Solvent-free Condensation of 2-Naphthol with Aromatic Aldehydes and Acetamide/Urea to 1-Amidoalkyl-2-naphthols

In recent years, the use of solid-supported reagents has attained considerable attention both in industrial and academic research due to their unique properties such as enhanced reactivity as well as selectivity, efficiency, facile work-up, and eco-friendly reaction conditions.1–18 Silica-supported phosphorus-containing reagents are a class of interesting solid-supported reagents which are inexpensive, safe and environmentally benign, and have been used in various organic transformations.4–17 Nevertheless, most of the existing processes in organic synthesis involve toxic and volatile organic solvents as reaction media, and these are environmentally unacceptable from a green chemistry view point. One of the most effective techniques to solve this problem is solvent-free conditions which leads to remarkable decreases in reaction times, increased yields, easier workup,3 and prevents waste, hazards, and toxicity due to solvents.18 Consequently, it is noteworthy that the combination of safe reagents with the use of solvent-free reaction conditions represents a suitable way toward the so-called “ideal synthesis”.1–18 1-Amidoalkyl-2-naphthol derivatives are of importance as they can be easily hydrolyzed to 1-aminoalkyl-2-naphthols. 1-Aminoalkyl-2-naphthols have been frequently applied as hypotensive and bradycardiac agents.19–20 1-Amidoalkyl-2-naphthols can also be converted to 1,3-oxazine derivatives.21 1,3-Oxazines have demonstrated a variety of biological activities including antibiotic,22 antitumor,23 antipsychotic,24 and antianginal.25 One-pot multi-component condensations of 2-naphthol with aromatic aldehydes and amide derivatives or acetonitrile has been used as a practical synthetic route toward 1-amidoalkyl-2-naphthols.26–38 Several reagents including Ce(SO4)2, K5CoW12O40. 3H2O, montmorillonite K-10,28 iodine,29 FeCl3/SiO2, Fe(HSO4)3, sulfamic acid/ ultrasound,32 p-toluenesulfonic acid,33 N-(4-sulfonic acid)butyl triethylammonium bisulfate,34 H3PW12O40, Sr(OTf)2, trityl chloride,37 and 4-(1-imidazolium)butanesulfonate38 have been used for this transformation. However, some of these reported methods suffer from one or more of the following drawbacks: (i) low product yield, (ii)

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