Carboxylic acid acetonyl esters

The direct introduction of the two-carbon unit of the heterocyclic ring, ortho to an existing carboxylic acid (ester) can be achieved in two ways ort/io-bromobenzoates can be coupled with 7r-(2-methoxyallyl)nickel bromide for the introduction of acetonyl, or thallation of benzoic acids, ortho to the carboxyl, can be followed by palladium-catalysed coupling with alkenes. Benzoates carrying an ortho acetylenic substituent can be ring closed using mercuric acetate, as shown below." ... 

Three papers published during 1979 describe the synthetic approach adopted by Woodward and his colleagues for the total synthesis of the penems. As with some of the other syntheses, which appear to be modelled on Woodward s methods, the central feature is the use of an intramolecular Wittig reaction to produce the bicyclic ring system. Illustrative of the method is the synthesis of the parent penem-3-carboxylic acid molecule as shown in Scheme 135. The use of the acetonyl group as ester protection contributed greatly to the success of the synthesis. 

A methylene group between the oxadiazole ring and a carboxyl group acts like a malonic acid with respect to ease of loss of the carboxyl group. Likewise the acetyl group from an acetonyl side chain at C-5 is lost in basic solution, analogous to the ketone split in a fi-keto ester or / -diketone. This was recognized in the early work on oxadiazoles by Tiemann,48 Wiese,49 Richter,47 and Schubart.50... 

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