Stereoselective synthesis of new pyran-dioxane based polycycles from glycal derived vinyl epoxide.

Chiral heteropolycyclic structures are widespread in compounds of high pharmaceutical relevance. In particular, linear fused pyran-dioxane based polycycles can be found in several naturally occurring molecules, and among them, cardiac glycosides and antibiotic spectinomycin are characterized by a cis-cisoid-trans geometry. Then, the stereocontrol in the synthesis of this type of polycyclic scaffold is of primary importance. Herein, we present two novel linear fused pyran-dioxane based bi- and tricycles, synthesized with total stereoselectivity from a glycal derived vinyl epoxide. The straightforward methodology described involves a substrate-dependent stereospecific glycosylation step followed by an intramolecular SN2' conjugate addition process, leading to a pyran-dioxane-cyclohexane tricycle with a cis-cisoid-trans stereochemistry, in agreement with the geometry of many natural products. The stereochemical analysis of these compounds, which was realized by a combined NMR/computational approach, is also reported.