Noteworthy Chemistry

October 26, 2009

Functional coatings for biomolecule immobilization
Unusual color produced in a demethylation reaction
A simple route to substituted isoxazoles
Ionic liquid–based selectors for chiral chromatographic separations
The first small-molecule avian flu entry inhibitors
Filler modification effects in electrospun polymeric fibers
Continuous nitration to avoid unstable dinitropyrazoles
Synthesis of a potential anticancer compound, saliniketal B

These functional coatings promote biomolecule immobilization. D. Sung, S. Park, and S. Jon* at the Gwangju Institute of Science and Technology (Korea) and the Korea Institute of Science and Technology (Seoul) used rationally designed polymers as functional surface coatings for immobilizing biomolecules on plastic surfaces. This immobilization is essential in biochip and biosensor technology.

The amphiphilic polymers contained a hydrophobic unit [alkyl (1) or benzyl (2)] to promote adhesion to the plastic substrate, a carboxyl acid linker to selectively attach biomolecules, and a protein-resistant domain [poly(ethylene glycol), PEG] to limit biofouling. Using a simple dip-coating method (1 h, room temperature), the researchers applied the polymeric coatings to polystyrene substrates. They confirmed conformal adhesion by measuring a change in the static contact angle. The angle decreased from 130 ± 7° to 74 ± 1° (1) or 79 ± 2° (2); the decrease indicated exposure of hydrophilic PEG units.

Polymer coating 2 was more stable when incubated in the presence of a surfactant. Both of the functional, amphiphilic materials exhibited nonspecific protein adsorption by the PEG domains. Both also selectively immobilized biomolecules (biotin–streptavidin and a large model protein) via microcontact printing.

The authors coated a commercially available 96-well polystyrene plate with functional polymers and modified it with biotin–streptavidin. An enzyme-linked immunosorbent assay for biotin-conjugated mouse immunoglobulin G was performed and compared with uncoated commercial 96-well polystyrene plates with blocking proteins. The amphiphilic coating had detection limits comparable with the control without the need for a blocking strategy. (Langmuir 2009, 25, 11289–11294; LaShanda Korley)

Investigate an unusual color produced during a demethylation reaction. E. Merifield and co-workers at AstraZeneca R&D Charnwood (Loughborough, UK) noticed that an orange solid was produced during the demethylation of 5-(4-methoyxphenyl)thiophene-2-sulfonic acid tert-butylamide with PhSH and K₂CO₃ in N-methylpyrrolidone. The sulfonamide was prepared by Suzuki coupling of 4-methoxyphenylboronic acid with 5-bromothiophene2-sulfonic acid tert-butylamide as part of the synthesis of a drug for asthma and rhinitis.

Analysis of the orange solid revealed that it contained 26% palladium. This serendipitous result provided a simple method for removing the residual palladium that had been carried over from the previous step. (Org. Process Res. Dev. 2009, 13, 751–759; Will Watson)

A simple route to substituted isoxazoles uses readily available starting materials. The value of isoxazole scaffolds for important natural products and pharmaceuticals is well established. X. She and coauthors at Lanzhou University (China) and the Chinese Academy of Sciences (Lanzhou) developed a simple one-step synthesis of 3-monosubstituted and 3,5-disubstituted isoxazoles by the reaction of α,β-unsaturated aldehydes or ketones with N-protected hydroxylamines.

Optimizing the reaction conditions and selecting the optimum base led to yields of the substituted isoxazoles as high as 92%. Aldehyde substrates provided 3-substituted products (e.g., 1), and ketones gave the corresponding regioselective 3,5-products (2). The authors attribute the success of this reaction to the p-toluenesulfonyl (Ts) protecting group, which makes the hydroxylamine nitrogen more nucleophilic for conjugate additions. This method is especially useful for the 3-substituted isoxazoles, which are not easy to prepare by other means. (Org. Lett. 2009, 11, 3982–3985; W. Jerry Patterson)

Separate amino acid enantiomers chromatographically with ionic liquid–based chiral selectors. Chiral separation is a hot topic in the pharmaceutical industry because of the increasing awareness that enantiomers may exhibit different pharmacological properties. To avoid undesirable side effects, it is imperative to isolate the pure therapeutically active form of a chiral drug. This calls for the development of effective processes for enantioseparation of racemic drugs.

