Reagent-mediated

As described above for the synthesis of thiophenes, the microwave assisted Lawesson s reagent mediated cyclization of various 1,4-dicarbonyl compounds yielded the desired 2-alkoxythiazoles in 90% yields [1] (Scheme 9). 

Thiophenes of type 31 (X-Y = CH) were generated via Lawesson s reagent-mediated cyclization of 1,4-dicarbonyl compounds 30 under microwave irradiation in the absence of solvent [37]. The reaction was carried by mixing the two solid reagents in a glass tube inserted inside a household microwave apparatus and irradiating until the evolution of H2S ceased. An interesting application of this method is the preparation of liquid crystals and other ferro- and antiferroelectric material such as compound 33 (Scheme 10). 

Figure 3.21 Eibre optic based, reagent-mediated sensor.

This chapter deals mainly with the 1,3-dipolar cycloaddition reactions of three 1,3-dipoles azomethine ylides, nitrile oxides, and nitrones. These three have been relatively well investigated, and examples of external reagent-mediated stereocontrolled cycloadditions of other 1,3-dipoles are quite limited. Both nitrile oxides and nitrones are 1,3-dipoles whose cycloaddition reactions with alkene dipolarophiles produce 2-isoxazolines and isoxazolidines, their dihydro derivatives. These two heterocycles have long been used as intermediates in a variety of synthetic applications because their rich functionality. When subjected to reductive cleavage of the N—O bonds of these heterocycles, for example, important building blocks such as p-hydroxy ketones (aldols), a,p-unsaturated ketones, y-amino alcohols, and so on are produced (7-12). Stereocontrolled and/or enantiocontrolled cycloadditions of nitrones are the most widely developed (6,13). Examples of enantioselective Lewis acid catalyzed 1,3-dipolar cycloadditions are summarized by J0rgensen in Chapter 12 of this book, and will not be discussed further here. 

Kiryanov, A.A., Sampson, R and Seed, A.J., Synthesis of 2-alkoxy-substituted thiophenes, 1,3-thiazoles, and related S-heterocycles via LawessoiTs reagent-mediated cyclization under microwave irradiation applications for liquid crystal synthesis, /. Org. Chem., 2001, 66, 7925-7929. 

Pecunioso reports that trialkylaluminums, alkenyldialkylalanes and alkynyldialkylalanes add efficiently to a-nitroalkenes (33) to (35) to afford nitroalkanes,17 y-nitroalkenes176 and y-nitroalkynes,l7c respectively (Scheme 7). This method complements the organolithium and Grignard reagent mediated... 

Nucleophilic alkylation with Grignard reagents (mediated by Et2AlCl) on chiral perhydropyrido(2.1 A]pyrrolo[l,2-ii][l,3,4]oxadiazinc (25) proceeds via an 5 2 mechanism below — 80 °C, forming the inversion product (26) with high stereoselectivity.22 At higher temperatures the stereoselectivity shifts in favour of retention. 

The iridium cyclopropene complex 171 reacts with an additional equivalent of Ir(l) to open the strained ring, affording bimetallic iridacyclobutene complex 172 (Equation 89) <1994JA10032>. Labeling experiments were used to establish that the external Ir(l) reagent mediates the oxidative activation of the intact coordinated cyclopropene, with the original Ir(l)-cyclopropene coordination retained throughout the reaction. 

Araujo E, Rodriguez-Malaver AJ, Gonzalez AM, Rojas OJ, Penaloza N, Bullon J, Lara MA, Dmitrieva N. Fenton s reagent-mediated degradation of residual kraft black liquor. Appl Biochem Biotechnol 2002 97 91-103. 

It is important to maintain low cytotoxicity, since transfection reagent-mediated toxicity could potentially mask the true phenotype of a target gene being studied. This chapter will attempt to describe the use of several cationic lipid/polymer-based transfection reagents for the in vitro delivery of siRNA and DNA. Emphasis will be laid on the key parameters that affect transfection efficacy. 

Brown and coworkers have reported chiral organoboron reagent mediated asymmetric synthesis of primary amines of high optical purity (Scheme 32). Stereospecific synthesis of secondary amines and N-substituted aziridines by the use of organoboron reagents has also been reported. ... 

A chromium complex of benzaldehyde imine is also a good substrate for the Michael-type addition of organolithium reagents mediated by a stoichiometric amount of the chiral diether 6 in toluene to give the corresponding product in up to93%ee(Eq. (12.13)) [32]. 

ESR is the method used extensively to characterize directly changes in the oxidation state of melanins (75). Spectrophotometric or electrochemical methods have been useful in monitoring concentration changes of the reagents-mediators (oxidants, reductants). The electrochemical method allows the monitoring of nontransparent suspensions without separating the melanin that does not exchange electrons with the electrode. 

The reaction of diisopropyl 2-(methoxycarbonyl)ethylphosphonate with Grignard reagents mediated by Ti(Oz-Pr)4 or Mc IKOZ-Pr), in THF at 0°C is completely diastereoselective and gives diisopropyl 2-(2-substimted-l-hydroxy-cyclopropyl)ethylphosphonates in good yields (60-71%). ... 

For a recent example of highly enantioselective addition of Grignard reagents mediated... 

The Sm(OTf)3 reagent mediates the Grignard-type reaction in THF-HMPA. Alkylation, allylation and benzylation of ketones and aldehydes with alkyl, allyl or benzyl halides proceeded via stable organosamarium intermediates [69] (Scheme 29). 

Conlrarily, Grignard reagent-mediated hydromagnesiation of acetylenes in the presence of... 

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