(CF3CO)2O/CF3SO3H-mediated synthesis of 1,3-diketones from carboxylic acids and aromatic ketones

Summary A very simple and convenient reaction for 1,3-diketone preparation from carboxylic acids and aromatic ketones in TFAA/TfOH system is described. When the β-phenylpropionic acids were used as starting materials, they initially gave 1-indanones and then underwent further acylation with the formation of 2-(β-phenylpropionyl)-1-indanones as the main reaction products. In addition, the application of the proposed protocol allowed for the synthesis of selected polysubstituted pyrazoles in a one-pot procedure directly from acids and ketones.


Introduction
1,3-Diketones represent one of the most important class of organic compounds, since they are applied as key structural blocks in organic syntheses, exhibit different kinds of biological activities, and display a broad range of ionophoric properties [1][2][3]. The method most frequently used for 1,3-diketone synthesis is the Claisen condensation, which comprises the C-acylation of the α-position of ketones in the form of their metal enolates, enamines or silyl ethers, with or without a catalyst. To appear as an acylating agent one of the following compounds could be required: acyl halides and acid esters, including formates and oxalates, and acid anhydrides, dialkyl carbonates, methoxymagnesium methyl carbonate, N-acylimidazoles, acyl cyanides, and acylbenzotriazoles [4]. Though recently many modifications of this method have been proposed [5][6][7][8][9][10][11], it is noticeable that none of these techniques implements a direct synthesis of β-diketones from acids and ketones with an immediate activation of both carbonyl and methylene components in the course of the reaction.
Herein, we would like to describe a direct and operationally simple TFAA/TfOH-mediated synthesis of 1,3-diketones from unmodified carboxylic acids and ketones.
We concluded that in the work of reference [25], an apparently larger quantity of the super acidic TfOH was employed (4 equiv vs 0.25-1.5 equiv in our work), which possibly slowed down the reaction of ketone acylation. This is corroborated in the case of phenylpropionic acid 1а. Here, the yield of the diketone 3a decreased with an increase of the quantity of TfOH, whereas the yield of 1-indanone (2a) increased and reached 94% with 1.5 equiv of TfOH (Table 1, entry 4).
In the case of β-(4-bromophenyl)propionic acid (1b) with 0.5 equiv of TfOH, the reaction chemoselectively proceeds to give the only diketone 3b (59%, Table 1, entry 6). The maximal yield of the diketone 3b is obtained with 1 equiv of TfOH (70%, Table 1, entry 7), and a further increase of the TfOH quantity to 3 equiv results in the indanone 2b as the only product (62%, Table 1, entry 9). Evidently, the intra-and intermolecular acylation of β-phenylpropionic acids in this reaction is dependent on the used TfOH quantity and the nature of the substituent in the phenyl moiety.
On the basis of the above results, we supposed that the acylation of ketones with carboxylic acids in a TFAA/TfOH/CH 2 Cl 2 system could be applied as an effective method for the synthesis of β-diketone. It turned out that the acylation of different alkyl aryl ketones 2a-k (indanones, tetralone, acetophenones, 2-acetylthiophene and methyl benzyl ketone) with alkanoic acids RCOOH 1d-h (where R = 1-adamantylmethyl, neopentyl, isopropyl, methyl, phenyl) gave the corresponding β-diketones 3c-t in 37-86% yields ( Table 2).  In most cases reactions were carried out at the molar ratios of acid:ketone:TFAA:TfOH = 1:1:6:0.5. For the reaction of 1-indanone (2а) and 1-adamantylacetic acid (1d) it was shown that in the absence of TfOH the yield of diketone 3c significantly dropped (<2%). With 0.25-1.5 equiv of TfOH the yield of the diketone 3c reached its maximal value (~80%) and decreased with a greater excess (3 equiv) of TfOH (57%). Apparently, the reduction of the TFAA excess (from 6 to 3 equiv) slightly lowered the yield of the diketone 3c to 68%, although the quantity of TFAA was not optimized for these reactions. An excess of the acid 1e (2 equiv vs 1 equiv) in the acylation of acetophenone (2e) only modestly increase the yield of diketone 3d from 64 to 69% (Table 2, entry 11), and usually we added equimolecular quantities of an acid and a ketone. An excess of acetic acid in acylation reactions (Table 2, entries 4, 8 and 13) was employed considering that this acid was partially self-acylated under the reaction conditions. A probable mechanism of the reaction of ketone acylation by acids in TFAA/ TfOH media may be that after the acyl trifluoroacetates are generated in situ, triflic acid becomes involved in the enoliza-Scheme 2: Scope and limitations.
tion of ketones and increases the acylating ability of acyl trifluoroacetates. However, with an increase of the TfOH quantity (>1.5 equiv) a retardation of the ketone acylation is observed, presumably as a result of its protonation.
The acylation of the methyl benzyl ketone (2k) by 1-adamantylacetic acid (1d) proceeded with regioselectivity at the α-СН 2group and gave the diketone 3t in good yield ( Table 2, entry  18), which is apparently associated with the enolization ability of 2k in the benzyl fragment of the molecule.
To enhance the application of the considered reaction, we investigated the acylation of the ketones by several functionally substituted carboxylic acids. Whereas glycine did not react with the ketones, β-alanine (1i) reacting with 1-indanone, 1-tetralone and acetophenone formed the corresponding trifluoroacylated β-aminodiketones 3u-w (reaction 1 in Scheme 2). A fuller acylation for 2a and 2c was achieved when 1.5 equiv of the acid 1i and 1 equiv of TfOH were used, whereas the yield of diketone 3w still remained low under these conditions. The hydrolysis of trifluoroacetates 3u, 3v under reflux in dilute HCl was followed by the intramolecular cyclization and led to the unknown heterocycles 4а,b.
Unambiguous evidence for the structure of heterocycle 4a was obtained by X-ray diffraction analysis [26]. The crystal struc-ture of 4a is mediated by hydrogen bonds with a cation, a chloride anion and a solvating water molecule as participants ( Figure 1).

