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-alkenylcyclope ntan-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 reacti ons either unsuccessful or low yielding, by reason of formation of a relati vely strained trans-fused 5/5 ring system. In contrast, iodocyclisations of cis-alkenylcyclopentanols work well but only show useful stereoselection i n the case of the cis-alkenylcyclopentanol 16a. All examples involving the cyclohexanols 14 and 16 are high yielding and generally lead to single isom ers of perhydro-3-iodobenzofurans 18, 20, 23 and 26. Transition state confo rmations are proposed which account for these observations and which should be useful in applying this type of cyclisation to related substrates.