Soluble Bifunctional Reagents

Bifunctional reagents have recently been used to facilitate separations in the Mitsunobu reaction.39 Mitsunobu products are often hard to separate from excess reagents and byproducts, including phosphines and phosphine oxides. The tagged phosphine 21 and azodicarboxylate 22 and the byproducts formed from these are converted to the carboxylic acid forms by treatment with trifluoroacetic acid (TFA) at the end of the reaction. The excess reagents and byproducts could then be captured on an ion exchange resin for convenient removal. 

3-Mercaptopropionic acid has been used as a scavenger in detritylation of protected imidazoles. In the purification, A-21 tertiary amine-functionalized resin 23 sequestered excess of this acidic reagent, the tritylated byproduct formed from it, and TFA (reaction 6).32 

The role of bifunctional (tagged) reagents in the transformations described above is similar to that of polymer-immobilized reagents. However, they exhibit more reliable reaction kinetics because they react in the solution phase. Moreover, manipulation of these reagents is more easily automated since transfer of solutions is much easier than transfer of solids. 

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Poorly reactive (poorly sequestrable) byproducts are more frequently encountered than poorly sequestrable reactants. A few reports have appeared describing the use of soluble bifunctional linking reagents to chemically tag... 

Photochemical reagents have been devised for crosslinking both soluble and particulate proteins (Chapter 5). In a recent study, Johnson et al. (1981) were able to crosslink radiolabeled glucagon to its receptor, merely by adding a bifunctional reagent that first reacted with amino groups and could subsequently be induced to form crosslinks by photolysis. 

Noncovalent binding of HRP to antibody, also known as unlabeled antibody binding, is described in great detail by Sternberger (1). Instead of the use of bifunctional reagents, IgG class antibodies to HRP are used to form a soluble semicyclic immune complex consisting of two antibody and three enzyme molecules. The molecular weight of the peroxidase antiperoxidase, PAP complex is 400 - 430 kDa. 

Covalent coupling between synthetic NAD+-derivatives, water-soluble polymer matrices, and enzyme molecules has combined both techniques mentioned above. For example, polyethyleneglycol carrying covalently bound NAD+-units was activated with a bifunctional reagent (25) and covalently attached to malate [288] or glucose dehydrogenase [290] (Fig. 22). 

Glutaraldehyde (GA), a bifunctional reagent for bridging amino groups, is the most widely used reagent for cross-linking proteins. GA, a linear, 5-carbon dialdehyde, is a clear, colorless to pale straw-colored, pungent oily liquid that is soluble in all proportions... 

Alkynyl halides are possible monomers for the cross-coupling polymerization, in which boronic acids are used as the organometallic counterparts. For example, bifunctional boronic acid 46 is allowed to react with l,4-di(bromoethy-nyl)benzene 138 to afford the corresponding PAE 139 as shown in Equation (64). Polymerization proceeds at room temperature in toluene in the presence of silver(i) oxide as an activator of the boron reagent. The polymer 139 is obtained in 30-50% yield showing color of red-brown to deep red-brown and slight solubility in toluene (<0.1 wt.%). The molecular weight (Mr of 139 was 1700-4300 (PDI = 1.3-3.6). 

Subsequently, following the successful isolation of stable phosphaalkene-con-taining polymers 156, the use of the same substituent for the preparation of a related polymer featuring P=P moieties was attempted. It is known that dipho-sphenes M can be prepared by dimerization of transient phosphinidenes J generated by photolysis of phospha-Wittig reagents L (Scheme 4.43) [80e]. Photolysis at room temperature or thermolysis (neat, 250 °C, 2 min) of bifunctional compound 155 does indeed result in the formation of polymer 157 in near quantitative yield (Scheme 4.43) [80d], This soluble material was characterized by NMR spectroscopy and GPC analysis, which revealed a rather low molecular weight (Mn = 5900). The UV-Vis spectrum of 157 shows a jt-jt transition (435 nm) accompa-... 

Reaction of magnesium metal with anthracene or its derivatives in THF yields magnesium anthracene reagents, which have versatile applications. In synthetic reactions, these compounds have an ambivalent character they can react both as a soluble form of magnesium or as bifunctional... 

In another approach, the C-2 position of taxol is the site of manipulation. Thus, a two-step enzymatic acylation in organic solvents has been employed to synthesize water-soluble taxol (paclitaxd) derivatives. In the first step, taxol 241 is reacted with the bifunctional acylating reagent 282, as catalyzed by thermolysin (from Bacillus thermoproteolyticus rokko), to give the activated acyl derivative 283. This is then used as a complex acyl donor in the second step [204]. 

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