Halogenation of Keto Acid Phosphoranes: Synthesis of Halo Enol Lactones and Haloallenes

Article abstract of DOI:10.1021/jo00058a039

Halolactonization of the keto acid phosphoranes 6a-f, 40, and 41 takes place with either Br2 or SO2Cl2 and Et3N to give the E- and Z-halo enol lactones 10-15, 42, and 43 in good yields.The cyclization proceeds via a halo phosphonium salt, e.g. 19.Halo phosphonium salts yield a halo allene when cyclization is not favoured as in the formation of the bromoallenes 24 and 37.The configuration of the halo enol lactones was determined by 1H and 13C NMR spectroscopy and via single-crystal X-ray determinations on 11a, 14c, and 42b.The barrier to interconversion of the biphenyl conformations of the bromo enol lactones 42b and 43b was determined by 1H NMR spectroscopy at elevated temperatures.