5-Amino-2-furancarboxylic acid methyl ester undergoes a facile Diels−Alder cycloaddition with a variety of dienophiles to afford ring-opened cycloadducts that are readily dehydrated using BF3· OEt2 to give polysubstituted anilines. In each case, the cycloaddition proceeds with high regioselectivity, with the electron-withdrawing group being located ortho to the amino group. The most favorable FMO interaction is between the HOMO of the furanamine and the LUMO of the dienophile. The atomic coefficient at the ester carbon of the furan is larger than that at the amino center, and this nicely accommodates the observed regioselectivity. The [4 + 2]-cycloaddition of N-(5-nitrofuranyl)morpholine with methyl vinyl ketone affords a mixture of three phenols. One of the phenols is derived from a Diels−Alder reaction followed by nitro group ejection and subsequent aromatization. The remaining two phenols are the result of cleavage of the initially formed oxabicyclic intermediate with concomitant migration of the nitro group. The mild reaction conditions with which furan-2-carbamic acid tert-butyl ester undergoes Diels−Alder cycloaddition with N-phenylmaleimide allow for the ready isolation of the initial oxybridged cycloadduct.

Acylation of 5-amino-2-furancarboxylic acid methyl ester with alkenoyl acid chlorides gives 2-amidofurans that undergo intramolecular Diels−Alder cycloadditions. The reactions occur at 165 °C in toluene or at 100 °C when 4 M ethereal LiClO4 was used as the solvent. The resultant dihydroindoles are formed by the nitrogen lone pair assisted ring opening of the initial oxa-bridged cycloadducts, followed by loss of water. Under certain conditions, alternative cationic cyclization routes become important pathways. Several members of the pyrrolophenanthridine class of alkaloids were obtained by a short, efficient method based on the intramolecular Diels−Alder furan cycloaddition of 2-amidofurans containing a tethered alkenyl group. The resulting dihydroindoles were elaborated in one step to the 1H-pyrrolo[3,2,1-de]phenanthridine ring system by a free radical induced cyclization using bis(tributyltin).