Synthesis and Fungicidal Activity of Novel Benzothiophene-Fused Pyrido[1,2-a]pyrimidine Derivatives

A series of novel benzothieno[3', 2': 2,3]pyrido[4,5-d]pyrido[1,2-a]pyrimidines are prepared via Pictet-Spengler reaction of 2-(3-aminobenzothiophene-2-yl)-4H-pyrido[1,2-a]pyrimidin-4-one using sulfamic acid as a catalyst, which in turn were obtained from the Thorpe-Ziegler isomerization of 2-(chloromethyl)-4H-pyrido[1,2-a]pyrimidin-4-one with 2-mercaptobenzonitrile. The structures of the products were characterized by FT-IR, H-1 NMR, C-13 NMR spectra and elemental analysis. The fungicidal activities of the prepared compounds were also preliminarily evaluated. For example, 5b exhibited more than 96% inhibition rate to Botrytis cinerea and Gibberella zeae at 50 mg/L, 5f exhibited 98% inhibition rate to Sclerotonia sclerotiorum at 50 mg/L, and 5g, 5i exhibited more than 93% inhibition rate to Alternaria alternata at 50 mg/L.