A study of the stereochemical features of 5-endo-trig iodocyclisations of 2-alkenylcycloalkan-1-ols

Overall 5-endo-trig iodocyclisations of all isomers of the 2-alkenylcyclopentan-1-ols and -cyclohexan-1-ols 11, 12, 14 and 16 have been examined. Only in the cases of the trans-alkenylcyclopentanols 11a and 12a are the reactions either unsuccessful or low yielding, by reason of formation of a relatively strained trans-fused 5/5 ring system. In contrast, iodocyclisations of cis-alkenylcyclopentanols work well but only show useful stereoselection in the case of the cis-alkenylcyclopentanol 16a. All examples involving the cyclohexanols 14 and 16 are high yielding and generally lead to single isomers of perhydro-3-iodobenzofurans 18, 20, 23 and 26. Transition state conformations are proposed which account for these observations and which should be useful in applying this type of cyclisation to related substrates.