The efficiency of ring‐closing metathesis (RCM) reactions of various acrylates to form 2 (5 H)‐furanones, catalyzed by second‐generation ruthenium complexes, is investigated in detail. In particular the effects of substrate concentration and substrate structure were studied for indenylidene and benzylidene catalysts, both of which are shown to catalyze the RCM reaction in good yields. The starting substrate concentration is shown to be an important factor in the reaction, as, at excessive initial substrate concentrations, irreversible catalyst deactivation seems to occur, whereas high catalyst loadings appear to be unnecessary.
Abstract2,5‐Disubstituted furans were synthesized by one‐flask Heck arylation/oxidation sequences. The starting materials are 2‐substituted 2,3‐dihydrofurans, conveniently available by RCM/isomerization sequences, and arenediazonium salts. These react in ligand‐free Heck reactions to afford 2,5‐disubstituted 2,5‐dihydrofurans, which are oxidized to the corresponding furans without isolation or intermediate workup. The oxidation is conveniently achieved with chloranil or DDQ, depending on the substrate.
SummaryThe catalytic performance of NHC-ligated Ru-indenylidene or benzylidene complexes bearing a tricyclohexylphosphine or a pyridine ligand in ring closing metathesis (RCM), cross metathesis, and ring closing enyne metathesis (RCEYM) reactions is compared. While the PCy3 complexes perform significantly better in RCM and RCEYM reactions than the pyridine complex, all catalysts show similar activity in cross metathesis reactions.
Butenolides, obtained by ring‐closing metathesis (RCM) of acrylates, undergo quantitative deprotonation with amide bases. Trapping of the resulting anions with electrophiles, for example, chlorophosphates, give furans. Subsequent Diels–Alder reaction and acid‐catalysed rearrangement of the resulting oxabicyclonorbornadienes give substituted benzenes.
Oxidation. -A new route to 2,5-disubstituted furans is presented starting from simple diallyl ethers. Ring closure is achieved by metathesis and Heck coupling. Subsequent oxidation reactions occur with chloranil or DDQ. This procedure is also applicable to synthesize unsymmetrical 2,5-diaryl furans which play an important role in medicinal chemistry [cf. (VII)]. -(SCHMIDT*, B.; GEISSLER, D.; Eur. J. Org. Chem. 2011, 25, 4814-4822, http://dx.
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