HALOGENATION OF KETO ACID PHOSPHORANES - SYNTHESIS OF HALO ENOL LACTONES AND HALOALLENES

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 H-1 and C-13 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 H-1 NMR spectroscopy at elevated temperatures.