New synthesis of 2,6-anhydro-β-d-fructofuranoses, pivotal [2.2.1] bicyclic acetals for the conversion of d-fructose into 2,2,5-trisubstituted tetrahydrofurans

2,6-Anhydro-β-d-fructofuranose derivatives were prepared by a novel tin-promoted 2,5-cyclisation of phenyl 2-thio-β-d-fructopyranosides. They were regioselectively opened by allyltrimethylsilane in the presence of catalytic Sc(OTf)3 to give 2,2,5-trisubstituted tetrahydrofurans in high yields and major α-stereoselectivity.