Influence of Lewis Acids on the Diels–Alder Reaction. V. The Reaction of Furan with Dimethyl Acetylenedicarboxylate

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Abstract
The Diels–Alder addition of furan to dimethyl acetylenedicarboxylate (DMAD) affords a monoadduct 1. The position and steric course of further addition of furan to the latter are temperature dependent. At temperatures of 50° or below addition occurs mainly at the activated double bond to give endo-endo4 and endo-exo5 diadducts. At 100° addition occurs at the less substituted double bond of the monoadduct to give analogous endo-endo7 and endo-exo9 diadducts; further addition of furan to the latter at 100° affords four triadducts 12–15. The configurations of all these products were determined by p.m.r. and assignments confirmed by simulation of the spectra.AlCl3 causes enormous rate enhancement of the Diels–Alder additions of furan to DMAD and to the activated double bond of the monoadduct. In the presence of the Lewis acid the predominant diadduct formed is the endo-exo isomer 5.

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