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Synthesis of isochromenes

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An effective, Ru-catalyzed cycloisomerization of benzannulated homo- and bis-homopropargylic alcohols affords benzofurans and isochromenes chemo- and regioselectively (5-, and 6-endo cyclizations). The presence of an amine/ammonium base-acid pair is crucial for the catalytic cycle.
A. Varela-Fernández, C. González-Rodríguez, J. A. Varela, L. Castedo, Carlos Saá, Org. Lett., 2009, 11, 5350-5353.


A versatile and efficient access to 1H-isochromenes via a domino cycloisomerization/reduction reaction process starting from a wide range of functionalized ortho-alkynylbenzaldehydes is catalyzed by dichloro(2-pyridinecarboxylato)gold ([AuCl2(Pic)]).
E. Tomás-Mendivil, J. Starck, J.-C. Ortuno, V. Michelet, Org. Lett., 2015, 17, 6126-6129.


Gold-catalyzed oxidative cyclizations of o-(alkynyl)phenyl propargyl ether derivatives enable a convenient synthesis of functionalized 1H-isochromene and 2H-pyran derivatives via highly regioselective oxidation, followed by 1,2-migration of an enynyl group and nucleophlic addition. Oxidative cleavage of the exocyclic double bond of the obtained 1H-isochromenes provides isocoumarins.
J. Zhao, W. Xu, X. Xie, N. Sun, X. Li, Y. Liu, Org. Lett., 2018, 20, 5461-5465.


A synthesis of 3-substituted-1-alkoxyisochromenes starting from 2-alkynylbenzaldehydes and different alcohols is catalyzed by a silver(I) complex with an original macrocyclic pyridine-containing ligand. The convenient approach is characterized by absolute regioselectivity, mild reaction conditions and good to excellent reaction yields.
M. Dell'Acqua, B. Castano, C. Cecchini, T. Pedrazzini, V. Pirovano, E. Rossi, A. Caseli, G. Abbiati, J. Org. Chem., 2014, 79, 3494-3505.