LiClO4- or LiOTf-accelerated 1,3-dipolar cycloaddition reactions: a facile synthesis of cis-fused chromano[4,3-c]isoxazoles

Acetonitrile solutions of lithium perchlorate or lithium triflate are found to accelerate considerably the intramolecular 1,3-dipolar cycloaddition reactions of nitrones derived in situ from hydroxylamines and the O-prenyl derivatives of salicylaldehydes to afford enhanced rates and improved yields of tetrahydrochromano[4,3-c]isoxazole derivatives with high diastereoselectivity. The stereochemistry of the products has been assigned by using extensive NMR studies.