Cellulose Sulfuric Acid as an Eco-Friendly Catalyst for Novel Synthesis of Pyrido [ 2 , 3-d ] [ 1 , 2 , 4 ] triazolo [ 4 , 3-a ] pyrimidin-5-ones

A novel synthesis of a series of pyrido[2,3-d][1,2,4]triazolo[4,3-a]pyrimidin-5-ones has been developed from reactions of 1-(5-methyl-1-phenyl-1H-pyrazol-4-yl)-3-arylprop-2-en-1-ones and 7-amino-1,3-disubstituted[1,2,4]triazolo[4,3-a]pyrimidin-5(1H)-ones in dioxane under thermal conditions, using cellulose sulfuric acid as an eco-friendly acid catalyst. The reaction mechanism was proposed and the structures of the newly synthesized compounds were established on the basis of spectral data (mass spectrometry, infrared, H and C nuclear magnetic resonance) and elemental analyses.

Recently, the direction of science and technology has been shifting more towards eco-friendly, natural product resources and reusable catalysts. 7][10][11] On the basis of these precedents, we attempted to report a new and efficient method for the synthesis of pyrido [2,3-d] [1,2,4]triazolo [4,3-a]pyrimidines using cellulose sulfuric acid as an eco-friendly biopolymeric solid supported acid catalyst.
We have found that, in non-polar aprotic solvents (benzene, toluene, and xylene) the product yields are low (runs 1-3).On the other hand, by using polar aprotic solvent (acetonitrile, DMF, and dioxane) the product yields improve (runs 4-6).Thus, dioxane was found to be the most suitable solvent for the formation of product 6a.The relationship between the yields of annelated product 6a and the amount of acid-catalyst was investigated.As shown in Table 1, the yield of 6a increased with an increase of the amount of catalyst (0.01 g up to 0.05 g) (runs 7-11) and then the yield decreased (run 12).
The efficiency of the cellulose sulfuric acid reagent compared to various acidic catalysts was also examined (Table 2).In this study, it was found that cellulose sulfuric acid is more efficient and superior catalyst (run 1) over other acidic catalysts (runs 2-4) with respect to the percent yield of the desired product.It was also observed that the yield of the product was only 10% in the absence of the cellulose sulfuric acid reagent (run 5).

Scheme 1 .
Scheme 1. Proposed mechanism for the synthesis of pyridotriazolopyrimidinone 6a.

Table 1 .
Effect of solvent and amount of acid-catalyst on the product yields

Table 2 .
Effect of acid-catalysts on the product yields