Quite surprisingly, the ratio of erythro to threo products proved essentially independent of the Lewis acid used; all Lewis acids which successfully promoted reaction gave ca. 4:l mixtures of diastereomeric products in which the er- ythro product predominated. For the case of R = cyclo- hexyl, erythro:threo ratios were between 77:23 and 79:21 for all of the following Lewis acids (isolated yields in parentheses') by using the reaction protocol previously described ...
