Treatment of penta-N-benzyloxycarbonylparomomycin with benzaldehyde and excess zinc chloride gives a dibenzylidene derivative in high yield. This consists of the 4′,6′-O-benzylidene 4′′′,6′′′-N,O-benzylidene (dihydrooxazine) derivative of penta-N-benzyloxycarbonylparomomycin. Chemical evidence is presented to support this structure and model studies are reported for the formation of dihydrooxazine and oxazolidine derivatives of benzyloxycarbonylamino sugars containing suitably situated hydroxyl groups. The easily obtained dihydrooxazine derivative of paromomycin constitutes an interesting, preferentially blocked derivative, that is useful for the chemical modification of the parent antibiotic.