Stereoselective Z- and E-Bromo Enol Lactonization of Alkynoic Acids

We have found that treatment of the silver salt of a 4- or 5-terminal alkynoic acid with bromine results in clean formation of the corresponding Z-bromo enol lactone, the result of a formal cis addition of carboxylate and bromine across the triple bond. This Z-bromo enol lactonization is highly stereoselective and gives good yields in systems that bear substituents on the internal methylene groups; yields with unsubstituted or terminally substituted alkynoic acids are modest. The E-bromo enol lactonization reaction, reported by us previously (Krafft, G. A.; Katzenellenbogen, J. A. J. Am. Chem. Soc. 1981, 103, 5459), has a broader scope and, with modifications, can be performed with high reliability. Mercury(II) salts equilibrate the Z- and E-bromo enol lactones, presumedly by a mercuric ion addition-elimination mechanism. These three reactions provide access to an array of stereoisomeric bromo enol lactone systems.