Multicomponent synthesis of pyrimido[4,5-b] quinolines over a carbocationic catalytic system

Trityl chloride (TrCl) was efficiently used as a neutral catalyst for the multicomponent cyclization reaction of an aldehyde with dimedone and 6-amino-1,3-dimethyluracil for the preparation of pyrimido[4,5-b] quinolines in chloroform under reflux condition.


Multicomponent
Organic catalysts with high stability, insensitivity to moisture and oxygen, specific molecular structure, low toxicity and the absence of metal in their structure are notable for the catalyze of the organic reactions 1,2 .
Recently, the application of trityl chloride as an organic catalyst has been considered in organic synthesis due to some advantages such as cheapness and commercially available.Also, neutral and mild reaction condition in terms of temperature and pressure is another benefits for trityl chloride as a catalyst 3 .
Pyrimido [4,5-b]quinolones are important compounds in medicinal chemistry due to some biological properties such as antifungal 13 , antimalarial 14 , anticancer 15,16 , antiviral 17 , antihistaminic 18 , anti-oxidant, anti-microbial 19 , and anti-inflammatory activities 19,20 .The one pot multi-component synthesis of pyrimido [4,5-b]quinolones by the reaction of aldehyde with dimedone and 6-amino-1,3-dimethyluracil is an important method for the preparation of these compounds 19 .Multi-component reactions are notable for the chemists due to prepare of the target product in one step without the produce of side products.High atomic economy, high yields, short reaction times, saving energy, reaction times, materials and solvents and compliance with green chemistry protocols are some important advantages of these reactions [21][22][23][24][25][26] .
In continues to our investigation on the catalytic application of carbocationic system to carry out the organic reactions, we have used of trityl chloride (TrCl) as a neutral catalyst for the preparation of pyrimido [4,5-b] quinolines by the multi component reaction of aromatic aldehydes with dimedone and 6-amino-1,3-dimethyluracil (Fig. 1).

Results and discussion
In the first step, to find the best reaction condition, the reaction of 2, 4-dichlorobenzaldehyde with dimedone and 6-amino-1,3-dimethyluracil was selected as a model reaction and kinds of catalysts, catalyst amounts and temperature were studied on this reaction.Also, the model reaction was carried out in various solvents such as chloroform, ethanol, ethyl acetate, n-hexane, acetone, dichloromethane and acetonitrile in comparison with solvent-free condition.According that, the best result was obtained using 10 mol % of trityl chloride as a catalyst in chloroform under reflux condition (Table 1).
The catalytic effect of trityl chloride has been compared with its other derivatives such as trityl alcohol, monomethoxytrityl chloride and dimethoxytrityl chloride on the three-component reaction of dimedone, 6-amino-1,3-dimethyluracil and 2, 4-dichlorobenzaldehyde.In this study, trityl chloride showed the best catalytic performance and in the presence of this catalyst, the shortest reaction time and the highest efficiency for the desired product were obtained.The presence of electron-donating groups on the trityl chloride ring stabilizes the carbocation and reduces its acid strength.The order of stability and acid strength of some carbocations of the trityl derivatives is specified in Fig. 2 44 .
Turn-over frequency (TOF) and Turn-over number (TON) of TrCl in comparison with other derivatives of TrCl such as monomethoxytrityl chloride and dimethoxytrityl chloride on the three-component reaction of dimedone, 6-amino-1,3-dimethyluracil and 2, 4-dichlorobenzaldehyde to show the superiority of TrCl as a catalyst for the synthesis of pyrimido [4,5-b]quinolines were given in Fig. 3.As it is shown in Fig. 3, indicates that TrCl is more efficient than the other catalysts in terms of TOF and TON for the preparation of this compounds.
After the optimization of the reaction condition, various aromatic aldehydes containing releasing or withdrawing groups and halogen on their ring, were reacted with dimedone and 6-amino-1,3-dimethyluracil to give pyrimido [4,5-b]quinolines derivatives.All products were obtained in high yields and short reaction times (Table 2).
To discuss the mechanism for the preparation of the pyrimido [4,5-b]quinolines compounds in the presence of carbocationic catalytic system under neutral media, as it is reported on previous literature [3][4][5][6][7][8][9][10][11][12] , firstly, aldehyde was converted to activated forms (I and II) by the reaction with trityl cation which in situ generated from trityl chloride.Then, dimedone in enole form reacted with activated forms of aldehyde (I and II) to give (III) which could be converted to (IV) and (V).Then, 6-amino-1,3-dimethyluracil was reacted with intermediate (V) to prepare (VI).Lastly, by the intramolecular nucleophilic attack in intermediate (VI), the expected product was synthesized after removing of one molecule of H 2 O. Trityl chloride was regenerated by the reaction of trytil alcohol with (VII) and removing of one molecule of H 2 O (Fig. 4).

Materials and methods
All chemicals were purchased from Merck or Fluka Chemical Companies.The known products 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 (250 MHz) and 13 C NMR (62.5 MHz) were recorded on a Bruker Avance DPX FT-NMR spectrometer (δ in ppm).Melting points were recorded on a Büchi B-545 apparatus in open capillary tubes.

Conclusions
In summary, TrCl was used as a homogeneous organocatalyst for the synthesis of pyrimido [4,5-b]quinolines derivatives in neutral condition.The products were prepared in high yields and short reaction times.The low cost of trityl choride, commercially availability of the catalyst, short reaction times and the avoidance of harsh acidic conditions are important advantages of this work.

Figure 2 .
Figure 2. The comparison of trityl carbocation derivatives with each other in term of acidity and stability.

Table 2 .
The preparation of pyrimido[4,5-b]quinolones in the presence of TrCl. a The isolated yield.