highly stereospecific. Experiments on simpler models are also described.