Preparative flash vacuum thermolysis. Homolytic cleavage of isomannide and isosorbide dinitrate to 1,4,3,6-dianhydrohexoses

1,4,3,6-dianhydromannopyranose (3) can be conveniently prepared by flash vacuum thermolysis of isomannide dinitrate (1), while its epimer isosorbide dinitrate (2) yields a mixture of (3) and 1,4,3,6-dianhydrofructose (5), reflecting the restricted number of rearrangements, in one case with inversion of configuration, possible in the intermediate cis-fused furofuran radicals.