Anhydride activity

Direct Borohydride Reduction of Alcohols to Alkanes with Phosphonium Anhydride Activation N-Proovlbenzene. To a solution of 5.56 g (20 mmol) of triphenylphosphine oxide in 30mL of dry methylene chloride at CfC was added dropwise a solution of 1.57 mL (10 mmol) of triflic anhydride in 30mL of dry methylene chloride. After 15 min when the precipitate appeared, a solution of 1.36g (10 mmol) of 3-phenyl-1-propanol in 10 mL of dry methylene chloride was added and the precipitate vanished in 5 min. An amount of 1.5g (40 mmol) of sodium borohydride was added as a solid all at once and the slurry was stirred at room temperature for... 

While triflic anhydride activation of sulfoxides is the norm, other activation procedures have been employed. In the synthesis of the ciclamycin 0 trisaccharide, catalytic triflic acid (TfOH) was employed.19 Trimethylsilyl triflate (TMSOTf), in conjunction with triethyl phosphite (a scavenger for phenylsulfenic acid), has also been used.20... 

The A-chain synthesis by Meienhofer et al.[8l used the same 1-9 and 10-21 fragments, but they employed azide or anhydride activation for the various condensations. Likewise, Kung et al.[9] used the 1-9 and 10-21 condensation at the last step of their synthesis. 

On the other hand, polymeric carriers can also be modified to introduce reactive groups. Polysaccharides such as dextran and inulin may be activated [149] by periodate oxidation to create aldehyde groups, by succinic anhydride activation to create carboxylic groups, or by p-nitrophenyl chloroformate activation to create reactive ester groups. 

Boos and Flauschildt90) obtained for the model copolymerization of phenylglycidyl ether with hexahydrophthalic anhydride activation energies of 96 kJ/mol up to 75% conversion and 27 kJ/mol for higher conversions. Frequency factors are also very different (log A = 13.7 and 5.5, respectively). The frequency factors as well as the temperature coefficients of the solution viscosities depended on the initiator concentration. The activation energy determined by the same authors 90) for the curing of epoxy resins at conversions lower than 75% was 86.4 kJ/mol and the frequency factor log A = 11.8 whereas at higher conversions these values were not obtained. 

PAMAM]. The final step of this functionalization relied on activation and cross-linking of attached dendrimers with a homobifunctional spacer (DSG or PDITC). Alternatively, after attachment of dendrimers to the surface, glutaric anhydride activated with V-h ydrox vsucc i n i m i de can be used. This surface modification yields a thin, chemically reactive polymer film, which is covalently attached to the glass support and can be directly used for the covalent attachment of amino-modified components, such as DNA or peptides (Fig. 14.2b). 

Succinic anhydride activation. Carboxylic groups can be easily introduced in polysaccharides (P-OH) by reaction with succinic anhydride (III). They can then be transformed into reactive esters and finally coupled with drugs ... 

As with trifluoroacetic anhydride, activation of dimethyl sulfoxide with thionyl chloride must be carried out at low temperatures as the reaction is highly exothenmic. Besides the higher yields, a further advantage of thionyl chloride to activate dimethyl sulfoxide over anhydrides is the lack of Pummerer rearrangement products or of esters formed as by-products (as long as Ae reactions are carried out below -60 C). This is amply demonstrated by the oxidation of (-)-bomeol which proceeds in an excellent 99% yield (equation 12). ... 

In peptide chemistry, in addition to the initially used acid chloride methodb h A -Phth amino acids can be coupled by the entire repertoire of coupling methods, i.e. mixed anhydrides, symmetrical anhydride, active ester, and the azide method.W Due to the add stability of A -phthaloyl derivatives, the acid hydrolyses of esters are readily performed.b h... 

Despite the relatively high reactivity of the serine hydroxy group, serine esters and serine-containing peptides with free hydroxy groups have been used as amine components in solution synthesis. Syntheses with free serine hydroxy groups can be performed with azides, without the risk of 0-acylation.b l Mixed anhydrides, active esters, and carbodiimides have also been successfully used for the coupling reactions. Therefore, for this couphng reaction, any over activation has to be avoided. 

To prevent racemization of sensitive amino acid derivatives, the activation time should be kept as short as possible (5 min) and the activation should be run at 4 °C rather than room temperature. Optimized conditions were developed for intermediate formation of active esters (DIC/HOBt or DIC/HOAt in DMF/CH2CI2, 1 1, for 5 min) and symmetrical anhydrides (activation in DMF/CH2CI2, 9 1, and coupling in DMF/CH2CI2, 1 1). 

Like the mixed anhydrides, activated esters may also be represented by the general formula (26). The only difference between mixed anhydrides and activated esters lies in the fact that HOX corresponds to a carboxylic or inorganic proton acid in the case of the mixed anhydrides and to an (acidified) alcohol in... 

Ester and amide synthesis. Nicotinic anhydride activates carboxyhc acids (by forming mixed anhydrides) to be transformed into esters and amides, with alcohols and amines, respectively (catalytic DMAP). 

Another y-hydrazide derivative of MTX was prepared by Rosowsky and Forsch [333] from a-t-butyl MTX by a mixed anhydride activation of the y-carboxyl group (/-BuOCOCl-Et3N) followed by addition of biotin hydrazide and treatment with trifluoroacetic acid (room temperature, 10 min) to obtain the biotinyl derivative (VIII.228). The overall yield for the coupling and deprotection steps was approximately 30%. 

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