1,2-Acyloxyl Migration in Pyranosyl Radicals

Article abstract of DOI:10.1021/jo00253a032

Tetra-O-acetylgalactosyl radical (5) and tetra-O-acetylglucosyl radical (10) undergo a 1,2-migration of acetoxyl groups to the corresponding 2-deoxytetra-O-acetylpyranosan-2-yl radicals.The activation parameters for the rearrangement of tetra-O-acetylgalactosyl radical, as determined by ESR spectroscopy, are ΔH(excit.) = 12.2 +/- 0.3 kcal/mol and ΔS(excit.) = -6.7 +/- 1.0 eu.Labeling studies with 18 O support a five-membered cyclic transition state for the rearrangement with exchange of the oxygen atoms of the carboxy group.The driving force for the rearrangement, which is unfavorable in terms of the stability of the radical centers, derives from the gain in anomeric stabilization of the product radical.