Reaction of 4- O-sulfonylated 2,3- O-isopropylidene- d- ribo- or - d- lyxo-1,5-lactams with tetrabutylammonium cyanide gave 4-amino-5- C-cyano-4,5-dideoxy-2,3- O-isopropylidene- l- lyxo- 5 or - l- ribo- 15-1,4-lactams, respectively. A stereospecific ring contraction with inversion at C-4 had taken place in each case. Reduction of the cyano-lactams with LiAlH 4 gave 6-amino-1,4,5,6-tetradeoxy-1,4-imino- l- lyxo- 6 or - l- ribo- 16-hexitol, respectively. The 6-amino-1,4,5,6-tetradeoxy-1,4-imino- l- ribo-hexitol 16 was found to be a moderate inhibitor of α- l-fucosidase with a K i of 110 μM.