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Enantioselective Bromoaminocyclization Reactions Promoted by Organocatalysts Derived from Cinchona Alkaloids
Jullyane Emi Matsushima
Universidade Federal de São Paulo
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Crie um tópicoIn 2016 Pan and coworkers1 reported the development of a methodology for enantioselective bromoaminocyclization reactions of several carbamates promoted by Sc(OTf)3 catalysts. The present work aims the development of an organocatalyzed alternative to the reactions reported by Pan, considering the use of metals and the high cost of the reagents used. The optimization of the bromoaminocyclization reactions used carbamate 1 as a model and the reactions were carried out on a 0.1 mmol scale. A series of organocatalysts derived from Cinchona alkaloids had their efficiency evaluated along with the variation of temperature, solvent, concentration, and bromine source. Among the organocatalysts that were evaluated, organocatalyst I was the most efficient, in the presence of PhMe:CHCl3 (1:1) as solvent and NBS (1.2 eq.) as bromine source at -50 ºC, providing exclusively the six-membered cyclic carbamate 1a with an enantiomeric excess of 60% (Scheme 1a). After the optimization step a series of structurally more complex allyl carbamates (Scheme 1b) were synthesized to evaluate the methodology. At the present moment the reaction scope is under way, and preliminary results indicated that the substituent affects the reaction regioselectivity, as carbamates 2, 3 and 12 provided the corresponding five membered heterocycle 2a, 3a and 12a in 70%, 37% and 35% respectively.
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