Oxidative cleavage of N-acetyl-neuraminic acid (Neu5Ac, 1) leads to the open chain octonic acid (ADOA, 2), which under various conditions is transformed into an acyclic acid or ester or into a six-or five-membered octonic acid lactone and lactam, respectively. All reactions proceed with high selectivity and in good yields. Strikingly, a simple switch of reaction conditions from basic to acidic catalysis leads to a complete switch in regioselectivity during a crucial cyclization step.