pressure the residue is placed on 80 g of silica gel and eluted with chloroform to remove polar reaction products the material eluted from the column is purified by preparative silica gel thin layer chromatography using methylene chloride to give 60 mg of yellow gum elution of this through 1 5 g of silica gel with benzene gives 44 mg 1 of 3 3 dimethyl 6 u03b2 phenoxyacetyl amino 7 thioxo 4 thia 1 azabicyclo 3 2 0 heptane 2 carboxylic acid 2 2 2 trichloroethyl ester as a yellow foam the ester from example 1 16 mg 0 03 mmoles is dissolved in 1 ml of 90 aqueous acetic acid and zinc dust 20 mg 30 mmoles is added the mixture is allowed to stir at 0 u00b0 for 2 hours the reaction mixture is poured into 5ml of water and 20 ml of ethyl acetate the layers are separated and the organic layer is washed with 5 ml of cold water cold water 10 ml is added and the ph is adjusted to 8 with cold aqueous sodium hydroxide the aqueous layer is separated and treated with 20 ml of cold ethyl acetate the ph is adjusted to 2 by the addition of concentrated hydrochloric acid the organic layer is separated dried over magnesium sulfate filtered and the solvent is removed under reduced pressure to yield 4 mg 25.