Bismuth triflate (0.1 mol %) is a highly efficient catalyst for the cyclization of citronellal 1, a reaction that yields a ratio of 80:20 of isopulegol 2 and neoisopulegol 3. This methodology has also been extended to the synthesis of substituted piperidines. The bismuth triflate catalyzed ene reaction of aldehyde 4 gives a 70:30 mixture of piperidines 5 and 6. The advantages of these methods include the use of a highly efficient catalyst that is relatively nontoxic, cheap and easy to handle.
Mohan, Ram; Anderson, Erin; Ernat, Justin; Nguyen, Mai; and Palma, Ann, "Environment Friendly Organic Synthesis Using Bismuth Compounds. An Efficient Method for Carbonyl-ene Reactions Catalyzed by Bismuth Triflate" (2005). Scholarship. 7.