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Synlett 2011(5): 635-638  
DOI: 10.1055/s-0030-1259691
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Catalytic Synthesis of Coumarins via Direct Annulations of α,β-Unsaturated Aldehydes and Salicylaldehydes

Ulrike Grossa,b, Patrick J. Grossa,b, Min Shi*b, Stefan Bräse*a
a Institute of Organic Chemistry, Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany
e-Mail: stefan.braese@kit.edu;
b State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry (SIOC), Chinese Academy of Sciences, 354 Fenglin Lu, Shanghai 200032, P. R. of China
e-Mail: mshi@mail.sioc.ac.cn;
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Publikationsverlauf

Received 4 December 2010
Publikationsdatum:
25. Februar 2011 (online)

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Abstract

The first organocatalytic approach towards substituted coumarins is reported. Catalytic amounts of in situ generated N-­heterocyclic carbenes (NHC) catalyze a one-pot redox esterification of α,β-unsaturated aldehydes with simultaneous aldol condensation.

Key words

domino reactions - heterocycles - umpolung

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11

Typical Procedure
To a round-bottom flask with reflux condenser under argon atmosphere were added salicylaldehyde (20 µL, 23 mg, 0.19 mmol), crotonaldehyde (23 µL, 19 mg, 0.28 mmol, 1.5 equiv), 1,3-diisopropyl-imidazolium chloride (7.0 mg, 37 µmol, 20 mol%), Cs2CO3 (60 mg, 0.19 mmol, 1.0 equiv) and o-xylene (1.0 mL). The reaction mixture was heated to 120 ˚C for 12 h. Then H2O (10 mL) was added, and the resulting mixture was extracted with EtOAc (3 × 10 mL). The combined organic extracts were washed with brine (10 ml), dried over MgSO4, and evaporated under reduced pressure. Purification by chromatography on silica gel, eluting with PE-EtOAc (20:1), yielded the desired product 4ac (26 mg, 81%) as a pale yellow solid.