## 3 Results and Discussion  
Recently, Mal et al. [41] showed that the 3,4-dihydropyrimidines can be accessed in two steps under ball milling through a combination of a catalytic amount of potassium bromide (KBr), oxone and TEMPO (2,2,6,6-Tetramethyl-1-piperdihydroxy).  
In this paper, we present an efficient one-pot, three-component solvent-free and catalyst-free approach to synthesize 3,4-dihydropyrimidine derivatives by direct condensation of equimolar quantities of benzaldehyde derivatives, ethyl acetoacetate, and urea/thiourea in a simple planetary ball mill at 750 rpm without adding any solvent or catalyst (Scheme 1). The progress of the reactions was monitored every 10 min of the milling cycle through thin-layer chromatography (TLC).  
We examined different ball ratios (Table 1) to improve the efficiency of the ball milling approach for the Biginelli reaction. Equimolar quantities (0.02 mol) of benzaldehyde (**1a**), ethyl acetoacetate (**2**), and urea (**3a**) (with a total mass of 5.92 g) were introduced in the planetary ball milling, and several milling times and ball weights were tested [42].  
Accordingly, our investigation showed that the reaction does not change even after 12 h of milling when the ratio (ball weight/reagent weight) is equal to 1. The increasing value of the ball weight causes the conversion rate to increase to the optimal value for the ball weight-to-reagent weight ratio, which is equal to 8 (47.36 g of balls) (Table 1). The protocol also provides simple access to 3,4-dihydropyrimidine derivatives (**4a-j**) (Table 2).  
This approach exhibits the advantages of high yield, short reaction time (30 min), and easy workup. It is also environmentally benign. All of the synthesized products have been characterized through NMR (\({}^{1}\)H and \({}^{13}\)C), IR, and elemental analysis.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Entry \({}^{*}\)** & **BRR (Balls-to-Reagents Weight Ratio)** & **Time (min)** & **Conversion (\%)** \\ \hline
1 & 1:1 & 720 & 00 \\
2 & 2:1 & 720 & 00 \\
3 & 3:1 & 720 & 20 \\
4 & 4:1 & 720 & 40 \\
5 & 5:1 & 300 & 70 \\
6 & 6:1 & 360 & 100 \\
7 & 7:1 & 180 & 100 \\
8 & 8:1 & 30 & 100 \\ \hline \hline \end{tabular} \({}^{*}\) 0.02 mol of benzaldehyde, ethyl acetoacetate, and urea (5.92 g).  
\end{table}
Table 1: Milling parameters and the conversion rate for the synthesis of 3,4-dihydropyrimidines derivatives.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Entry** & **R** & **X** & **Product** & **Yield (\%)** & \multicolumn{2}{c}{**M.P. (\({}^{\circ}\)C)**} \\ \cline{5-6}  & & & & & **Found Reported References** \\ \hline
1 & C\({}_{6}\)H\({}_{5}\)– & O & **4a** & \textgreater{}98 & 20–202 (202–204) [40] \\
2 & 4-CH\({}_{3}\)–C\({}_{6}\)H\({}_{4}\)– & O & **4b** & \textgreater{}98 & 214–216 (215–217) [39] \\
3 & 4-Cl–C\({}_{6}\)H\({}_{4}\)– & O & **4c** & \textgreater{}98 & 208–210 (207–210) [40] \\
4 & 4-CH\({}_{3}\)O–C\({}_{6}\)H\({}_{4}\)– & O & **4d** & \textgreater{}98 & 204–207 (203–205) [40] \\
5 & 4-OH–C\({}_{6}\)H\({}_{4}\)– & O & **4e** & \textgreater{}98 & 221–223 (224–227) [40] \\
6 & 4-O\({}_{2}\)N–C\({}_{6}\)H\({}_{4}\)– & O & **4f** & \textgreater{}98 & 208–210 (206–208) [39] \\
7 & \({}_{6}\)H\({}_{5}\)– & S & **4g** & \textgreater{}98 & 209–211 (206–208) [39] \\
8 & 4-CH\({}_{3}\)–C\({}_{6}\)H\({}_{4}\)– & S & **4h** & \textgreater{}98 & 187–189 (186–188) [39] \\
9 & 4-Cl–C\({}_{6}\)H\({}_{4}\)– & S & **4i** & \textgreater{}98 & 182–184 (181–183) [39] \\
10 & 4-CH\({}_{3}\)O–C\({}_{6}\)H\({}_{4}\)– & S & **4j** & \textgreater{}98 & 155–157 (151–153) [39] \\
11 & 4-OH–C\({}_{6}\)H\({}_{4}\)– & S & **4k** & \textgreater{}98 & 206–208 (200–202) [40] \\
12 & 4-O\({}_{2}\)N–C\({}_{6}\)H\({}_{4}\)– & S & **4l** & \textgreater{}98 & 197–199 (191–193) [39] \\ \hline \hline \end{tabular}
\end{table}
Table 2: Synthesis of 3,4-dihydropyrimidine derivatives using planetary ball milling technique.