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Photoremovable chiral auxiliary1 Viju B. Kammath, Peter Šebej, Tomáš Slanina, Zdeněk Kříž, Petr Klán Department of Chemistry, Faculty of Science, Masaryk University, Kamenice 5/A8, 625 00, Brno, Czech Republic Chiral auxiliaries are optically active groups, temporarily covalently attached to the prochiral substrate, that induce a selective formation of one of the enantiomeric products during a stereoselective reaction. When the reaction stereochemistry is achieved, the auxiliary is removed. The benzoin group has already been used as a photoremovable protecting group2 for various functionalities, such as carboxylates,3 hydroxy compounds,6 and phosphates.4,5 Here we introduce a concept of photoremovable chiral auxiliary as a novel strategy in the field of asymmetric organic synthesis, which offers an auxiliary that can be removed upon irradiation. Enantiopure benzoin derivatives were utilized as such auxiliaries: A dienophile linked to this group underwent a Diels-Alder reaction with a diene in the presence of various catalysts and the auxiliary was removed photochemically. The released acids were obtained with very high enantiomeric excess in some cases. O O * * O O * O R various conditions * * CO2Me * endoa * exoa 2. esterifcation * * O * * exob O R= H, OMe, OH, CO2Me * CO2Me * 1. h, MeOH 2 eq. O R * O R CO2Me * * * CO2Me endob (ee exo up to 96%) 1 Balachandran Kammath V., Šebej P., Slanina T., Køíž Z., Klán P., Phoremovable Chiral Auxiliary, Org. Lett. (submitted). 2 Greene T.W., Puts P.G.M.: Protective groups in organic synthesis - 3rd ed., John Wiley & Sons, New York, USA, 1999 3 Sheehan, J. C.; Wilson, R. M. Journal of the American Chemical Society 1964, 86, 5277. 4 Givens, R. S.; Athey, P. S.; Kueper, L. W.; Matuszewski, B.; Xue, J. Y. Journal of the American Chemical Society 1992, 114, 8708-8710. 5 Pirrung, M. C.; Shuey, S. W. Journal of Organic Chemistry 1994, 59, 38903897. 6 Pirrung, M. C.; Bradley, J. C. Journal of Organic Chemistry 1995, 60, 11161117.