Useful intermediates for preparing a 1-dethia-1-oxacephalosporin and represented by the following formula: ##STR1## (wherein R is a monovalent group resulting from the elimination of the carbonyl function of an acyl group derived from a carboxylic or carbonic acid;COB is carboxy or protected carboxy; andX is hydrogen or a nucleophilic group)are prepared from the corresponding penicillin 1-oxides of the following formula: ##STR2## (wherein R, COB, and X are as defined above) by heating, if required in the presence of a desulfurizing reagent, or by exchanging the X group with another one under the condition of an appropriate nucleophilic substitution.