Enantioselective Synthesis of exo-4-Nitroprolinates from Nitro­alkenes and Azomethine Ylides Catalyzed by Chiral Phosphor­amidite·Silver(I) or Copper(II) Complexes

Chiral complexes formed by privileged phosphoramidites derived from chiral binol and optically pure Davies’ amines, and copper(II) triflate, silver(I) triflate or silver(I) benzoate are excellent catalysts for the general 1,3-dipolar cycloaddition between nitroalkenes and azomethine ylides generated from α-amino acid derived imino esters. These three methods can be conducted at room temperature to afford the exo-cycloadducts (4,5-trans-2,5-cis-4-nitroprolinates) with high diastereoselectivity and high enantioselectivity. In general, the three procedures are complementary but silver catalysts are more versatile and less sensitive to sterically congested starting materials.

This work has been supported by the Spanish Ministerio de Ciencia e Innovación (MICINN) (Consolider INGENIO 2010 CSD2007-00006, CTQ2010-20387) and Ministerio de Economía y Competitvidad (MINECO) (CTQ2013-43446P, CTQ2014-51912-REDC), FEDER, Generalitat Valenciana (PROMETEO/2009/039 and 2014/017), and by the University of Alicante. L.M.C. thanks the MICINN for a FPI fellowship. Financial support was also provided by the MICINN and FEDER (projects CTQ2010-16959/BQU), the University of the Basque Country (UPV/EHU, UFI11/22QOSYC) and the Basque Government (GV/EJ, grant IT-324-07).