Synthesis of Coumarins
An efficient annulation of phenolic acetates with acrylates in the presence of [Rh2(OAc)4] as catalyst and formic acid as reducing agent provides high yield of coumarin derivatives via C-H bond activation. The addition of NaOAc as a base increased the yield of the products. The reaction is quite successful for both electron-rich as well as electron-deficient phenolic acetates, affording coumarins with excellent regioselectivity.
S. K. Gadakh, S. Dey, A. Sudalai, J. Org. Chem., 2015, 80, 11544-11550.
Palladium-catalyzed oxidative Heck coupling reaction of coumarins and arylboronic acids allows a direct synthesis of 4-arylcoumarins in good yields. The reaction also showed tolerance toward functional groups such as hydro, methoxy, diethylamino, nitro, and chloro groups.
Y. Li, Z. Qi, H. Wang, X. Fu, C. Duan, J. Org. Chem., 2012, 77, 2053-2057.
The two discrete photochemical activation modes of (-)-riboflavin sequentially induce isomerization and cyclization by energy transfer (ET) and single-electron transfer (SET) activation pathways in an emulation of the coumarin biosynthesis pathway via a key photochemical E → Z isomerization step. The ensuing SET-based cyclization eliminates the need for a prefunctionalized aryl ring.
J. B. Metternich, R. Gilmour, J. Am. Chem. Soc., 2016, 138, 1040-1045.
The employment of hydrophobic ionic liquids dramatically enhanced the activity of metal triflates in Friedel-Crafts alkenylations of aromatic compounds with various alkyl- and aryl-substituted alkynes.
C. E. Song, D.-U. Jung, S. Y. Choung, E. J. Roh, S.-G. Lee, Angew. Chem., 2004, 116, 6309-6311.
Arylpropionic acid methyl esters having a MOM-protected hydroxy group at the ortho position underwent hydroarylation with various arylboronic acids in MeOH at ambient temperature in the presence of a catalytic amount of CuOAc, resulting in the formation of 4-arylcoumarins in high yields after the acidic workup.
Y. Yamamoto, N. Kirai, Org. Lett., 2008, 10, 5513-5516.
The use of cyanuric chloride (2,4,6-trichloro-1,3,5-triazine) and N-methyl morpholine enables an efficient and general protocol for a rapid synthesis of substituted 3-aryl coumarins. A series of substituted phenyl acetic acids have been successfully reacted with substituted 2-hydroxy benzaldehydes to afford 3-aryl coumarins in good to excellent yields.
K. V. Sashidhara, G. R. Palnati, S. R. Avula, A. Kumar, Synlett, 2012, 23, 611-621.
A direct carboxylation of alkenyl C-H bonds of 2-hydroxystyrenes in the presence of a catalytic amount of Pd(OAc)2 and Cs2CO3 as base under atmospheric pressure of CO2 afforded coumarins in good yield. The reaction undergoes a reversible nucleophilic addition of the alkenylpalladium intermediate to CO2.
K. Sasano, J. Takaya, N. Iwasawa, J. Am. Chem. Soc., 2013, 135, 10954-10957.
Palladium-catalyzed oxidative cyclocarbonylation of 2-vinylphenols enables a direct synthesis of various coumarins in good yields in the presence of low pressures of CO, and air or 1,4- benzoquinone as the oxidant. The reaction conditions are attractive in terms of environmental considerations and operational simplicity.
J. Ferguson, F. Zeng, H. Alper, Org. Lett., 2012, 14, 5602-5605.
The basic ionic liquid 1-butyl-3-methylimidazolium hydroxide, [bmIm]OH, efficiently catalyzes the Knoevenagel condensation of various aliphatic and aromatic aldehydes and ketones with active methylenes at room temperature without requirement of any organic solvent.
B. C. Ranu, R. Jana, Eur. J. Org. Chem., 2006, 3767-3770.
A facile, convenient, efficient, and high yielding synthesis of a combinatorial library of 3-aroylcoumarins has been developed by the condensation of easily available -aroylketene dithioacetals and 2-hydroxybenzaldehydes in the presence of catalytic amount of piperidine in THF reflux.
H. S. P. Rao, S. Sivakumar, J. Org. Chem., 2006, 71, 8715-8723.
The ionic liquid 1-butyl-3-methylimidazonium tetrafluoroborate [bmim]BF4 was used for ethylenediammonium diacetate (EDDA)-catalyzed Knoevenagel condensation between aldehydes or ketones with active methylene compounds. Catalyst and solvent can be recycled.
C. Su, Z.-C. Chen, Q.-G. Zhen, Synthesis, 2003, 555-559.
An efficient metal-free tandem acylation/cyclization of alkynoates with aldehydes enables the synthesis of 3-acyl-4-arylcoumarins via addition of acyl radical to alkynes and a C-H bond functionalization to form two new C-C bonds simultaneously.
X. Mi, C. Wang, M. Huang, Y. Wu, Y. Wu, J. Org. Chem., 2015, 80, 148-155.
4-Carboxyalkyl-8-formyl coumarins can be synthesized from 2-hydroxybenzaldehydes, triphenylphosphine and dialkyl acetylenedicarboxylate in good yields via vinyltriphenylphosphonium salt mediated aromatic electrophilic substitution.
K. C. Majumdar, I. Ansary, S. Samanta, B. Roy, Synlett, 2011, 694-698.
A new carbamoyl Baker-Venkataraman rearrangement allows a general synthesis of substituted 4-hydroxycoumarins in good overall yields. Intermediate arylketones are efficiently prepared via a Directed ortho Metalation - Negishi cross coupling protocol from arylcarbamates. The overall sequence provides a regiospecific anionic Friedel-Crafts complement for the construction of ortho-acyl phenols and coumarins.
A. V. Kalinin, A. J. M. Da Silva, C. C. Lopes, R. S. C. Lopes, V. Snieckus, Tetrahedron. Lett., 1998, 39, 4995-4998.
An exclusive 6-endo-dig iodocyclization of 3-ethoxy-1-(2-alkoxyphenyl)-2-yn-1-ols gives 4-substituted 3-iodocoumarins, whereas a 5-endo-dig iodocyclization of 1-alkoxy-4-ethoxy-3-yn-1,2-diols gives 3-iodobutenolides respectively. The reactions are carried out under very mild conditions using I2 in DCM or toluene at room temperature.
M. S. Reddy, N. Thirupathi, M. H. Babu, S. Puri, J. Org. Chem., 2013, 78, 5878-5888.