Table of Contents

Use polymer imprinting for selective uranium extraction. P. H. Walton and co-authors have used the uranyl complex of chloroacrylic acid as a template and copolymerized this structure with ethylene glycol dimethacrylate as a cross-linking agent.

HIV crisis spurs new approaches to artificial blood substitutes. In the 1980s, the HIV health crisis compromised the nation’s blood supply and prompted increased emphasis on developing safe and effective blood substitutes. T.M.S. Chang summarizes the status of this effort, which has been based primarily on modified hemoglobin and fluorochemicals. The hemoglobin molecule is the best oxygen carrier at present and can be readily extracted from red blood cells; however, when the unmodified form is infused into the body, it breaks down into subunits that cause toxic effects in the kidneys. One promising modification uses bifunctional monomers (glutaraldehyde or o-raffinose) to cross-link the hemoglobin molecules, preventing breakdown in the body. A form of this material is in phase III clinical trials. Other approaches cross-link the hemoglobin molecule internally with bis(3,5-dibromosalicyl) fumarate to block breakdown or use recombinant technology to form fused single hemoglobin molecules.

A new generation of blood substitutes based on the idea of a completely artificial blood cell shows great promise. The author points to recent efforts on encapsulating hemoglobin—with blood chemistry enzymes—that use a biode gradable polylactide membrane to form a 150-nm nanocapsule. The polymer converts readily to water and CO₂ after use, precluding accumulation in the body. Blood substitutes, such as these innovative hemoglobin modifications, represent a promising area of research with a potentially big commercial payoff. (Chem. Ind. 2000, 8, 281–284; WJP)

Rearrange N-allylanilines to indoline derivatives. J. S. Yadav, B.V.S. Reddy, and co-authors found that this aza-Cope rearrangement proceeded in good yields when N-allyl anilines are subjected to microwave irradiation (2450 MHz) in the presence of a Zn²⁺ montmorillonite catalyst. The micro wave-promoted reactions took place within minutes, compared with 8–10 h for comparable yields when the reactions were attempted with conventional heating at 130 °C.

Other catalysts such as KSF clay or ZnCl₂ were equally effective. The catalyst is reusable after reactivation. This kind of microwave-assisted dry reaction on high surface area solids is appearing more frequently in the literature. The technique offers certain operational, economical, and environmental benefits over conventional methods and should be considered by chemists conducting large-scale reactions. (Synlett 2000, 4, 487–488; RM)

This catalyst promotes Suzuki coupling of halides and triflates with arylboronic acids under mild conditions. A. F. Littke, C. Dai, and G. C. Fu* report that the catalyst system Pd₂(dba)₃–P(t-Bu)₃ allowed phenyl-, pyridyl-, and thiophenyl-based aryls to be coupled to the arylboronic groups in THF at room temp in 77–99% yields.

Dba is dibenzylideneacetone. Sterically hindered couplings can be accomplished by running the reaction at 60 °C. The authors also demonstrated that molecules that bear more than one group capable of coupling can be selectively coupled, depending on the reaction conditions used.

Cy is cyclohexyl; Tf is trifluoromethanesulfonyl. They noted that the Pd(OAc)₂–PCy₃ catalyst system worked better for the aryl triflates. Using low-cost KF as a replacement for CsF in the workup of reaction products is another improvement in this approach. (J. Am. Chem. Soc. 2000, 122, 4020–4028; DAS)

Catalyst promotes high efficiency in Friedel–Crafts acylation. Lewis acid catalysts such as AlCl₃ have often been used to carry out this reaction on an industrial scale, although they generate very large amounts of waste products. Lanthanide triflates, such as Hf(OTf)₄, function as Friedel–Crafts catalysts in some cases. J. Matsuo, K. Odashima, and S. Kobayashi* report an exciting new catalyst, gallium nonafluorobutanesulfonate, or Ga(ONf)₃, which seems to give excellent yields of aryl ketones with most aromatic compounds including deactivated aromatics. When m-xylene is treated with benzoyl chloride as the acylating agent, the yield of the ketone product is 99%. Typical reaction conditions are 5 mol% catalyst, reflux, 24 h; the reaction is less successful using acetic anhydride as acylating agent. A plus for this method is the conversion of inactivated aromatics (e.g., fluoro benzene, dichlorobenzene) to the corresponding aromatic ketones in good yields. (Synlett 2000, 3, 403–405; RM)

