Benzyl 6-.beta.-[bistrifluoromethanesulfonyl]amidopenicillanate, 4, was prepared by reaction of benzyl 6-.beta.-aminopenicillanate, 3, with 2 equivalents of trifluoromethane sulfonic anhydride. In the presence of aqueous base, 4 undergoes detriflation to yield 7 and in the presence of basic methanol, 4 yields the .alpha.-methoxymonotriflamide 6. The free acid 5 obtained by hydrogenolysis of 4 has [bacterial] .beta.-lactamase inhibitory properties.