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N-triflylphosphoramide (NTPA) was found to be an efficient Brønsted acid catalyst for the practical synthesis of 2,5-disubstituted 1,3-dioxolane-4-ones. Racemic and optically pure mandelic acid and lactic acid could be protected using several aldehydes in high selectivity and moderate yield in the presence of NTPA without azeotropic distillation or use of a dehydrating reagent.