Abstract
Halolactonization of the keto acid phosphoranes 6a-f, 40, and 41 takes place with either Br 2 or SO 2 Cl 2 and Et 3 N 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 1 H and 13 C 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 lactaones 42b and 43b was determined by 1 H NMR spectroscopy at elevated temperatures
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