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Synthesis of 2H-chromenes (2H-benzopyrans)

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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 solvent-controlled and rhodium(III)-catalyzed C-H activation/[3 + 3] annulation sequence enables an efficient and redox-neutral synthesis of 2H-chromene-3-carboxylic acids from N-phenoxyacetamides and methyleneoxetanones as a three-carbon source.
Z. Zhou, M. Bian, L. Zhao, H. Gao, J. Huang, X. Liu, Y. Yu, X. Li, W. Yi, Org. Lett., 2018, 20, 3892-3896.

The Petasis condensation of vinylic or aromatic boronic acids, salicylaldehydes, and amines is assisted by the hydroxy group adjacent to the aldehyde moiety. A subsequent cyclization with ejection of amine upon heating provides 2H-chromenes. A catalytic method using a resin-bound amine enables a convenient preparation of 2H-chromenes.
Q. Wang, M. G. Finn, Org. Lett., 2000, 2, 4063-4065.

Diarylprolinol silyl ether catalyzes an asymmetric enantioselective domino oxa-Michael-Mannich-[1,3]-amino rearrangement reaction of N-tosylsalicylimines with a wide range of α,β-unsaturated aldehydes to produce the corresponding chair N-tosylimines-chromenes with excellent enantioselectivity and yield. The reaction tolerates a range of functional groups.
S. Hu, J. Wang, G. Huang, K. Zhu, F. Chen, J. Org. Chem., 2020, 85, 4011-4018.

A structurally simple and easily accessible l-proline derived aminocatalyst provides 2-alkyl/aryl-3-nitro-2H-chromenes in excellent enantioselectivity within a short reaction time via an asymmetric tandem oxa-Michael-Henry reaction of salicylaldehyde with conjugated nitroalkene.
R. Mohanta, G. Bez, J. Org. Chem., 2020, 85, 4627-4636.

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.