In response to this need, S. Yao’s group at Hunan University (China) explored the potential applications of ionic liquids in chiral separations. They developed a new process for effective enantioseparation of amino acid mixtures.

Yao’s process uses chiral 1-alkyl-3-methylimidazolium L-proline ligands coordinated with Cu(II). The enantioselectivity of the chiral separation is enhanced by increasing the alkyl chain length. Resolution is much higher (up to ~11-fold) than can be achieved by using “conventional” amino acid ligands. The authors believe that this superiority stems from the ion pairing between the alkylimidazolium cation and the L-proline anion on the surface of stationary phase or the capillary wall in the chromatographic process. (Chem. Eur. J. 2009, 15, 9889–9896; Ben Zhong Tang)

The first small-molecule avian flu entry inhibitors have been made. Avian influenza caused by the H5N1 virus continues to break out in Asia, Europe, and Africa, with high mortality rates. Current methods of controlling the spread of the virus are M2 ion channel inhibitors such as amantadine and rimantadine and neuraminidase inhibitors such as oseltamivir, zanamivir, and peramivir.

Because oseltamivir resistance is already being reported, there is a challenge to develop new kinds of inhibitors. Early in H5N1 infections, virus attachment to the host cell is mediated by the glycoprotein hemagglutinin (HA). Y. Li and coauthors at Ocean University of China (Qingdao), the Chinese Academy of Medical Sciences and Peking Union Medical College (Beijing), and the Medical College of Chinese People’s Armed Police Forces (Tianjin) report a series of saponin viral entry inhibitors that target HA.

Saponins 1 and 2 with β-chacotriosyl moieties showed inhibitory activity and were chosen as lead compounds for new compounds 3–9. Compounds 3–7 retained the β-chacotriosyl group, but the aglycone residue was changed to dihydrochlorogenin (3), dehydroisoandrosterone (4), methyl oleanolate (5), methyl ursolate (6), or stigmasterine (7). In compounds 8 and 9, the β-chacotriosyl moiety was simplified to evaluate the effect of the sugar chain.

The new compounds were tested by using an HIV-based pseudotyping system against two highly pathogenic H5N1 viruses and for specificity to H5N1. The inhibitory effect of ursolate saponin 6 was comparable with 1 and 2 and superior to other compounds in the series. This result showed that modifying the aglycone can strongly affect activity. Compounds 8 and 9 showed no activity, demonstrating the importance of the β-chacotriosyl group for antiviral activity. This is the first report of small-molecule inhibitors of H5N1 HA. (J. Med. Chem. 2009, 52, Article ASAP DOI: 10.1021/jm900275m; José C. Barros)

Filler modification controls heterogeneity and reinforcement in electrospun polymeric fibers. O. J. Rojas and colleagues at North Carolina State University (Raleigh) and Helsinki University of Technology (Finland) explored the reinforcing effect of hydrophilic cellulose nanocrystals (CNXs, 3–10 nm diam, 100–250 nm long) in hydrophobic poly(ε-caprolactone) (PCL, 80 kDa) electrospun nanofibers. They point out that CNXs are bioderived, inexpensive, biodegradable, and biocompatible; these properties make them an attractive option for improving the mechanical strength of PCL bioscaffolds.

The research team generated PCL nanofibers that contain unmodified and oligomeric PCL–grafted (2 kDa, 4.5% grafting density) CNXs at 2.5, 5.0, and 7.5 wt%. Except at 2.5 wt% (slightly below the percolation threshold), the diameters of the modified CNX fibers were greater than those of the unmodified fibers; this is most likely caused by filler aggregation. Overall, the PCL–unmodified CNX fibers exhibited similar morphology to neat PCL nanofibers.

