An expeditious and efficient synthesis of new enantiopure polyhydroxylated seven- and eight-membered carbocycles is described starting from 2,3.5-tri-O-benzyl-D-arabinose. The key cyclization step involves ring closing metathesis of 1,8- and 1,9-dienes using Grubbs' catalyst. All of the new carbasugar analogs synthesized were evaluated as glycosidase inhibitors. Contrary to Our expectations, (1 S,2S,3R,4R,5R)-1-(hydroxymethyl)-cyclohepta-1,2,3,4,5-pentol which has the beta-D-mannopyranose configuration for C(1)-C(5) inhibits alpha- and beta-glucosidases, whereas its diepimer (1S,2S,3R,4S,5S)-1-(hydroxymethyl)-cyclohepta-1,2,3,4,5-pentol, which has the alpha-D-glucopyranose configuration, is not recognised by these enzymes. (C) 2002 Elsevier Science Ltd. All rights reserved.