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Issue 41, 2014

Annulated and bridged tetrahydrofurans from alkenoxyl radical cyclization

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4-Pentenoxyl radicals sharing two or more carbon atoms with a cycloalkane cyclize in a predictable manner stereoselectively and regioselectively to afford in solutions of bromotrichloromethane cycloalkyl-fused or -bridged 2-bromomethyltetrahydrofurans in up to 95% yield. Stereoselectivity in alkenoxyl radical ring closures arises from cumulative steric effects. The substituent positioned the closest to the alkene carbon, which is being attacked by the oxygen radical, exerts the strongest stereodirecting effect. This principal inductor guides 5-exo-cyclization 2,3-trans- or 2,4-cis-selectively. The substituent located further from the attacked π-bond is the secondary inductor. A secondary inductor in the relative trans-configuration enhances stereodifferentiation by the primary inductor; a cis-configured secondary inductor decreases this effect. A secondary inductor is not able to overrule the guiding effect of a similar sized primary inductor. Intramolecular 4-pentenoxyl radical additions to a cyclohexene-bound exo-methylene group or to endocyclic double bonds proceed cis-specifically, as exemplified by synthesis of a diastereomerically pure bromobicyclo[2.2.1]heptyl-annulated tetrahydrofuran from the verbenylethyloxyl radical. According to theory, the experimental 2,3-cis-specificity in alkoxyl radical cyclization to an endocyclic π-bond arises from strain associated with the 2,3-trans-ring closure.

Graphical abstract: Annulated and bridged tetrahydrofurans from alkenoxyl radical cyclization

Supplementary files

Article information

17 Jun 2014
12 Aug 2014
First published
22 Aug 2014

This article is Open Access

Org. Biomol. Chem., 2014,12, 8288-8307
Article type

Annulated and bridged tetrahydrofurans from alkenoxyl radical cyclization

C. Schur, H. Kelm, T. Gottwald, A. Ludwig, R. Kneuer and J. Hartung, Org. Biomol. Chem., 2014, 12, 8288 DOI: 10.1039/C4OB01266F

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