The homoiminosugars alpha- and beta -homogalactonojirimycins were prepared from a common intermediate, tetra-O-benzyl-D-galacto-heptenitol 6, by way of highly stereoselective reaction sequences involving, as the key steps, an internal amidomercuralion (a-epimer) and a double reductive amination (beta -epimer). alpha -Homogalactonojirimycin retains a large part of the potent activity of the parent galactonojirimycin and 1-deoxygalactonojirimycin as an inhibitor of alpha -galaclosidases. However, by contrast with the parent iminosugars, it does not inhibit beta -galactosidases, with the exception of the Jack beans enzyme. beta -Homogalactonojirimycin is a weak cc-galactosidase inhibitor and is completely devoid of activity towards P-galactosidases. Thus, a marked selectivity toward one family of enzymes has been achieved by the addition of an alpha -CH2OH group in the structure of the parent iminosugars. (C) 2001 Elsevier Science Ltd. All rights reserved.