Cyclic ether formation in superacid media
The formation of ethers in superacids by interaction of a primary hydroxy group with a carbocation centre has been investigated by a study of cyclisation of suitable substrates, mainly 1-(2-hydroxyethyl)cyclohexanols to give hydrobenzo[b]furans. Cyclisation traps thermodynamically stable ionic species, with rates of reaction dependent upon the size of the ether ring formed. Three- and four-membered ether rings were not formed, five-membered ether rings formed readily, the reaction being comparable in rate with a 1,2-methyl shift. Six-membered rings formed a little less readily and seven-membered rings less readily still, though a yield of 34% from a suitable substrate has been recorded. An unexpected feature of the reactions was their stereospecificity; in one case, this is believed to result from a methyl shift concerted with attack of the primary hydroxy group, the reaction proceeding through hindered transition state, in which methyl loss, probably as CH3+, competes with a 1,2-methyl shift.