Prepare unprotected, N-linked glycopeptide building blocks using solid-phase synthesis. The synthesis of N-linked glycopeptides is a hot subject in glycochemistry, and several methods have been developed; however, the yields from unprotected carbohydrates are rather poor and require extensive purification. For this reason, various protection schemes must be used. This incon- venience can be largely overcome, according to L. Jobron and G. Hummel*, by attaching temporarily N-protected glycosyl amines to a suitable trityl-activated support at the primary hydroxyl position. This technique is known as spot synthesis. Condensation of this resin-bound substrate with an amino-acid–based acid chloride was followed by cleavage from the resin with trifluoroacetic acid, providing the glycosylated building blocks in good yields (70–80%) and >90% purity. Fmoc is 9-fluorenylmethoxycarbonyl; Pfp is pentafluoro phenyl; TFA is trifluoroacetic acid. (Angew. Chem., Int. Ed. 2000, 39, 1621–1624; OR)

Use enzymes to unlock a safety catch and release products bound to resins. In their work on stable yet easily cleaved linkers in solid-phase synthesis, U. Grether and H. Waldmann* have designed a convenient enzyme-based method. Because enzymatic reactions can be completed under very mild conditions (neutral pH, ambient temperature, etc.), they are attractive tools for releasing newly synthesized—and sometimes very labile—molecules from the resin. In the developed system, a hydrolytic enzyme (penicillin G acylase) was used in conjunction with a safety-catch strategy, in which the linker unit was completely stable until the catch was unlocked.

After the final solid-phase synthetic step, the product (1) was treated with the enzyme to selectively cleave a labile benzyl amide group and produce a free amine (2). The liberated amine could then undergo intramolecular ring closure to form the lactam (3) and simultaneously liberate the target molecule (4) from the resin. (Angew. Chem., Int. Ed. 2000, 39, 1629– 1632; OR)

Mesoporous silica provides a simple route for preparing silver nanowire. According to M. H. Huang, A. Chou drey, and P. Yang*, uniform silver nanowires can be synthesized within the channels of mesoporous silica SBA-15. They use a simple chemical approach that includes preparing an impregnated catalyst. They treated the SBA-15 matrix by soaking in 0.2 M AgNO₃ solution (2 mg solid/mL liquid) with stirring at room temperature overnight. The solid was then filtered, washed, and pyrolyzed at 300 °C in air for 2 h to allow chemical decomposition of AgNO₃ to form silver nanowires. TEM studies show that the silver nanowires are continuous and essentially follow the curvature of the nanoscale channels of the silica. The particle diameters are in the range of 5–6 nm with aspect ratios (length/diameter) between 100 and 1000. (Chem. Commun. 2000, 1063–1064; XSZ)

A new family of porous magnesium oxides displays high affinity for organic molecules. K. J. Klabunde and co-authors produced magnesium oxide aerogels by hydrolyzing Mg(OMe)₂, followed by supercritical drying and heat treatment. The MgO aerogels exist as 4-nm nanocrystals that aggregate into porous weblike structures ~1 mm in diameter and have at least 50% more surface area than classically prepared MgO. This very high surface area persists even when the aerogels are pressed into pellets, unless compressed under very high pressures. The authors describe the nanocrystals of MgO as “voracious adsorbents” of organics such as MeOH, because of their unique morphological features and very high surface areas. They also note that the pore structure can be controlled to some extent by compression techniques. (J. Am. Chem. Soc. 2000, 122, 4921–4925; DAS)

Can combinatorial chemistry techniques be adapted to air–water interfaces? R. M. Leblanc and co-workers have created combinatorial libraries at the surface of a Langmuir– Blodgett film. Their approach was to form synthetic carbohydrate receptors combinatorially arranged from an array of lipid-linked peptides. A tripeptide library composed of five amino acids—in which the N-terminals were attached to a stearic acid molecule—was synthesized by split-and-mix techniques.

This 250-compound library was subsequently applied to an air–liquid interface forming a Langmuir surface, where the peptides were exposed into the aqueous subphase. Adding maltose to the subphase allowed the peptide structures to rearrange and form pseudoreceptor structures that could bind the maltose structure. Deconvolution of the library by removing two of the amino acids each time (producing three sublibraries with 54 components each) revealed that polar charged residues (Glu) and aromatic residues (Tyr) were necessary, and a sublibrary enriched in these residues proved the most efficient. (Angew. Chem., Int. Ed. 2000, 39, 1854–1857; OR)