The successful use of molecular iodine as a catalyst in the intermolecular imino-Diels-Alder reaction is described. A onepot synthesis of pyrano [3,2-c]quinolines was achieved by threecomponent coupling of aldehydes and anilines with 2,3-dihydropyran catalyzed by iodine. The reactions could be carried out smoothly at room temperature within three to six hours to offer the target products in good yields. We also investigated solvent effect, substituent effect, and the amount of iodine required.Tetrahydroquinoline derivatives are a significant class of natural products and exhibit biological activities in various fields, 1 such as psychotropic, 2 antiallergenic, 3 antiinflammatory 4 and estrogenic activity, 5 besides, pyranoquinoline derivatives are also used as potent pharmaceuticals. 6 It has been established that imino-Diels-Alder reactions of N-arylimines (the heterodienes) with electron-rich alkenes are one of the most powerful routes to build the 1,2,3,4-tetrahydroquinoline scaffold. 7 Many Lewis acids such as InCl 3 8 , Yb(OTf) 3 , 9 Sc(OTf) 3 , 10 BF 3 ·OEt 2 , 11 GdCl 3 , 12 LiClO 4 , 13 LiBF 4 , 14 TFA 15 etc were found to catalyze this reaction. Montmorillonite 16 and fluorinated alcohols 17 were also reported to be efficient for the formation of tetrahydroquinolines. However, all these methods required a lengthy procedure to prepare the starting materials. Moreover, most imines are hygroscopic, unstable at high temperature, and difficult to purify by distillation or column chromatography. Therefore, it is necessary to develop a simple and effective approach to tetrahydroquinolines under mild conditions. The use of a convergent three-component reaction from aldehydes, anilines, and alkenes in which the heterocycles are assembled in a one-pot operation is an efficient method with particularly valuable application in the combinatorial synthesis of tetrahydroquinolines. Unfortunately, for many Lewis acids in a one-pot procedure, the presence of amines and water from the formation of imines can decompose or deactivate them. Even if the desired reactions proceed, more than stoichiometric amounts of the catalysts are needed, since they are trapped by nitrogen. 18 Although the one-pot operation using lanthanide triflate 19 as the catalyst was reported, its strong acidity and high cost prohibited its use as a versatile reagent in the synthesis of tetrahydroquinolines.Recently, molecular iodine has received considerable attention as an inexpensive and readily available catalyst for various organic transformations. 20 Due to its moderate Lewis acidity reactions catalyzed by iodine effectively take place in neutral media under very mild conditions. Moreover, its water-tolerance and weak ability to accept electrons make it an excellent candidate for reactions like the one-pot synthesis of tetrahydroquinolines. To the best of our knowledge, there is no report on the use of iodine as a catalyst for this type of reaction. Herein, we describe our first example of the preparation for pyrano [3,2-c]quinolines by an iodine...
