Synthesis of substituted aromatic compounds by Diels–Alder reactions of alkoxycyclobutenes produced via photocycloadditions between vinyl ethers and 1,3-dione enol esters

Abstract

The adduct (5) obtained from [2 + 2] photocycloaddition of ethyl vinyl ether to the 1,3-dione enol acetate (4) rapidly loses acetic acid producing the ethoxycyclobutene (7). On being warmed in the presence of a dienophile, the ethoxycyclobutene (7) undergoes electrocyclic ring-opening to give the ethoxydienone (8) followed by [4 + 2] cycloaddition to produce Diels–Alder adducts [e.g.(10) with dimethyl acetylenedicarboxylate]; elimination of ethanol from (10) gives the dimethyl phthalate (11). In a ‘one-pot’ reaction, heating a solution of (5) with 1,4-naphthaquinone in xylene containing alumina leads to the substituted anthraquinone (12) in 95% yield.

On storage at 0 °C for a few days the ethoxycyclobutene (7) is converted into the crystalline dimer (9)via the transient ethoxydienone (8); the structure of (9) followed from X-ray measurements. An analogous dimer (14) was produced during attempts to effect an intramolecular Diels–Alder reaction from the bicyclic ether (13).