In organic synthesis, when a reaction is to be carried out selectively at one functional group of a multifunctional compound, other reactive functional groups must be converted into inactive functional groups and protected temporarily. Therefore, appropriate use of protecting agents is very important to the synthetic strategy. Useful protecting agents must have properties as follows: - The protecting agents must selectively react with the desired functional group requiring protection. - The protecting groups must be introduced in high yields without any side reactions. - The protected functional groups should be stable under various reaction conditions. - The protecting groups must be chemoselectively deprotected under specific conditions without deprotection of other kinds of protecting groups. Especially in the synthesis of structurally complex compounds, designing the synthetic strategies such as selection of protecting groups and the efficient deprotection process are key steps to achieve. It is expected that a wide variety of protecting agents are available.