The effect of Lewis acids on the intramolecular Diels–Alder reaction of the furan diene

A general method is described for effecting the intramolecular Diels–Alder reaction of the furan diene in which the side arm connecting the diene to the dienophile contains four carbon atoms. The use of 1.1 equivalents of methylaluminum dichloride at −78 °C for 2–8 h shifts the Diels–Alder equilibrium towards the products and provides the oxatricyclo adducts in good to excellent yield. Catalytic quantities of methylaluminum dichloride (10 mol%) provided a higher quantity of adduct than excess Lewis acid when the enone was substituted with alkyl groups. The scope was extended to include a precursor containing a five carbon atom side arm, and two examples containing acetylenic dienophiles that were activated by a carbonyl moiety on the side arm. Precursors having a four carbon atom side arm provided only oxatricyclo adducts having the side arm syn to the oxygen bridge. The assignment of the stereochemistry of the oxatricyclo adducts is discussed in detail.