Abstract: With the increasing environmental concerns and the regulatory constraints faced in the chemical and pharmaceutical industries, development of environmentally benign organic reactions (also called Green Chemistry) has become a crucial and demanding research area in modem organic chemical research. Our contribution to this rapidly growing field involves use of certain lanthanide salts such as lanthanide(Ⅲ) trifluoromethanesulfonates as stable Lewis acids in protic solvent such as water or alcohols to catalyze a variety of organic transformations under nearly neutral conditions. This benign approach has the potential to replace many traditional reaction systems which are either conducted in strong acidic conditions or in environmentally less-friendly solvents. For example, aza Diels-Alder (DA) reaction in aqueous solution conveniently combines three reactant components (an aldehyde, an amine salt and a diene) in aqueous solution and generates nitrogen-containing heterocyclic products. However, this reaction is limited to either the smallest aldehyde or activated aldehydes such as glyoxylates. We have found that lanthanide(Ⅲ) triflates catalyze the aza DA reactions of a larger,inactivated aldehyde and an amine hydrochloride with a diene in aqueous solution[1]. Other examples of application of lanthanide triflates in protic solvents include Chicliibabin pyridine synthesis[2],condensation of indoles with aldehydes or ketones[3], and use of ion-exchange resin supported lanthanide catalysts[4].