Simple and Efficient Synthesis of Novel bis-Betti Bases via a One-Pot Pseudo-Five-Component Reaction

Background: A one-pot pseudo-five-component green synthesis of new bis-Betti bases is described. The reported bis-Betti bases are obtained via the condensation of one equivalent of 2,3- dihydroxynaphthalene, two equivalents of aryl aldehydes, and two equivalents of heteroaryl amines in the presence of formic acid catalyst under solvent-free conditions. The broad substrate scope, operational simplicity, clean reaction, high yield, easy work up and shortened reaction times make this procedure a practical and environmentally friendly method for the synthesis of new bis-(1-aminomethyl-2- hydroxy)naphthalenes. Methods: As a model reaction, one equivalent of 2,3-dihydroxynaphthalene, two equivalents of benzaldehyde and two equivalents of 2-aminopyrimidine were heated at a temperature of 60-110oC in the presence of formic acid as catalyst (10% mol) without the use of solvent. We were pleased to find that the reaction proceeded smoothly and almost complete conversion of reactants was observed at 80oC to afford the corresponding product in 83% yield within 80 min. We also varied the amount of formic acid catalyst and the results revealed that 10% mol gives excellent yield of the product in a short duration. Results: After optimizing the conditions, the generality of this method was examined by the reaction of different substituted aldehydes with various heteroaryl amines and 2,3-dihydroxynaphthalene for the synthesis of new bis-Betti bases via the one-pot pseudo-five-component reaction under solvent-free conditions. It was found that various aromatic aldehydes containing electron-donating or electronwithdrawing functional groups at different positions of the benzene ring show a difference in the reaction time but the yields of products were almost the same. Identification of the structures of the products was achieved by spectral analysis. The formation of the products in the present reaction is expected to involve the initial formation of the imine intermediate from an aldehyde and amine in the presence of formic acid catalyst. Then, in the second step, two equivalents of imine reacted with one equivalent of 2,3-dihydroxynaphthalene affording the corresponding bis-(1-aminomethyl-2-hydroxy)naphthalene. Conclusion: We have demonstrated a simple and efficient method for the synthesis of new bis-Betti bases. The attractive features of this protocol are: use of inexpensive catalyst, mild and environmentally benign reaction conditions, easy isolation of the product and almost quantitative conversion into the corresponding products.