Decarbonylation of Esters

Chlorosulfonic acid can be used for the decarbonylation of esters. For instance, when ethyl l,2-dimethylcyclopropene-3-carboxylate 42 is dissolved in chlorosulfonic acid at RT, carbon monoxide is evolved and the corresponding dimethyl-cyclopropenium ion 43 is formed in almost quantitative yield. (Equation 17).  

The procedure using chlorosulfonic acid is preferable to that originally proposed for the preparation of the diphenylcyclopropenium ion, which employed perchloric acid as the strongly acidic reagent. 

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