Orthoester Claisen rearrangement of a D-glucose-derived spirocyclic substrate

The Claisen rearrangement of a spiro compound 1 derived from 1,2:5,6-di-O-i sopropylidene-alpha -D-glucofuranose, with triethyl orthopropionate afforde d the rearrangement products 4, 5, and 6 as a 1:2.5:10 diastereomeric mixtu re. The reaction of 1 with trimethyl orthobutyrate provided a 1:3 mixture o f 7 and 8, In both cases, the sigma -bond formation proceeded predominantly from the beta -side. This stereochemical outcome was opposite to that obse rved in the case of the rearrangement of 1 with triethyl orthoacetate.