Acid-Catalyzed Rearrangement of 5,6-exo-Epoxy-7-oxabicyclohept-2-yl Derivatives. Migratory Aptitudes of Acyl vs. Alkyl Groups in Wagner-Meerwein Transpositions

Article abstract of DOI:10.1002/hlca.19870700706

In the presence of HSO3F/Ac2O in CH2Cl2, 2-exo- and 2-endo-cyano-5,6-exo-epoxy-7-bicyclo<2.2.1>hept-2-yl acetates (6a,b) gave products derived from the epoxide-ring opening and a 1,2-shift of the unsubstituted alkyl group (? bond C(3)-C(4)).In contrast, under similar conditions, the 5,6-exo-epoxy-7-oxabicyclo<2.2.1>heptan-2-one (6c) gave 5-oxo-2-oxabicyclo<2.2.1>heptane-3,7-diyl diacetates 20 and 21 arising from the 1,2-shift of the acyl group.Acid treatment of 5,6-exo-epoxy-2,2-dimethoxy-7-oxabicyclo<2.2.1>heptane (6d) and of 5,6-exo-epoxy-2,2-bis(benzyloxy)-7-oxabicyclo<2.2.1>heptane (6c) gave minor products arising from epoxide ring opening and the 1,2-shift of ? bond C(3)-C(4) and major products (25,29) arising from the 1,3-shift of a methoxy and benzyloxy group, respectively.Under similar conditions, 5,6-exo-epoxy-2,2-ethylenedioxy-7-oxabicyclo<2.2.2>heptane (6f) gave 1,1-(ethylenedioxy)-2-(2-furyl)ethyl acetate (32, major) and a minor product 33 arising from the 1,2-shift of ? bond C(3)-C(4).The following order of migratory aptitudes for 1,2-shifts toward electron-deficient centers has been established: acyl > alkyl > alkyl α-substituted with inductive electron-withdrawing groups.This order is valid for,competitive Wagner-Meerwein rearrangements involving equilibria between carbocation intermediates with similar exothermicites.