Efficient and regioselective chromium(0)-catalyzed reaction of 2-substituted furans with diazo compounds: stereoselective synthesis of (2E,4Z)-2-aryl-hexadienedioic acid diesters

Pentacarbonyl(η2-cis-cyclooctene)chromium(0) (1) catalyzes efficiently reactions of diazo compounds with electron-rich furans. The reaction of 2-methoxyfuran (2) with alkyl α-diazoarylacetate (3a–g) furnishes the (2E,4Z)-2-aryl-hexadienedioic acid diesters (4a–g) in excellent yields. These reactions are highly regioselective. The cyclopropanation intermediates formed from 1 and diazo compounds 3a–g always arise from a carbene addition to the less substituted Cdouble bond; length as m-dashC bond of 2. The resulting cyclopropanation product undergoes a ring opening reaction to form the corresponding (2E,4Z)-2-aryl-hexadienedioic acid diesters (4a–g). The pentacarbonylchromium(0)-catalyzed reactions of 2-alkylfuran (5a–b) with ethyl α-diazophenylacetate (3a) and 9-diazo-9H-fluorene (3h) produce the 1(E),3(E)-butadienes (6a–d) in very good yields.