Lactone formation in superacidic media

Abstract

The reaction of substituted 1-hydroxycyclohexanecarboxylic acids in fluorosulphuric acid has been studied. Cyclisation takes place around 0 °C, accompanied by rearrangement in appropriate cases, yielding the thermodynamically stable lactone or mixture of lactones. An unexpected feature of these reactions is that the carboxy-substituted cyclohexyl carbocation does not undergo ring contraction, unlike the unsubstituted cyclohexyl carbocation, although the cycloheptyl system contracts to cyclohexyl. We suggest that the cyclohexyl carbocation is strongly stabilised by carboxyl substitution, as a result of throug-space interaction between the carboxyl oxygen atom and the carbocation centre.