Strategy XV The Use of Ketenes in Synthesis

We met ketenes in chapter 31 where they were intermediates in the Arndt-Eistert chain extension procedure. Now we are going to take a wider view of their value in synthesis. A ketene 1 is very electrophilic at the curious sp carbon atom (marked in 1) and combines with nucleophiles 2 to give enolates that are protonated at carbon 3 to give acylated compounds 4. 

Ketenes are rarely isolated as they dimerise easily. Ketene itself 2 gives the lactone 5 but dimethylketene 6 gives the diketone 7. Other ketenes may give either type of dimer. Only a few ketenes, such as diphenyl ketene, are normally isolated. 

Ketenes are normally prepared by the base-catalysed elimination of HC1 from an acid chloride 9 or by elimination of chlorine from a chloroalkyl acid chloride with zinc dust, often assisted by ultrasound. For reactions with nucleophiles, the solution would already contain the nucleophile before the ketene 6 was generated. 

Unlike ordinary alkenes, ketenes do 2 + 2 cycloadditions with themselves—the dimerisation above—and with other alkenes.1 Reaction of dichloroketene with cyclobutadiene 11 to give the 

Organic Synthesis The Disconnection Approach. Second Edition Stuart Warren and Paul Wyatt 2008 John Wiley Sons, Ltd 

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