One-Pot Cascade Synthesis of Fused Nitrogen-Containing Heterocycles in Aqueous Media - Utility of N-Protective Groups in Intramolecular Diels-Alder Reaction of Furan

A metal-free, thermal, intramolecular Diels-Alder (IMDA) reaction of furan in aqueous media without the use of microwave irradiation was investigated. Protection of the amine functionality and the cycloaddition reaction were performed as a one-pot, two-component process. Various nitrogen-protecting groups and their electronic and steric effects on the cycloaddition reaction were studied. The protection-intramolecular Diels-Alder reaction sequence proceeds under environmentally benign aqueous conditions, which are tolerated by substrates with a broad range of nitrogen-protecting groups such as benzyloxycarbonyl (Cbz), trityl, tert-butoxycarbonyl (Boc), trifluoroacetyl, tosyl, mesyl, and p-nosyl [(4-nitrophenyl)sulfonyl]. This study allowed the development of a stereoselective, tandem allylamine isomerization-Diels-Alder cycloaddition sequence leading to the rapid assembly of complex nitrogen-containing heterocycles in a simple one-pot process.