Synthesis of Trisubstituted Furans from Activated Alkenes by P(III)/P(V) Redox Cycling Catalysis

The organocatalytic formation of an underrepresented family of trisubstituted and tetrasubstituted furans from activated alkenes and acyl chlorides is reported. In a reaction sequence based on P(III)/P(V) redox cycling catalysis, the cyclic phosphine catalysts react with diacylethenes or acyl acrylates in Michael addition, followed by acylation and either an intramolecular Wittig reaction or a ring closure reaction, liberating the furans. The formed phosphine oxides are reduced in situ by phenylsilane as a terminal reductant. In the first step, 12 diacylethenes were converted to the respective trisubstituted furans. The reaction of acyl acrylates showed a surprising, catalyst-dependent alternate reaction forming tetrasubstituted furans. Two additional methods were developed, giving 14 trisubstituted furans using a phospholene catalyst and an additional 6 tetrasubstituted furans using a phosphetane catalyst. This encompassed 19 newly described compounds.