Acetyl bromide → benzhydryl (6R,7R)-3-methyl-8-oxo-7-[(2-phenoxyacetyl)amino]-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylate

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(1S,3S,5R,6R) 2,2-Dimethyl-6-phenoxyacetamidopenam-3-carboxylic acid-1-oxide diphenylmethyl ester (55.06 gms, 100.00 mmoles) was dissolved in sieve-dried, peroxide-free dioxane (550 mls) at 25° C. with good agitation. To this solution, in order, were added pyridine (24.21 mls, 23.73 gms, 300.00 mmmoles), acetyl bromide (1.48 mls, 2.46 gms, 20.00 mmoles) and methylsilyl triisocyanate (89% pure, 21.30 mls, 25.34 gms, 133.33 mmoles). The slurry was heated to reflux (ca 100° C.) for 4 hours. The reaction mixture was then cooled to 25° C. and filtered, with the clear yellow filtrate being concentrated in vacuo at 50° C. to a highly viscous oil. The oil was dissolved in hot 2-propanol (~80° C., 3000 mls), filtered, and allowed to cool to 25° C. The slurry was cooled to 0°-5° C. for 2 hours, filtered, and washed with 2-propanol (0°-5° C., 400 mls), and oven-dried at 45° C. to constant weight. Yield: 38.6 gms, 75% of white crystalline title compound. The 80 MHz H' NMR spectrum was clean and consistent for the desired structure.

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