Efficient one-pot synthesis of pyrido[2,3-d]pyrimidines ...

The powder X-ray diffraction pattern of nanocrystalline MgO is shown in Fig. 17. All the diffraction peaks matched well with the face centered cubic structure of periclase MgO (JCPDS No. 87-). The major peaks at 2θ values of 37.1º, 43.0º, 62.4º, 74.8º and 78.6º can be indexed to the lattice planes of (111), (200), (220), (311) and (222) respectively.

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X-ray powder diffraction pattern of nanocrystalline MgO

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The surface and textural morphology of the product was studied by SEM analysis. A closer view reveals that most of the nanocrystals have uniform diameter (Fig. 18).

SEM micrograph of nanocrystalline MgO

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The particle size was also examined using TEM. Figure 19 displays TEM micrographs of nanocrystalline MgO, revealing that the particle size is approximately 50 nm.

TEM micrograph of nanocrystalline MgO

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From BET analysis, the specific surface area of nanocrystalline MgO sample was found to be 63.32 m2/g. A variety of pyrido[2,3-d]pyrimidines was prepared from arylaldehydes, 6-aminouracil, 6-amino-2-thiouracil or 6-amino-1,3-dimethyluracil and malononitrile in the presence of nano-MgO in water at 80 °C in excellent yields (Table 1, entries 1&#;16). It is worth mentioning that the corresponding pyrido[2,3-d]pyrimidines was isolated by crystallization from the crude filtrate. The results have been shown in Table 1. The reactions worked well with electron-donating and electron-withdrawing aldehydes. This three-component reaction proceeds via dual activation of substrates by nanocrystalline MgO which have a number of anionic oxidic Lewis basic O2&#;and Mg2+ as Lewis acid site [21]. The Lewis base moiety of the catalyst activates the methylene group of malononitrile. The carbonyl oxygen of aldehyde coordinates with the Lewis acid moiety increasing the electrophilicity of the carbonyl carbon and thereby making it possible to carry out the reaction in short time. In a plausible mechanism, it is assumed that the reaction may proceed initially through the Knoevenagel condensation between arylaldehyde and malononitrile to form intermediate I. Next, Michael addition of 6-aminouracils to intermediate I affords II. Intermediate II converts to III after tautomerization (nanocrystalline MgO can also act as a mild base for the deprotonation of an acidic proton of Intermediate II). Then, Intermediate III converts to IV via cyclization. Next, intermediate IV converts to V after tautomerization. Finally, the desired product VI is obtained after aromatization from V (Scheme 2).

A plausible mechanism for the pyrido[2,3-d]pyrimidines synthesis

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It is notable that the bulk MgO shows lower catalytic activity than nanocrystalline MgO in this reaction and the products were obtained with lower yields and longer time. The high efficiency of the nanoparticle oxides is caused not only by their high surface area but also by the high concentration of low-coordinated sites and structural defects on their surface [22].

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It is noteworthy to highlight that the nanocrystalline MgO could be recovered and reused without a significant loss of activity as illustrated in Table 2. After completion of the reaction, the product was extracted with ethyl acetate from the reaction mixture, and nanocrystalline MgO catalyst was separated out by centrifugation and the recovered catalyst was washed with ethanol followed by drying in an oven at 100 °C and reused as such for the subsequent reactions with fresh batch of reactants up to 5th run with only a slight loss its activity.

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