A 1:1 1,4-dioxane-tert-butyl acetate mixture and BF3 etherate were used for
an effective 6-aminopenicillanic acid conversion into the tert-butyl ester
(64 +/- 3% yield) which could be easily transformed into tert-butyl 6-oxo-
and 6-diazo-penicillanates.