Benzo furan reaction with carbenes

Fischer-type chromium carbene complexes of furans underwent Dotz benzannulation with alkynes to provide trisubstituted benzo [, ]furan derivatives. An example used in the synthesis of isodityrosine is depicted in Equation (39) <2005JOC7422>. The efficiency of the reaction could be improved by ultrasound sonication <19990L1721>. 

Vinyl-substituted benzo[c]furans can be prepared by reaction of n-alkynylbenzaldehydes with chromium Fischer carbene complexes. Initially a benzo[c]furan chromiumtricarbonyl complex is believed to be formed which is converted into an alkylidenephthalan derivative or can be trapped with electron-deficient dienophiles with excellent ( 3 ti-selectivity (Equation 128) <20000L1267>. More elaborate vinylidene Fischer carbene complexes yield dienyl benzo[c]furans that undergo [8-1-2] cycloaddition with DMAD to furnish furanophane derivatives <2003JA12720>. An equilibrium between 7] -(o-ethynylbenzoyl)rhenium complexes and rhenium benzo[f]furyl carbene complexes has been observed. These species behave like other benzo[< ]furans in the reaction with DMAD <20040M4121>. 

Thermal decomposition of the diazo compounds 116a,b in methanol or ethanol, in the presence of Cu(acac)2 as catalyst, resulted in an intramolecular carbene reaction the benzo[c]furans 117a,b were not isolated, but 117b could be trapped with A-phenylmaleimide and dimethyl acetylenedicarboxylate, to give compounds 118 (endo exo =1 1, 90%) and 121 (92%). Dimethyl fumarate yielded adduct 119 (15%) and supposedly 120 (83%) concerning this structure the same objections can be raised as in the case of... 

When the diazoesters (240) and (241) were thermally decomposed in the presence of copper(II) acetoacetate ester as catalyst, an intermolecular carbene reaction occurred, the resulting benzo[c]furans (242) and (243) were not isolated but were trapped as the Diels-Alder adducts with N- methylmaleimide and dimethyl acetylenedicarboxylate (76CL287). 

Diazo-l-oxoalkenes with Z-configuration on treatment with rhodium catalysts yield furans (Equation (7)). This reaction can be envisaged as an electrocyclization reaction of the corresponding carbene <83JOC38i3, 90TL6835> (see also Section 2.07.2.2.3). TTiis route is also useful in the benzo[6)-and benzo[c]furan series (see Sections 2.07.3 and 2.07.4). 

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