Synthesis and Herbicidal Activity of Muti-Substituted Pyrido[4,3-d]pyrimidin-4-ones

A series of novel muti‐substituted pyrido[4,3‐d]pyrimidin‐4‐one derivatives 5a, 5b, 5c, 5d, 5e, 5f, 5g, 5h, 5i, 5j, 5k, 5l were designed and synthesized by the muti‐step reaction. N,S‐acetal 1 reacted with acetyl acetamide in the presence of zinc nitrate to obtain muti‐substituted pyridine 2, which reacted with triethyl orthoformate to give 8‐cyano‐5‐methyl‐7‐methylthio‐pyrido[4,3‐d]pyrimidin‐4‐one 3; the target compounds 5 were obtained in good yields by the oxidation of 3 with H2O2 in a catalytic amount of sodium tungstate then by the substitution with various substituted phenols. Their structures were confirmed by IR, 1H NMR, EI‐MS, and elemental analyses. The preliminary bioassay indicated that some of them displayed moderate herbicidal activity against dicotyledonous weed Brassica campestris L. at the concentration of 100 mg/L. For example, compounds 5a, 5f, and 5g possessed 76.0%, 62.7%, and 60.2% inhibition against B. campestris at the concentration of 100 mg/L. Moreover, 5a exhibited 58.2% inhibition against B. campestris at the concentration of 10 mg/L.