Figure 1:
The molecular structure of 4a.
By acylation of 1-indanone (2a) with (diphenylphosphoryl)acetic acid (1j), the self-acylation of the acid 1j already occurred at room temperature, which complicated the separation of the desired dicarbonyl compound. When the acid 1j was heated under the conditions of TFAA/TfOH-mediated selfacylation of alkanoic acids recently reported by us [27] and decarboxylation was subsequently carried out, 1,3-diphenylphosphorylated acetone 5 could be obtained (reaction 2 in Scheme 2).
Finally, we used our method in a two-stage one-pot syntheses of pyrazoles, which find extensive use in the pharmaceutical industry [8,28]. Diketones 3a-c,f, essential for synthesis of the pyrazoles 6a-d, were obtained by one of the three routes (Scheme 3). The one-pot intra-and intermolecular acylation of β-phenylpropionic acids 1а,b gave 3а,b. The acylation of the 1-indanone (2а) by acetic acid (1g) yielded 3f. The selective intramolecular cyclization of acid 1а was used to obtain the intermediate ketone 2а, which was further acylated by the acid 1d to finally give 3c. Upon the formation of the diketones, the reaction mixtures were evaporated in vacuum, the residues were dissolved in ethanol, and the following reaction with hydrazine hydrate gave the pyrazoles 6a-d in 62-76% yield.

Conclusion
In summary, we proposed a simple and effective synthetic approach to 1,3-diketones based on the TFAA/TfOH-mediated acylation of ketones with carboxylic acids. Advantages of this approach are the ready availability of the starting materials, the simple operational procedure, and the possibility to realize the one-pot immediate syntheses of 1,3-diketones and pyrazoles directly from acids and ketones. Moreover, 1,3-diketones and pyrazoles can be obtained from unmodified β-phenylpropionic acids. Given the importance of 1,3-dicarbonyl compounds in general, we expect that this reaction has a wide application in the realm of synthetic chemistry.

Experimental
General procedure for the synthesis of diketones: A solution of carboxylic acid (1 mmol), ketone (1 mmol, if required) and TFAA (0.85 mL, 6 mmol) in dichloromethane (1 mL) was stirred for 15 min at rt. The required quantity of triflic acid (usually 44 μL, 0.5 mmol) was then added, and the resulting solution was stirred at rt for 1-4 h (24 h for 3v and 3w) under the conditions indicated in Scheme 1, Scheme 2, Table 1, and  Table 2 (TLC monitoring). The reaction mixture was evaporated under reduced pressure, and after quenching with water, the residue was redissolved in dichloromethane (10 mL), washed with 5% NaHCO 3 (2 × 3 mL), water (2 × 3 mL), and dried over MgSO 4 . The solvent was removed in vacuum, and the crude reaction mixture was purified by silica gel chromatography (n-hexane/CH 2 Cl 2 /MeOH).
As illustrative examples, compounds 3a and 3c are prepared as follows.  The synthesis of pyrazoles 6. As an illustrative example, compound 6a is prepared as follows. A solution of β-phenylpropionic acid (1a, 150 mg, 1 mmol) and TFAA (0.85 mL, 6 mmol) in 1 mL CH 2 Cl 2 was stirred for 15 min at rt. Then TfOH (44 μL, 0.5 mmol) was added, and the reaction mixture was kept for 2 h. On completion of the reaction, the solvent was removed under reduced pressure. The crude 3a was dissolved in 5 mL ethanol and heated under reflux with hydrazine hydrate (0.1 mL, 2 mmol). After 2 h the solvent was evaporated, and the remaining oil was dissolved in CH 2 Cl 2 , washed with 5% NaHCO 3 , water, and dried over MgSO 4 . The product was purified by means of column chromatography (SiO 2 60, eluent: CH 2 Cl 2 /MeOH 50:1). Yield: 69% (90 mg), brown solid; mp 108-110 °С; 1