Abstract

Pyranosides with N-benzyl-2,3-trans-oxazolidinone undergo anomerization from the beta form to the a form even in the presence of a weak Lewis acid. Experimental evidence for endocleavage, the breaking of the bond between the pyran-oxygen and anomeric carbon atoms, in the anomerization reactions was obtained. This unexpected phenomenon was investigated by quantum mechanical calculations, which found clear differences in the transition states between anomerized and non-anomerized substrates. The computations suggest that BF3 induces endo-cleavage followed by rotation of the C1-C2 bond to give the alpha form via lower-energy transition states.