A simple and efficient method for the synthesis of functionalized spirochromenes is explained using protic guanidinium ionic liquid as a catalyst under solvent-free conditions at room temperature. This procedure is simple, clean, and excellent yields are obtained in short reaction times.
Baghbanian, S. M., Tajbakhsh, M., & Farhang, M. (2014). Protic guanidinium ionic liquid as a green and highly efficient catalyst for the synthesis of functionalized spirochromenes under solvent-free conditions. Comptes Rendus Chimie, 17(12), 1160–1164. https://doi.org/10.1016/j.crci.2013.12.005