Intramolecular hydrogen bond activation: Thiourea-organocatalyzed enantioselective 1,3-dipolar cycloaddition of salicylaldehyde-derived azomethine ylides with nitroalkenes

An organocatalytic strategy for the synthesis of tetrasubstituted pyrrolidines with monoactivated azomethine ylides in high enantiomeric excess and excellent exo/endo selectivity is presented. The key to success is the intramolecular activation via hydrogen bonding through an o-hydroxy group, which allows the dipolar cycloaddition to take place in the presence of azomethine ylides bearing only one activating group. The intramolecular hydrogen bond in the azomethine ylide and the intermolecular hydrogen bond with the catalyst have been demonstrated by DFT calculations and mechanistic proofs to be crucial for the reaction to proceedThe Spanish Government (CTQ2015-64561-R, CTQ2016- 76061-P) and the European Research Council (ERC-CG, contract number 647550) are acknowledged. We acknowledge the generous allocation of computing time at the CCC (UAM). S.D.-T. gratefully acknowledges the “Ramón y Cajal” program (RYC-2010-07019). Financial support from the Spanish Ministry of Economy and Competitiveness, through the “Maria de Maeztu” Program of Excellence in R&D (MDM- 2014-0377