" The reactions were carried out at-78-0'C (1-2 h) in ether.* Determined by HPLC analysis. cErythro isomers were assigned by spectroscopic methods and by comparison with the authentic materials. Isolated yields. e Purified by column chromatography on silica gel./The ratio was lowered to 2.8: 1 by using THF as solvent. gThe ratio was lowered to 2.6: l in THF.