Ring-Closing Metathesis of Allylic O,O- and N,O-Acetals

A variety of allylic O,O- and N,O-acetals were synthesized using a mild palladium-catalyzed coupling of an alcohol or sulfonamide with an alkyl or aryl 1,2-propadienyl ether. The resulting linear acetals were used for the synthesis of unsaturated rings via ring-closing metathesis, in which the acetal carbon–a precursor for oxycarbenium or N-sulfonyliminium ions, respectively–served as a reactive center for further introduction of functional groups. The products–unsaturated oxygen and nitrogen heterocyclic scaffolds–offer multiple opportunities for derivatization as illustrated with the synthesis of substituted dihydropyrans, chromenes, enantiopure tetrahydropyridines and an enantiomerically pure quinolizidine amino acid.