Synthesis of Chromones and Flavones
A highly efficient and selective palladium-catalyzed ligand-free cyclocarbonylation reaction of o-iodophenols with terminal acetylenes under atmospheric CO pressure affords diversified chromones in very good yields. The use of a phosphonium salt ionic liquid as the reaction medium enhances the efficiency of the cyclocarbonylation reaction.
Q. Yang, H. Alper, J. Org. Chem., 2010, 75, 948-950.
A palladium complex of 1,3,5,7-tetramethyl-2,4,8-trioxa-6-phenyl-6-phosphaadamantane is an effective catalyst for a sequential microwave-assisted Sonogashira and carbonylative annulation reaction to give substituted flavones.
E. Awuah, A. Capretta, Org. Lett., 2009, 11, 3210-3213.
A Pd-catalyzed copper-free carbonylative Sonogashira coupling reaction at room temperature was achieved by using water as a solvent under balloon pressure of CO with Et3N as a base. The developed method was successfully applied to the synthesis of flavones.
B. Liang, M. Huang, Z. You, Z. Xiong, K. Lu, R. Fathi, J. Chen, Z. Yang, J. Org. Chem., 2005, 70, 6097-6100.
The carbonyl β-C(sp3)-H bond hydrogens of ortho-acyl phenols could be substituted by intramolecular phenolic hydroxyls to form O-heterocycles. A subsequent dehydrogenation provides flavonoids. This oxidant-free cascade reaction is catalyzed by Pd/C.
X. Zhao, J. Zhou, S. Lin, X. Jin, R. Liu, Org. Lett., 2017, 19, 976-979.
A one-pot Friedel-Crafts acylation of alkynes with suitably substituted benzoyl chlorides provides chromenones and thiochromenones. The ready availability of both starting materials, alkynes and benzoyl chlorides, coupled with the experimental simplicity makes the current synthetic method to (thio)chromenones fast, efficient, and practical.
H. Y. Kim, E. Song, K. Oh, Org. Lett., 2017, 19, 312-315.
Chromone derivatives were synthesized from 2,3-allenoic acids and benzynes in good yields under mild conditions. The benzyne intermediate undergoes 1,2-addition with the carbonyl group, followed by ring opening, conjugate addition, and protonolysis to afford chromone derivatives. This protocol allows the diversity due to the substituent-loading capability of 2,3-allenoic acids as well as benzynes.
G. Chai, Y. Qiu, C. Fu, S. Ma, Org. Lett., 2011, 13, 5196-5199.
A Rh(III)-catalyzed annulation between salicylaldehydes and diazo compounds offers good functional group tolerance, scalability, and a controllable chemoselectivity. AgNTf2 as additive favored benzofurans via a tandem C-H activation/decarbonylation/annulation process, while AcOH led to chromones through a C-H activation/annulation pathway.
P. Sun, S. Gao, C. Yang, S. Guo, A. Lin, H. Yao, Org. Lett., 2016, 18, 6464-6467.
The unusual alcohol mediated reaction of 4-hydroxycoumarins and β-nitroalkenes leads to 4-oxo-2-aryl-4H-chromene-3-carboxylate (flavone-3-carboxylate) derivatives. The transformation occurs via the in situ formation of a Michael adduct, followed by the alkoxide ion mediated rearrangement of the intermediate. The effects of different media on the reaction were investigated.
M. R. Zanwar, M. J. Raihan, S. D. Gawande, V. Kavala, D. Janreddy, C.-W. Kuo, R. Ambre, C.-F. Yao, J. Org. Chem., 2012, 77, 6495-6504.
A mild ICl-induced cyclization of heteroatom-substituted alkynones provides a simple, highly efficient approach to various 3-iodochromones, iodothiochromenones, iodoquinolinones and analogues in good to excellent yields. Subsequent palladium-catalyzed transformations afford a rapid increase in molecular complexity.
C. Zhou, A. V. Dubrovsky, R. C. Larock, J. Org. Chem., 2006, 71, 1626-1632.