Ring Contraction to Cyclopropane Carboxylic Acid

The Faworski-rearrangement of a-halocyclobutanones provides a very attractive and in practice, often particularly useful method [488-491], since it usually utilizes readily available acid chlorides and olefins as precursors. Not only the commercially important (fenpropathrin) tetramethyl cyclopropane carboxylic acid 247 as in Reaction scheme 169, but many others, particularly higher alkyl-substituted cyclopropane carboxylic acids are preferentially prepared in this way (Reaction scheme 170). 

The involvement of higher substituted chlorides sometimes provides an access to highly enriched single trans-isomers [492]  

Likewise, spirocyclic variations of important pyrethroid acids are readily available by this method  

Dichloroketene as a component for this 2 + 2-cycloaddition, however, introduces a new problem for the Faworski-reaction. The a,a-dihalocyclobutanones are not suited for the rearrangement of the carbon skeleton. Instead, the tertiary chlorine atom is transferred to the other a-position under basic conditions to give a,a -dihalocyclobutanones 248 [495] (Reaction schemes 172, 173 see also page 43). Even 

Instead of phenoxide dialkylamino, alkoxy and phenylthio may likewise be applied [276], 

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Synthesis of Other Important Cyclopropane Pyrethroid Acids 1.7.5 Ring Contraction to Cyclopropane Carboxylic Acid... 

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