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A magnetically separable core–shell CoFe2O4@SiO2‐SO3H nanocatalyst has been successfully exploited as a heterogeneous acid catalyst in the synthesis of diversely substituted biologically important spiro fused pyrrolo/indolo[1,2‐a]quinoxaline derivatives through the condensation of N‐(2‐aminophenyl)pyrroles/indoles and various cyclic conjugated 1,2‐diones in ethanol under ultrasonic irradiation. Room temperature synthesis, short reaction time, wide substrate scope, good to excellent yield of products and use of a magnetically separable and recyclable nanocatalyst make this method attractive and practicable. A magnetically separable CoFe2O4@SiO2‐SO3H nanocatalyst having core–shell morphology has been employed for the very first time in a benign strategy to synthesize a series of novel spiro fused quinoxaline derivatives under ultrasonic irradiation. Spiro fused quinoxaline derivatives derived from acenaphthoquinone, 9,10‐phenanthroquinone and [1,2]benzoquinone are reported. Room temperature synthesis, shorter reaction time, wide substrate scope, good to excellent yield and use of a magnetically separable and recyclable nanocatalyst make this synthetic protocol attractive. [ABSTRACT FROM AUTHOR]