Anhydrides resin-supported

In addition, minor variation of the catalyst in combination with immobilization on a resin support gave an analogous recyclable solid-supported organocatalyst. Varying the derivatization method by trapping the a-amino nitrile intermediate with formic acid and acetic anhydride gives the crystalline formamides 19 in excellent yield and with high enantioselectivity. These features of this catalytic process have been demonstrated by results from the synthesis of r-tert-leucine (Scheme 14.8) [49]. 

For the solid-phase synthesis of amides, it makes a significant difference whether the amine or the acid is linked to the support. Resin-bound amines are readily acy-lated by adding first a carboxylic acid and then a carbodiimide (Table 13.3). The acid/ carbodiimide ratio is not critical, because both the O-acylisourea (ratio 1 1) and the symmetric anhydride (ratio 2 1) will lead to N-acylation. It should, however, be borne in mind that the half-lives of O-acylisoureas are shorter than those of anhydrides, and for difficult couplings it might be advantageous to acylate with a symmetric anhydride (two equivalents of acid and one of carbodiimide). 

The esterification of support-bound carboxylic acids has not been investigated as thoroughly as the esterification of support-bound alcohols. Resin-bound activated acid derivatives that are well suited to the preparation of esters include O-acylisoureas (formed from acids and carbodiimides), acyl halides [23,226-228], and mixed anhydrides (Table 13.15). A-Acylurea formation does not compete with esterifications as efficiently as it does with the formation of amides from support-bound acids. Esters can also be prepared from carboxylic acids on insoluble supports by acid-catalyzed esterification [152,229]. Alternatively, support-bound carboxylic acids can be esteri-fied by O-alkylation, either with primary or secondary aliphatic alcohols under Mitsu-nobu conditions or with reactive alkyl halides or sulfonates (Table 13.15). 

A compound prepared and first described as nicotinium dichromate (NDC) by Palomo et al.,379 was later shown by X-ray-crystal analysis380 to be a betainic mixed anhydride of nicotinic and chromic acid (NACAA). Because of its unique structure, it deserves a close scrutiny of its oxidative properties.381 Replacement of the chloride anion in the quaternary ammonium resin, Dowex 1-X8, for the dichromate anion, leads to a polymer supported dichromate, which is able to make selective benzylic oxidations.382 Finally, poly[vinyl(pyridinium dichromate)] (PVPDC), a polymeric analogue of PDC, must be mentioned whose use in the oxidation of alcohols allows for a very easy work-up.383... 

Natural supports (agarose, dextran, cellulose, porous glass, silica, the optical fiber itself or alumina) and synthetic resins (acrylamide-based polymers, methacrylic acid-based polymers, maleic anhydride-based polymers, styrene-based polymers or nylon, to name a few) have been applied for covalent attachment of enzymes. These materials must display a high biocatalyst binding capacity (as the linearity and the limit of detection of the sensing layers will be influenced by this value), good mechanical and chemical stability, low cost, and ease of preparation. 

Discussion In principle, acetals are cleaved by acid-catalyzed hydrolysis. In most cases aqueous acetic acid, aqueous trifluoracetic acid, dilute HC1 in THF or DOWEX 50W (H+) resin are used. Thus, treatment of 6 with DOWEX ion exchange resin in methanol rapidly furnishes the corresponding 1,2-diol without any further chromatographic purification steps. Generally, polymer supported reagents benefit from the ease of removal from the reaction mixture just by filtration of the insoluble resin. The resulting diol is acetylated by addition of acetic anhydride and pyridine. Final acetal exchange is achieved by acetic anhydride and catalytic amounts of concentrated sulfuric acid. A mixture (2 1) of anomers is obtained. 

However, there was clear evidence for elution of Co from the resin during reaction. Since leaching of Co was promoted by the water formed in the autoxidation, some acetic anhydride was added to trap the water. Even then, leaching remained substantial, and the anhydride caused some polymerization. Clearly, since acetic acid is a corrosive medium and dissolves Co compounds well, it is probably impossible to find a supported equivalent of Co catalysis in acetic acid. Similar problems of leaching by acid might arise in the autoxidation of propionaldehyde to give perpropionic acid with Co- or Mn-exchanged resins as catalysts (169). 

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