Enantiomerically pure 2,8-diazabicyclo[3.2.1]oct-2-ene derivatives (+)-5 and (-)-5 have been obtained from 2-azido-3-tosyl-7-azabicyclo[2.2.1]heptanes (+)-1 and (-)-2 and their enantiomers, by ring expansion under radical conditions. Compounds (+)-5 and (-)-5 were transformed into hemiaminals 9 ((3S,4R,5R)- and 10 ((3R,4S,5S)-5-(2-aminoethyl)-2,3,4-trihydroxypyrrolidine) that are good inhibitors of alpha-mannosidases. (C) 2003 Published by Elsevier Science Ltd.