Melamine Trisulfonic Acid as a Highly Efficient and Reusable Catalyst for the Synthesis of β-Acetamido Ketones

A highly efficient protocol for the one-pot multi-component condensation of acetophenones with aromatic aldehydes, acetonitrile and acetyl chloride in the presence of melamine trisulfonic acid (MTSA) as a highly efficient and recyclable sulfonic acid-containing catalyst at room temperature is described. In this method, β-acetamido ketone derivatives are obtained in high to excellent yields and in relatively short reaction times.


Introduction
β-Acetamido ketones are useful building blocks for a number of biologically and pharmaceutically valuable compounds [1][2][3][4][5][6][7] .For example, they are precursors of molecules such as 1,3-amino alcohols [1][2][3] and γ-lactams 4 , as well as biologically attractive compounds such as nikkomycins or neopolyoxins 5,6 .Moreover, it is reported that β-acetamido ketones can act as aglucosidase inhibitors 7 .The one-pot multi-component condensation of acetophenones with aromatic aldehydes, acetonitrile and acetyl chloride has been used as a most common synthetic route towards β-acetamido ketones.Some catalysts have been utilized to perform this transformations, e.g.La(OTf) 3   7   , CoCl 2 8 , heteropolyacids 9 , Zr(HSO 4 ) 4 10  {Selectfluor TM } 13 , ZnO 14 , ZrOCl 2 .8H 2 O 15 , polyaniline-supported salts 16 , and CeCl 3 •7H 2 O 17 .β-Acetamido ketones have been also prepared from acetophenones, aldehydes and amides 18 .However, the reported methods for the synthesis of these compounds are associated with one or more of the following drawbacks: (i) low yields, (ii) long reaction times, (iii) the use of large amount of catalyst, (iv) the use of toxic or expensive catalysts, (v) tedious work-up procedure, (vi) harsh reaction conditions, and (vii) performance the reaction under certain special conditions.So, search for finding a protocol for the preparation of β-acetamido ketones which are not associated with the above-mentioned disadvantages is still relevant.
Multi-component reactions (MCRs) play an important role in combinatorial chemistry because of the ability to synthesize target compounds with greater efficiency and atom economy by generating structural complexity in a single step from three or more reactants 19- 23 .Moreover, MCRs have many advantages over conventional synthetic methodologies, such as higher productivity, simple procedures and facile execution [19][20][21][22][23] .In recent years, there has been a rapid growth in the development of novel heterogeneous and homogeneous sulfonic acid-containing catalysts and reagents in organic synthesis, because of their unique properties such as efficiency, high reactivity, easy availability of their starting materials and ability to promote a wide range of reactions 10,[23][24][25][26][27][28][29][30][31][32][33][34][35] .They are also green, non-toxic, non-corrosive and inexpensive.Melamine trisulfonic acid (MTSA) is certainly one of the most interesting sulfonic acid-containing catalysts which has been recently synthesized, and used in some organic transformations including synthesis of coumarins 32 , synthesis of 3,4-dihydropyrimidin-2(1H)-ones/thiones 33 , acetylation of alcohols, phenols, and amines 34 and chemoselective oxathioacetalyzation of aldehydes 35 .In this paper, we report a new, highly efficient and simple method for the synthesis of βacetamido ketones via the one-pot multi-component condensation between acetophenones, arylaldehydes, acetonitrile and acetyl chloride using melamine trisulfonic acid (MTSA) as an inexpensive and reusable SO 3 H-containing catalyst at room temperature (Scheme 1).It is worth noting that this method has none of the above-mentioned drawbacks at all.

Experimental
All chemicals were purchased from Merck or Fluka Chemical Companies.All known compounds were identified by comparison of their melting points and spectral data with those reported in the literature.Progress of the reactions was monitored by TLC using silica gel SIL G/UV 254 plates.The 1 H NMR (300 MHz) and 13 C NMR (75 MHz) were run on a Bruker Avance DPX-250, FT-NMR spectrometer (δ in ppm).Melting points were recorded on a Büchi B-545 apparatus in open capillary tubes.

