Synthesis of 2H-chromenes (2H-benzopyrans)
A mild, electrophilic cyclization of substituted propargylic aryl ethers by I2, ICl, and PhSeBr produces 3,4-disubstituted 2H-benzopyrans in excellent yields. This methodology tolerates various functional groups, such as methoxy, alcohol, aldehyde, and nitro groups.
S. A. Worlikar, T. Kesharwani, T. Yao, R. C. Larock, J. Org. Chem., 2007, 72, 1347-1353.
Under base-free conditions, readily accessible 2-ethoxy-2H-chromenes undergo C-O activation and C-C bond formation in the presence of an inexpensive nickel catalyst and boronic acids. This modular and highly efficient protocol enables broad access to 2-substituted-2H-chromenes and has been applied to the late-stage incorporation of complex molecules.
T. J. A. Graham, A. G. Doyle, Org. Lett., 2012, 14, 1616-1619.
A transition-metal-free coupling of various triorganoindium reagents with chromene and isochroman acetals in the presence of BF3ˇOEt2 afforded 2-substituted chromenes and 1-substituted isochromans, respectively, in good yields. The reactions proceed with only 50 mol % of the organometallic, thus demonstrating the efficiency of these species.
J. M. Gil-Negrete, J. P. Sestelo, L. A. Sarandeses, Org. Lett., 2016, 18, 4316-4319.
A cascade oxa-Michael-Henry reaction of salicylaldehyde derivatives with β-nitrostyrenes is catalyzed by potassium carbonate using solvent-free ball milling to provide 3-nitro-2H-chromenes in good yields. This method offers short reaction times and mild conditions.
S.-X. Liu, C.-M. Jia, B.-Y. Yao, X.-L. Chen, Q. Zhang, Synthesis, 2016, 48, 407-412.