Preparation of activated donors

The activated donors in transglycosylation reactions fulfil three roles [6]  

Galactosyl fluoride donors are reported to be more soluble in aqueous media than the corresponding nitrophenyl compound [10]. 3-D-Galactopyranosyl fluoride can be prepared by reaction of acetylated galactopyranosyl bromide with silver fluoride [ 10] or by reaction of penta-G-acetyl-p-D-galactopyranoside with hydrogen fluoride in pyridine [11]. 

Disaccharides, such as lactose [12, 13], have also been used as donors in which the reducing end sugar acts as the leaving group. 

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Allyloxycarbonyl)benzyl (ACB) Glycosides New Latent Donor for the Preparation of Active 2-Azido-2-deoxy BC Glycosyl Donors... 

The use of liquid ammonia solutions of Eu or Yb enables the preparation of active catalysts for transfer hydrogenation in which ammonia is a preferred hydrogen donor. Catalytic transfer hydrogenation using hydrogen donors shows some interesting features which are of potential synthetic importance and use (Johnstone and Wilby 1985, Hartner 1980, Brieger and Nestrick 1974, Furst et al. 1965). 

Esters such as ben2oates and phthalates are also used in the preparation of high activity catalysts for olefin polymeri2ation. They appear to function as electron donors in the catalyst complex, and play a significant role in catalyst performance (115). 

Diastereoselective preparation of a-alkyl-a-amino acids is also possible using chiral Schiff base nickel(II) complexes of a-amino acids as Michael donors. The synthetic route to glutamic acid derivatives consists of the addition of the nickel(II) complex of the imine derived from (.S )-,V-[2-(phenylcarbonyl)phenyl]-l-benzyl-2-pyrrolidinecarboxamide and glycine to various activated olefins, i.e., 2-propenal, 3-phenyl-2-propenal and a,(f-unsaturated esters93- A... 

Several methods for asymmetric C —C bond formation have been developed based on the 1,4-addition of chiral nonracemic azaenolates derived from optically active imines or enamines. These methods are closely related to the Enders and Schollkopf procedures. A notable advantage of all these methods is the ready removal of the auxiliary group. Two types of auxiliaries were generally used to prepare the Michael donor chiral ketones, such as camphor or 2-hydroxy-3-pinanone chiral amines, in particular 1-phenylethanamine, and amino alcohol and amino acid derivatives. 

Sterically bulky P,0-donor ligands have been used to prepare highly active catalysts, many of which are also capable of incorporating polar co-monomers. For example, complex (115) has... 

Not only phosphines or phosphites but also phosphoric acid trisdialkyl-amides (40), sulfoxides (41), etc. have been used as electron donors in the preparation of the catalyst. In addition, the catalytic activity of tetra-methylcyclobutadienenickel dichloride and alkylaluminum halides has been studied in detail (42, 43). 

Since its discovery in 1980,7 the Sharpless expoxidation of allylic alcohols has become a benchmark classic method in asymmetric synthesis. A wide variety of primary allylic alcohols have been epoxidized with over 90% optical yield and 70-90% chemical yield using TBHP (r-BuOOH) as the oxygen donor and titanium isopropoxide-diethyl tartrate (DET, the most frequently used dialkyl tartrate) as the catalyst. One factor that simplifies the standard epoxidation reaction is that the active chiral catalyst is generated in situ, which means that the pre-preparation of the active catalyst is not required. 

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