Procedure for the preparation of melamine trisulfonic acid (MTSA)
A 250 mL suction flask charged with chlorosulfonic acid (5 mL, 75.2 mmol) was equipped with a gas inlet tube for conducting HCl gas over an adsorbing solution, i.e. water.Melamine (3.16 g, 25.07 mmol) was added in small portions over a period of 30 min at room temperature.HCl gas evolved from the reaction vessel immediately.After completion of the addition of melamine, the mixture was shaken for 30 min; meanwhile, the residual HCl was exhausted by suction.The mixture was triturated with n-hexane (10 mL) and then filtered.The solid residue was washed with n-hexane (10 mL) and dried under vacuum.MTSA (7.9 g, 87%) was obtained as a white solid, which was stored in a capped bottle 32 .

General procedure for the synthesis of β-acetamido ketones
To a mixture of acetophenone (1 mmol), aldehyde (1 mmol), acetonitrile (3 mL) and acetyl chloride (0.3 mL) in a 10 mL round-bottomed flask, was added MTSA (0.018 g, 0.05 mmol), and the resulting mixture was stirred at room temperature.After completion of the reaction, as monitored by TLC, the solvent (acetonitrile) was evaporated, EtOAc (5 mL) was added, and the catalyst was recovered by filtration, washed with EtOAc (5 mL), dried and reused (MTSA was recovered three times without significant loss of its activity).After evaporation of the EtOAc from the filtrate, crushed ice (10 mL) was added to the mixture and stirred thoroughly.On solidification, the crude product was filtered, dried, and purified by short column chromatography on silica gel eluted with EtOAc/n-hexane (1/4).

Results and Discussion
Initially, melamine trisulfonic acid was prepared from the reaction of melamine with chlorosulfonic acid according to the reported procedure 32 .Considering the high efficacy of this SO 3 H-containing compound to catalyze some organic transformation [32][33][34][35] , and also the high importance of β-acetamido ketones, we decided to examine the applicability of this catalyst for the synthesis of β-acetamido ketones.For this purpose, as a model, the reaction of acetophenone (1 mmol) with benzaldehyde (1 mmol), acetonitrile (3 mL) and acetyl chloride (0.3 ml) was examined in the presence of different molar ratios of MTSA at room temperature.The results are summarized in Table 1.As it can be seen in Table 1, higher yields of the product and shorter reaction times were obtained when the reaction was performed using 5 mol% of the catalyst.Moreover, increasing the amount of MTSA didn't affect significantly on the reaction results.To assess efficiency and scope of the method, different acetophenones (processing electronreleasing substituents, electron-withdrawing substituents and halogens on their aromatic ring) were condensed with structurally and electronically diverse aromatic aldehydes (bearing electron-releasing substituents, electron-withdrawing substituents and halogens on their aromatic ring), acetonitrile and acetyl chloride under the optimized reaction conditions.The respective results are displayed in Table 2.As Table 2 indicates, all reactions proceeded efficiently and the desired products were obtained in good to excellent yields in relatively short reaction times; thus, the protocol was general and efficient.Furthermore, β-acetamido ketones were obtained in high yields and in relatively short reaction times when 2naphthaldehyde and anthracene-10-carbaldehyde were used in the reaction instead of benzaldehydes (Table 2, compounds 1n-r).Interestingly, the condensation of 4-bromoacetophenone (3.2 eq.) with a tris-aldehyde (1 eq.), acetonitrile (9 mL) and acetyl chloride (0.9 mL) using MTSA (10 mol%) at room temperature afforded complex compound 2a in 83% yield within 120 min (Scheme 2).This is the first report of the synthesis of this class of β-acetamido ketones.To compare the efficiency of MTSA with the reported catalysts for the synthesis of βacetamido ketones, we have tabulated the results of these catalysts to perform the condensation of acetophenone with benzaldehyde, acetonitrile and acetyl chloride, in Table Scheme 1

a
Isolated yield.b This compound is new.

Table 1 .
The reaction of acetophenone with benzaldehyde, acetonitrile and acetyl chloride using different molar ratios of MTSA at room temperature.

Table 3 .
Comparison of the results of the reaction of acetophenone with benzaldehyde, acetonitrile and acetyl chloride catalyzed by MTSA, with those obtained by the reported catalysts.In this case, the reaction time and yield is related to the condensation of acetophenone with 4-nitrobenzaldehyde, acetonitrile and acetyl chloride.
a Our catalyst.b