Synthesis of β-keto carboxylic acids, esters and amides
An (E)- and (Z)-stereocomplementary preparative method for α,β-disubstituted α,β-unsaturated esters is performed via three general and robust reaction sequences: Ti-Claisen condensation (formylation) of esters to give α-formyl esters, (E)- and (Z)-stereocomplementary enol tosylation using TsCl-N-methylimidazole-Et3N and LiOH, and stereoretentive Suzuki-Miyaura cross-coupling.
H. Nakatsuji, H. Nishikado, K. Ueno, Y. Tanabe, Org. Lett., 2009, 11, 4258-4261.
Pentafluorophenylammonium triflate (PFPAT) successfully catalyzed the C-acylation of enol silyl ethers with acid chloride to produce various β-diketones in good yield. Similarly, C-acylation of ketene silyl acetals or ketene silyl thioacetals proceeded smoothly to provide also thermodynamically unfavorable α,α-dialkylated β-keto (thio)esters in good yield.
A. Iida, J. Osada, R. Nagase, T. Misaki, Y. Tanabe, Org. Lett., 2007, 9, 1859-1862.
A Ti-crossed Claisen condensation between ketene silyl (thio)acetals and acid chlorides gave α-monoalkylated (thio)esters and thermodynamically unfavorable α,α-dialkylated β-keto (thio)esters in good yield. The protocol was extended to the direct condensation of ketene silyl acetals with carboxylic acids.
A. Iida, S. Nakazawa, T. Okabayashi, A. Horii, T. Misako, Y. Tanabe, Org. Lett., 2006, 8, 5215-5218.
Enamination of acetoacetamides with Boc-monoprotected ethylenediamine provides β-enamino amides, which can be acylated at the α-carbon with excellent selectivity. These C-acylated derivatives undergo domino fragmentation in acidic media to give the corresponding β-keto amides accompanied by 2-methyl-4,5-dihydro-1H-imidazole.
P. Angelov, Synlett, 2010, 1273-1275.
Thioesters undergo chemoselective soft enolization and acylation by N-acylbenzotriazoles on treatment with MgBr2ˇOEt2 and i-Pr2NEt to give β-keto thioesters without prior enolate formation. The reaction is conducted using untreated CH2Cl2 open to the air. The coupled products can be converted directly into β-keto esters, β-keto amides, and β-diketones under mild conditions.
G. Zhou, D. Lim, D. M. Coltart, Org. Lett., 2008, 10, 3809-3812.
Aromatic, aliphatic, and heterocyclic aldehydes are successfully condensed with ethyl diazoacetate in the presence of molybdenum(VI) dichloride dioxide as catalyst to obtain the corresponding β-keto esters in high yields at room temperature.
K. Jeyakumar, D. K. Chand, Synthesis, 2008, 1685-1687.
Aldehydes react readily with ethyl diazoacetate in the presence of 5 mol% of NbCl5 in dichloromethane to produce the corresponding β-keto esters in good yields with high selectivity. This method allows the preparation of β-keto esters from various aldehydes under mild reaction conditions.
J. S. Yadav, B. V. S. Reddy, B. Eeshwaraiah, P. N. Reddy, Tetrahedron, 2005, 61, 875-878.
Chelated enolates are good nucleophiles for reactions with acyl halides and imidazolides affording α-amino-β-keto esters. In most cases, the reactions are over after a few minutes and preparatively useful yields are obtained, independent of the protecting groups and electrophile used. With chloroformates as acylating agents, various protected amino malonates become accessible.
K. Schultz, L. Stief, U. Kazmaier, Synthesis, 2012, 44, 600-604.
TiCl4/Et3N promotes a condensation of acetate and formate esters to provide (E)-β-alkoxy- and (E)-β-aryloxyacrylate moieties. A plausible mechanism involving a bimetallic titanium intermediate for this type of transformation.
J. M. Álvarez-Calero, Z. D. Jorge, G. M. Massanet, Org. Lett., 2016, 18, 6344-6347.