A novel and efficient conversion of D-glycono-1,5-lactones into the corresponding L-sugars is described. The important intermediate, delta-hydroxyalkoxamates, was provided by a practical alkoxyamination of D-glycono-1,5-lactones mediated by Me3Al. In contrast to the preparation of beta-lactam skeletons from beta-hydroxyalkoxamates, the cyclization of delta-hydroxyalkoxamates under Mitsunobu conditions resulted in O-alkylation rather than N-alkylation. It is noteworthy that delta-hydroxyalkoxamates derived from D-mannono-1,5-lactones afforded the O-alkylation product in 91% yield. None of the N-alkylation product was detected in this case. These O-cyclized oximes, in which the inversion of the configuration at C5 was secured, were efficiently converted into the L-sugars.