Synthetic and Theoretical Investigations on the Construction of Oxanorbornenes by a Michael Addition and Intramolecular Diels-Alder Furan Reaction

The conjugate addition of nucleophiles such as allylmercaptan, allyl- and homoallylmalonate and diallylamine to beta-furyl enones and acrylate, provides the Michael adducts in good yield. A facile intramolecular Diets-Alder reaction between the unsaturated tether and the furan diene ensues when these adducts are heated in a solvent such as toluene or xylene to afford the cycloadducts in good yield and excellent stereoselectivity in most cases. The structure and stereochemistry of these cycloadducts were confirmed by extensive NMR experiments and X-ray crystallography. Quantum chemical calculations on the transition state and product geometries suggest that the formation of the cycloadducts, in which the newly formed ring is exo-fused to the oxanorbornene framework, is favored over the endo-fused product due to less strain in the former and its transition state. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)