Supported reagents types

Another example of microwave-assisted PSR chemistry involves the rapid conversion of amides to thioamides by use of a polystyrene-supported Lawesson-type thio-nating reagent. By use of microwave irradiation at 200 °C in sealed vessels (monomode reactor), a range of secondary and tertiary amides was converted within... 

Supported Ziegler-type neodymium surface species (54, see below) have been prepared by mixing molecular components composed of [Nd(naph)3] (derived from naphthenonic acids) and alkyl aluminium reagents such as Al2Et3Cl3, Al( Bu)3 and/or Al( Bu)2H at 50-60°C with silica (source QiLu Petrochemicals Co., China) [158-160]. Although the immobihzed neodymium species are iU-defined, the materials display interesting properties in butadiene polymerization (Section 12.4.1.2). 

Cyanuric chloride (27) has been loaded on different types of NH2-functionalized resins 26 to give a new supported reagent 28. This reagent has been employed for the solution-phase synthesis of different amides and dipeptides 29 <990L1355>. 

A number of N-brominated and N-chlorinated heterocycles also provide sources of electrophilic bromine. Examples include 1-chlorobenzotriazole (82JOC4895 87JOC173 88CHE36) and various HBr and Br2 adducts of pyridines, or pyridine perbromides [84SC939 85JAP(K)60/87264], Polymer-supported reagents of this type include 1-cyclohexylpyridinium perbromide linked to polystyrene, effective for the bromination of 1-methylindole, benzo[fc]furan, and benzo[6]thiophene (89T7869). 

Polymer-supported reagent. HMPT supported on a polystyrene-type resin is a catalyst for SN2 reactions5-7 and for reduction of ketones by NaBIi4.7 It also has a marked effect on the alkylation of ethyl acetoacetate with diethyl sulfate. In the presence of solid HMPT the enolales undergo 60 70% O-alkylation. In the absence of HMPT, the lithium enolate does not react and the sodium and potassium enolates undergo C-alkylation (90-100%). There is some difference in the effect of solid and liquid I1MPT The solid HMPT increases the reactivity of the K. enolate more than the liquid form, whereas the reverse is true with the Li enolate.8... 

Similar para-quinone derivatives can be obtained by oxidative demethyla-tions of phenol ethers which can be performed in water. Either [bis(trifluo-roacetoxy)iodo]benzene 4 or the polymer-supported reagent 19 can be used for the generation of para-quinones 38 from 1,4-dimethoxybenzene derivatives of type 39, Scheme 19 [93]. 

SCHEME 47. Polymer-supported Stille-type reagents... 

Substrates of this type which afforded ge/n-difluorides are shown in Table 6.2. The yields reported were substantially improved by using a polymer-supported reagent which had the added advantage that no separation from iodobenzene was needed however, fluorination of the iodinated polystyrene requires the use of the expensive xenon difluoride [47, 48]. 

Rosiglitazone 9, an agonist of peroxisome proliferator activated receptor-y (PPARy), is a recently introduced antihyperglycemic thiazolidinedione effective in the treatment of noninsulin dependant diabetes mellitus (type II diabetes). A seven-step synthesis was developed (Scheme 3), which utilised supported reagents in combination with in-line SPE purifications. The introduction of the pyridine moiety provided a convenient molecular handle by which to purify the molecule throughout the synthesis. Notably, the overall yield (46%) for this synthesis was higher than the yield reported in the initial shorter solution-phase synthesis (31%). ... 

This polymer-supported reagent was used in an improved synthesis of d-bioliri, an important compound in the treatment of type 2 diabetes following the Hoff-mann-Roche approach [56], the key synthetic step introducing stereogenic centers into the molecule was performed on a 20-g scale using the polymer-supported version of CBS to afford the desired intermediate 47 in high yield and excellent enantiomeric excess (Scheme 4.12). 

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