Asymmetric Aerobic Oxidation of α-Hydroxy Acid Derivatives by C₄-Symmetric, Vanadate-Centered, Tetrakisvanadyl(V) Clusters Derived from N-Salicylidene-α-aminocarboxylates

Abstract

A series of chiral vanadyl(V) methoxides bearing 3-t-butyl-5-substituted N-salicylene-L-valinate and L-t-leucinate as chiral auxiliaries has been prepared. In all cases except the 3,5-di-t-butyl analogue, they exist as monomers both in solution and in the single crystal state. In the case of the 3,5-di-t-butyl analogue, the architectural nature of the vanadyl(V) complex highly depends on the base used during the complex formation event. A pentanuclear C4-symmetric complex was formed when potassium salts were employed instead of the corresponding sodium salts. A central vanadate(V) unit serves to grip four identical chiral monomeric vanadyl(V) units together, by which a potassium ion sits on top of the four flanking units through carbonyl coordinations and serves to hold the whole cluster by cooperation with the central vanadate(V) unit. In comparison with the corresponding monomeric vanadyl(V) methoxide complex, the cluster complex was utilized to facilitate the asymmetric aerobic oxidations of various racemic alpha-hydroxyesters, -amides, and -thioesters with excellent selectivity factors (krel 40 to >500).

Catalytic Non‐Enzymatic Kinetic Resolution

“…This new protocol was particularly efficient when applied to α‐aryl‐α‐hydroxy esters and essentially all kinds of α‐hydroxy amides. Excellent selectivity factors of up to 500 were also obtained by these authors for the KR of a range of other α‐hydroxy acid derivatives including α‐hydroxy thioesters by using catalyst 35 82. It must be concluded from these studies that the best enantioselectivities were obtained through judicious selections of the C‐3 and C‐5 substituents in the N ‐salicylidene template of the catalysts employed.…”

Oxidative Thioesterification of Alkenes Mediated by 1,3‐Dibromo‐5,5‐dimethylhydantoin and DMSO for the Synthesis of α‐Ketothioesters

“…Traditionally, α‐ketothioesters were obtained by means of the hydrolysis of trimethyl α‐oxo trithioorthoesters, the aerobic oxidation of α‐hydroxythioesters, and the hydrolysis of β‐keto α, α‐dichloro sulfides. [2a], These classic procedures mentioned above still suffer from some apparent drawbacks, such as the use of the flammable reagent ( tert ‐butyllithium) and the demand of expensive metal reagents. The reactions involving metal reagents usually are heterogeneous and produce a large amount of residue, which may restrict their application in industry.…”

An Efficient and Mild Method for the Synthesis and Hydrazinolysis of N‐Glyoxylamino Acid Esters

“…The glyoxylamides are compounds of widespread interest in organic chemistry and present in many biologically active compounds [1][2][3][4][5][6][7][8]. In parallel with the application of the glyoxylamide in medicinal chemistry, numerous synthetic methods have been described in the literature [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. Other authors have also reported that synthesis of the glyoxylamide fragment could be achieved by the ring opening of Nacetylisatin 1 by attacking an amine at C2-carbonyl group of N-acetylisatin (Scheme 1) [27][28][29][30][31][32][33][34][35].…”