The present invention relates an improved process for the preparation of piperonal (2), which is useful as an intermediate for the preparation of Tadalafil of formula (1).

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The present invention provides a curable composition capable of securing delayed curing time for carrying out a work such as laminating or tightening after carrying out irradiation with an energy ray or heating and capable of exhibiting delayed curing property after that, regardless of whether an adherend is transparent or not. The present invention is a curable composition comprising components (A) to (C): a component (A): a compound having a (meth)acryloyl group in a molecule; a component (B): saccharin; and a component (C): at least one of a photocationic catalyst as a component (C-1) and a thermal cationic catalyst as a component (C-2).