Polymer supported reagents acidic

Polymer supported reagents, catalysts, protecting groups, and mediators can be used in place of the corresponding small molecule materials (Sherrington, 1991 Sundell and Nasman, 1993). The reactive species is tightly bound to a macromolecular support which immobilizes it. This generally makes toxic, noxious, or corrosive materials much safer. The use of polystyrene sulfonic acid catalyst for the manufacture of methyl r-butyl... 

Weik and Rademann have described the use of phosphoranes as polymer-bound acylation equivalents [65]. The authors chose a norstatine isostere as a synthetic target and employed classical polymer-bound triphenylphosphine in their studies (Scheme 7.54). Initial alkylation of the polymer-supported reagent was achieved with bromoacetonitrile under microwave irradiation. Simple treatment with triethyl-amine transformed the polymer-bound phosphonium salt into the corresponding stable phosphorane, which could be efficiently coupled with various protected amino acids. In this acylation step, the exclusion of water was crucial. 

Polymer-supported persulfonic acid was prepared from potassium persulfate and the cation exchange resin P—SO3H in water. The authors reported various applications of this new oxidizing reagent such as epoxidation of olefins, Baeyer-Villiger reaction and cleavage of disulfide and Af-formylamino acids. 

As a final example it should be mentioned that precondensed enones, prepared by standard Knoevenagel condensation of the aldehyde with the CH-acidic carbonyl component, when reacted with thioureas provided 1,3-thiazines 37, which are isomeric to thio-Biginelli compounds of the general formula 14 (see Figure 4.5). A published report describes the combinatorial synthesis of a library of 29 derivatives of thiazines 37 utilizing polymer-supported reagents and catalysts [168]. 

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. 

As this area of polymer supported reagents continues to expand the complexity of the polymeric supported resins has increased [18]. Although electrophilic supported reagents like the isocyanate 5 and acid chloride 12 have been shown to be efficient reagents for the covalent capture for primary and secondary amines (Table 1), they are not without their difficulties. The isocyanate resin is particularly expensive and the loading is rather low (approximately 1 mmol NCO/gram). 

A one-step synthesis of 1,3,4-oxadiazoles from readily available carboxylic acids and acid hydrazides was mediated by the polymer-supported reagent... 

Furthermore, in this synthesis the polymer-supported reagents were encapsulated in sealed pouches to aid work-up. The reaction sequence starting from the acid chloride 2 through to the intermediate nitroalkene 3 could thus be performed in a one-pot procedure. The reaction progress as effected by the pouched reagent could be easily monitored by... 

Organic hydroperoxides are generally used for the preparation of sulfoxides from sulfides, - while sulfones can be obtained in neutral organic solvents in the presence of metal catalysts such as V, Mo and Ti oxides at 50-70 C. Two polymer-supported reagents which involve peroxy acid groups and bound hypervalent vanadium(V) and molybdenum(VI) compounds have been developed for facile oxidation of sulfoxides to sulfones. 

Microwave chemistry is beginning to play a greater role also in solid-phase chemistry and in cases where polymer-supported reagents are utilized. The main drawback with these methodologies concerns the slow kinetics, which makes reaction scouting and optimization tedious. An impressive example of a microwave accelerated and polymer assisted process was recently described by Linclau. In this report, a set of carboxylic acids were fully 0-alkylated after only 3-5 min of microwave heating (Reaction Scheme 4). 

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