An Efficient Synthesis of 5-Alkoxycarbonyl-4-aryl-3,4-dihydropyrimidin-2(1H)-ones

A mild and efficient catalytic method for synthesis of 5-alkoxycarbonyl-4-aryl-3,4- dihydropyrimidin-2(1H)-ones using KSF montmorillonite as catalyst is described.


Introduction
4-Aryl-3,4-dihydropyrimidin-2(1H)-ones 4 ("Biginelli compounds", DHPMs) have been reported to possess diverse biological activities such as antiviral, antibacterial, antihypertensive and anti-tumor effects [1][2][3].More recently, DHPMs have emerged as the integral backbones of several calcium channel blockers [4].Furthermore, several marine alkaloids with interesting biological activities containing the dihydropyrimidine-5-carboxylate core have been isolated [5].Consequently a great need still exists for versatile and simple processes whereby compounds 4 may be formed under very mild conditions.However, Biginelli's initial one-pot method of refluxing a -keto ester 1, aryl aldehyde 2 and urea 3 with a catalytic amount of acid frequently afforded low (20-60%) yields of the desired target molecules 4 [6].This has led to the recent disclosure of several improved reaction protocols for the synthesis of DHPMs, either by modification of the classical one-pot Biginelli approach itself [7][8][9][10], or by the development of novel, but more complex multistep strategies [11,12].In addition, several combinatorial approaches towards DHPMs 4 have been reported [13,14].
Montomorillonite clays have been widely used in organic synthesis due to their ready availability, ease of set up and of work up, mild experimental condition and high yield and selectivity.Recently, Bigi [15] reported that the Biginelli reaction could be performed under solventless conditions using KSF montmorillonite, although the procedures described suffer limitations such as long reaction times and moderate yields.Our group has exploited the catalytic potential of montomorillonite for various synthetic organic transformations [16].While optimizing the reaction conditions of the Biginelli reaction, we found that treatment of -keto ester, aryl aldehyde and urea with montmorillonite in methanol afforded DHPMs in good to excellent yields (Table 1).

Results and Discussion
Our results show that it is necessary to control the molar ratio of the reactant.When the molar ratio of -keto ester (1): aryl aldehyde (2): urea (3) is 1:1:1.2,our method produced higher yields.The same reaction when run in methanol gave better yields than in tetrahydrofuran.A variety of montomorillonites, including montomorillonite K 10 and montomorillonite KSF showed catalytic activity for Biginelli reaction, although montomorillonite KSF reagent proved superior to montomorillonite K 10. c Method C: Hu improved conditions (BF 3 •OEt 2 /CuCl in THF, reflux 18h) [7] d Method D: Bigi improved conditions (montmorillonite KSF without solvent at 130Û& Our approach not only preserved the simplicity of Biginelli's one-pot reaction, but also consistently provided excellent yields of dihydropyrimidinones 4, regardless of the keto ester or aldehyde substituents tested.Montmorillonite KSF has many advantages over catalysts like BF 3 •OEt 2 /CuCl such as ease of handling, low cost and elimination of metal wastes (use of an environmentally-friendly catalyst).When compared to Bigi's conditions (Table 1), our method consistently produced higher yields and required shorter reaction times.Detailed work on the mechanism of the transformation is still in progress.

General
Melting points were determined using a XT 4A melting point apparatus and were uncorrected.The 1 H-and 13 C-NMR spectra were acquired on a Bruker Avance 300 spectrometer in DMSO with TMS as internal standard.IR spectra were recorded on a Nicolet Axatar IR 360 spectrometer.Elemental analysis data were taken on a Perkin-Elmer 240C elemental analytical instrument.

Syntheses
A mixture of -keto ester (20mmol), aryl aldehyde (20mmol), urea (24mmol) and montmorillonite KSF (0.6g) was refluxed in dry methanol (15mL), The completion of the reaction was monitored vide tlc.Montomorillonite KSF filtered off and methanol was removed under reduced pressure.The product was crystallised from ethanol.The results were summarized in Table 1.All the products except entry 1 are known compounds, which were satisfactorily characterised by melting point, 1 H-NMR, 13 C-NMR and IR spectra.

Table 1 .
Experimental results and physical data of dihydropyrimidinones 4.