Stereochemistry of ethyl 2, 6-dimethyl-4-oxocyclohex-2-enecarboxylate (6-methyl hagemann's ester) and its products of conjugate addition by vinylmagnesium bromide/copper(i) iodide

Given the ready availability and widespread use of 6-alkyl Hagemann's esters as building blocks for organic synthesis, it is remarkable (if not disconcerting) that no one has reported the fact that the 6-methyl derivative (ethyl 2, 6-dimethyl-4-oxocyclohex-2-enecarboxylate, 1) exists as a readily interconverted pair of diastereomers. This is all the more true in the specific instance in which 1 was assumed to be of trans stereochemistry and then used as the starting material in a synthesis of botryodiplodin (2), thus “proving” the 3, 4-cis relationship in the antibiotic.¹ In another study² it was recognized that the preparation of 1 gave a mixture of two isomers (which were characterized and separated as their 2, 4-DNP derivatives), but these were assigned, incorrectly, as the regioisomers 1 and 3 and the stereochemical issue was again ignored.