The thermomechanical behavior of the PCL was enhanced when unmodified CNXs were incorporated at 2.5 and 7.5 wt%. The authors emphasize the complex interplay between fiber diameter, surface area, filler organization, and mechanical strength. Incorporating PCL-grafted CNXs into the PCL matrix, however, produced electrospun fibers (under identical spinning conditions) with heterogeneous microstructures that resulted from jet instabilities, rheological mismatch, and differences in PCL (matrix and oligomeric grafts) crystallization rates, leading to poor mechanical reinforcement. PCL crystallinity and melting temperature also decreased in the PCL–grafted CNX nanofibers. (ACS Appl. Mater. Interfaces 2009, 1, 1996–2004; LaShanda Korley)

Continuous nitration of 3-alkylpyrazoles avoids formation of unstable dinitropyrazoles. The semibatch nitration of 3-alkylpyrazoles with HNO₃ in H₂SO₄ is traditionally carried out at 140 °C. J. Pelleter* and F. Renaud at AstraZeneca (Reims, France) improved this process by continuously nitrating 3-alkylpyrazoles at 65 °C.

The process change avoids the formation of unstable dinitropyrazoles that are produced only at temperatures >80 °C. Simple standard laboratory equipment can be used: the only specialized unit is the mixer. The authors also describe the continuous bromination of pyridines. (Org. Process Res. Dev. 2009, 13, 698–705; Will Watson)

Here’s a highly efficient synthesis of a potential anticancer compound, saliniketal B. The polyketide saliniketal B (1), first isolated from the marine actinomycete Salinispora arenicola, can inhibit ornithine decarboxylase (ODC) induction. This biochemical mechanism is a potential target for chemotherapeutic or chemopreventive intervention. Compounds such as 1 function by attenuating tumor-promoter-mediated induction of ODC. A flexible and efficient strategy for synthesizing 1 would promote future structural variants and enhance structure–function and mode-of-action studies.

J. Liu and J. K. De Brabander* at the University of Texas Southwestern Medical Center at Dallas report a strategy based on the synthesis of novel coupling partners ethyl ketone 2 and dihydropyranone-functionalized aldehyde 3. Their reaction ultimately leads to the desired compound 1, which has nine stereocenters, eight of which are contiguous.

Starting with a commercially available alkynol, the asymmetric synthesis of 2 proceeded in eight steps with a 45% overall yield. An unusual feature of 2 is the 2,8-dioxabicyclo[3.2.1]octane moiety, which remains intact in the final target compound 1. The asymmetric synthesis of the other coupling intermediate 3 was carried out with similar efficiency: seven steps with an overall 39% yield. The formation of the key dihydropyranone moiety in 3 was accomplished via ring-closing metathesis.

The aldol coupling of 2 and 3 was promoted by lithium hexamethyldisilazide (LiHMDS) to yield β-hydroxy ketone 4. Reduction of 4 with a modified borohydride provided anti diol 5 in almost optically pure form (>20:1 dr). Finally, fluoride-mediated desilylation of 5 and fragmentation of the dihydropyranone ring were followed by in situ amidation of the generated carboxylic acid to produce target structure 1. HOBt is 1-hydroxybenzotriazole; EDC is
1-ethyl-3-(3-dimethylaminopropyl)carbodiimide.

The outstanding features of this synthesis are

a 23% overall yield for the synthesis,
the use of platinum-catalyzed cycloisomerization method for forming the novel bicyclooctane core of intermediate 2, and
an unusual one-pot desilylation–dihydropyranone fragmentation–amidation sequence to convert 5 to 1.

(J. Am. Chem. Soc. 2009, 131, 12562–12563; W. Jerry Patterson)