Asymmetric synthesis of chrysanthemic acid. An application of copper carbenoid reaction

Chiral N-salicyl-β-amino alcohol Schiff bases are important ligands widely employed in asymmetric catalysis. These are referred to as ‘tridentate salen ligands’ and are considered as a member of the well-known ‘privileged ligands’ group. The desire to understand the roles of weak non-covalent interactions in catalysis has greatly expanded the scope of structural studies on catalytic complexes. This review focuses on the structural and supramolecular features of the metal complexes derived from chiral N-salicyl-β-amino alcohol Schiff base ligands. Metal complexes of varying nuclearities, such as, mono-, di-, tri- and tetra-nuclear complexes, have been reported from this group of ligands. The geometries of the metal centers present in these complexes vary from tetrahedral, square-planar, square-pyramidal, octahedral to trigonal prismatic. The available structural data clearly indicate the coordination versatility of this group of ligands towards various metal ions. In addition, these complexes exhibit a variety of weak bonding interactions leading to the formation of supramolecular structures such as helices, 1-D chains, 3-D networks, etc. in solid state. The articles discussed here highlight the competence of chiral N-salicyl-β-amino alcohol Schiff base ligands to support various structural and supramolecular features in their complexes, but the role of these features in determining their catalytic efficiencies have not yet been studied in detail. We believe that the present review may inspire many such studies in future.

An efficient and selective method for cyclopropanation of styrene derivatives with ethyl diazoacetate (EDA) catalyzed by tin(IV) tetraphenylporphyrinato trifluoromethanesulfonate, [Sn^IV(TPP)(OTf)₂], and tin(IV)tetraphenylporphyrinato tetrafluoroborate, [Sn^IV(TPP)(BF₄)₂] is reported. These electron-deficient catalysts catalyzed the cyclopropanation of styrene derivatives in high yields and short reaction times under mild conditions. The reactions were highly selective and only trans-isomers were produced. Electron-rich styrenes were reacted faster than electron-poor ones. The catalysts were reused several times without loss of their catalytic activity and diastereoselectivity.

This chapter describes the use of catalytically generated transition-metal carbenoid complexes in the synthesis of enantioenriched products. The reactions discussed include cyclopropanations, C–H insertion reactions, reactions involving ylide formation, and aromatic addition/substitution. Although the principal focus in each area is on catalyst-controlled enantioselectivity, substrate-controlled reactions are also considered as appropriate. Within each transformation, the historical development of the stereoselective reactions is included, in addition to an overview of the application of the reactions to stereoselective synthesis of significant compounds, using examples from the recent literature.

Water mediates synthesis of novel hindered Schiff bases via transmination protocol. The feature of the procedure is that it allows both tuning of steric and electronic factors of the substituents and mono-imination of vicinal diamines, which potentially facilitates one-pot approach to unsymmetrical metal ligands.

An enantioselective total synthesis of (−)-strychnine was accomplished through the use of the highly practical catalytic asymmetric Michael reaction (0.1 mol% of (R)-ALB, greater than kilogram scale, without chromatography, 91% yield and >99% ee), and a domino cyclization that simultaneously constructed the B- and D- rings of strychnine (>77% yield). Newly-developed reaction conditions for thionium ion cyclization, NaBH₃CN reduction of the imine moiety in the presence of a Lewis acid to prevent the ring-opening reaction, and chemoselective reduction of the thioether (desulfurization) in the presence of exocyclic olefin were pivotal to complete the synthesis. The described chemistry paves the way for the synthesis of more advanced Strychnos alkaloids.

A remarkable increase in catalytic activity is found for the asymmetric cyclopropanation of 2,5-dimethyl-2,4-hexadiene with diazoacetate by use of the chiral copper Schiff-base complexes, which are derived from substituted salicylaldehydes, chiral aminoalcohols, and copper acetate monohydrate. Furthermore, a combination of a chiral copper Schiff-base with a Lewis acid showed an increase in yield (up to 90%) and in enantioselectivity (up to 90% ee) for the asymmetric cyclopropanation of the diene with t-butyl diazoacetate at 20 °C.