Syntheses of Medium-Sized Cyclic Ethers from Carbohydrates via an ­Intramolecular Nitrile Oxide-Alkene Cycloaddition Strategy

The regioselectivity of intramolecu1ar 1,3-dipolar cyclo­additions of nitrile oxides prepared from 3-O-alkenyl-1,2-O-iso­propylidene--d-pentodialdofuranoses is dependent on the length of the alkenyl chain. 3-O-Homoallyl nitrile oxide afforded exo-cyclization oxepane exclusively. 3-O-Pentenyl nitrile oxide produced a mixture of exo- and endocyclization adducts (oxocane and oxonane) whereas 3-O-hexenyl, heptenyl, and octenyl oxides all gave endo-cyclization adducts (ten-, eleven-, and twelve-membered ­cyclic ethers, respectively) exclusively.