Catalyst-free Mannich reaction of hydroxyanthraquinone: facile access to emodin Mannich bases and anthraoxazines

A simple and efficient method for derivatization of hydroxyanthraquinone natural product emodin through catalyst-free Mannich reaction is described. The method allows the modification of emodin skeleton at 2-position. This new derivatization strategy to emodin provides a clear advantage over traditional approaches, due to its easy operation and efficiency that does not involve the protection and deprotection.