2,3-cis-Cyclization of 4-pentenoxyl radicals

Abstract

4-Pentenoxyl radicals cyclize 2,3-cis-selectively, when substituted by an allylic hydroxy, acetyloxy, or benzoyloxy group. Additional substituents increase or decrease the fraction of 2,3-cis-cyclized product, depending on relative configuration, positioning, and their chemical nature. The preference for 3-acceptor-subsituted pentenoxyl radicals to furnish products of 2,3-cis-ring closure arises from a secondary orbital interaction between the allylic oxygen substituent and the alkene entity, kinetically disfavoring the 2,3-trans-mode of 5-exo-cyclization. Aligning the β-C,O-bond in anticline orientation to the plane of the alkene, which is the preferred conformation for transition structures for 2,3-trans-cyclization, stabilizes the double bond by delocalizing π-electrons into the σ*(C,O)-orbital. Along with energy decreases the affinity of π-electrons for forming a σ (C,O)-bond with the oxygen radical. In 2,3-cis-cyclization, a similar stabilizing effect cannot occur, because the allylic oxygen substituent and the alkene align synperiplanar. The kinetic effect of an allylic oxygen substituent becomes furthermore apparent in cyclization of the 3-hydroxynona-1,8-dien-5-oxyl radical, favoring intramolecular addition to the unsubstituted allylic double bond by a factor three.