The mechanism of the β-acyloxyalkyl radical rearrangement. Part 2: β-acyloxytetrahydropyranyl radicals

Abstract

The rate of rearrangement of the 3-butanoyloxytetrahydropyran-2-yl radical (4) to give the product 5 of 1,2-migration of the acyloxy group has been determined by competition against the reaction of 4 with tributylstannane. The rate constant is larger than that for rearrangement of the acylic radical 18 but less than that for the substituted cholestanyl radical 19. Experiments with ¹⁷O- and ¹⁸O-labelled substrates indicate that the rearrangement of 4 proceeds with ca. 33% transposition of the ether and carbonyl oxygen atoms, while there is also a small amount of scrambling in the product 12 formed by direct reduction of 4. The isotope labelling studies and the Arrhenius parameters (log [A/s^–1]= 12.7, E_act= 58 kJ mol^–1) are consistent with the view that the reaction proceeds, at least in part, via dissociation into a tight anion radical-cation pair 22. A pathway via the three-membered transition structure 21 might also be involved.