An efficient, one-pot synthesis of benzo[b]pyrans catalyzed by vitamin B₁ functionalized Fe₂O₃@SiO₂ NPs under ultrasonic conditions is reported herein. The nano-organocatalyst was synthesized and characterized by FT-IR, TEM, EDS, powder XRD, etc. Several benzo[b]pyran derivatives were prepared with satisfactory yields and the nanocatalyst was magnetically retrieved and recycled. The salient features of this protocol include convenient work-up procedure, mild reaction conditions, easy catalyst recovery, shorter reaction time and use of green solvents. Ultrasound energy has been meticulously exploited for the synthesis of a nano-organocatalyst as well as for the preparation of several benzo[b]pyran derivatives. The biological activity of the synthesized benzo[b]pyrans were investigated and one of the derivatives was found to demonstrate profound activity in suppressing the side-effects of chlorpyrifos, an insecticide on the morphology of the external gills of the Duttaphrynus melanostictus tadpoles.