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[DCS seminar] Carmen Nájera ,Miguel Yus (Universidad de Alicante,Spain,,) -- Chiral N-Sulfinyl Imines: New Discoveries in Asymmetric Synthesis,Coinage Metal Complexes as Chiral Catalysts for 1,3-Dipolar Cycloadditions |
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The use of different chiral catalysts derived from binap and phosphoramidites and silver, gold(I) and copper(II) salts for the enantioselective synthesis of highly substituted prolines by 1,3-dipolar cycloadditons (1,3-DC) of azomethine ylides, derived from imino esteres with dipolarophiles is presented. The 1,3-DC using bidentate binap complexes give good enantioselectivities for azomethine ylides when silver salts monomeric complexes and dimeric gold trifluoroacetates are used yielding endo-cycloadducts. In the case of monodentate phosphoramidites, metal complexes derived from silver salts have been used for the general 1,3-DC of different imino esters and dipolarophiles to afford endo-cycloadducts. In the case of using nitroalkenes as dipolarophiles phosphoramidites and either copper(II) triflate or silver salts are the most appropriate catalysts affording exo-cycloadducts. In addition, computational studies have also been carried out in order to explain the high enantioselection exhibited by these chiral complexes. This methodology has been applied to the synthesis of hepatitis C virus inhibitors blocking the viral RNA-dependent RNA-polymerase and for the preparation of exo-4-nitroprolinates, which are excellent chiral organocatalysts for the aldol reaction.
References
For a recent account, see: Nájera, C. and Sansano, J. M. J. Organomet. Chem. 2014, 771, 78.
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Chiral N-sulfinyl imines, especially the corresponding N-tert-butyl substituted derivatives 1, are interesting starting materials in asymmetric synthesis because (a) they are easily accessible in both enantiomerically pure form, (b) the sulfinyl group activates the imine moiety towards nucleophilic substitution so, in the reaction with different nucleophiles an asymmetric induction takes place giving an diastereoenriched product, which can be easily separated into the corresponding pure diastereomers, and (c) the deprotection of the amino group, after the addition of the nucleophile can be easily achieved by simple treatment with hydrochloric acid. In this lecture, the reactivity and synthetic applications of these materials in the (1) ruthenium-catalyzed hydrogen transfer, (2) addition of alkyl zincates, (3) indium-promoted allylation, and (4) multicomponent reactions involving cycloalkenones and dialkyl-zinc reagents will be presented (Scheme 1). Especial attention is paid to the synthetic applications of the mentioned processes, mainly for the preparation of natural or unnatural alkaloids and amino acids. |