Science Concentrates

Allylic ether synthesis leads to mosquito repellent

An improved stereospecific synthesis of allylic ethers combined with a ring-closing metathesis reaction has led to a new route to make disubstituted cyclic ethers of varying ring size [Angew. Chem. Int. Ed., 43, 4788 (2004)]. Chemistry professor P. Andrew Evans and his group at Indiana University, Bloomington, demonstrate the utility of their strategy by carrying out the seven-step total synthesis of gaur acid (shown, R = protecting groups, R´ = tert-butyl). The acid is found in an oily secretion of the gaur, a wild ox in Asia, and is being investigated as a repellent for the Aedes mosquito that spreads the yellow-fever virus. The synthesis begins by reacting an allylic secondary alcohol with copper iodide to form an alkoxide. The copper alkoxide is subsequently coupled with an allylic carbonate using a rhodium catalyst to form an ether with unsymmetrical alkenyl groups. The gaur acid synthesis is completed by a ring-closing metathesis of the alkenyl ether. The total synthesis allowed the team to pin down the previously unknown absolute configuration of gaur acid, thus uncovering its biologically active form.

Toxicity of C₆₀ species varies

The toxicity of water-soluble fullerene species in two different human cell lines has been reported by a team from Rice University and Georgia Tech [Nano Lett., published online Sept. 11, http://dx.doi.org/10.1021/nl0489586]. Rice chemist Vicki L. Colvin and colleagues found that the sparingly soluble colloids of C₆₀ that form in water--dubbed nano-C₆₀ by the group--exhibit cytotoxicity in human skin and liver carcinoma cells at concentrations as low as 20 ppb. They also found that this toxicity decreases considerably with surface modification of the C₆₀ molecule. For example, the fullerene deriva- tive C₆₀(OH)₂₄ shows no toxicity up to the limits of its solubility--greater than 5,000,000 ppb. The group attributes nano-C₆₀'s toxicity to the compound's ability to generate superoxide anions. These oxygen radicals damage cell membranes and lead to cell death. Colvin and colleagues say this work demonstrates a strategy for enhancing fullerene toxicity for use as a bactericide or cancer therapeutic. However, they note that as C₆₀ finds widespread use in consumer products, it will be important to consider the effects that unintentional generation of nano-C₆₀ may have on the environment.

Nitric oxide delays flowers

Plants put off flowering under the influence of nitric oxide. Whether the source is NO-spiked plant food, supplemented air surrounding the plant, or an internal gene set to overproduce the molecule, NO represses what is called the floral transition in Arabidopsis thaliana, says Zhen-Ming Pei, assistant professor of biology at Duke University, who led the study [Science, 305, 1968 (2004)]. Researchers have long known that plants overproduce NO under stressful conditions: drought, salt, and microbial invasion. Pei's group established that NO acts as a growth regulator to inhibit flowering. Pei says the fact that plants respond to an external or an internal NO signal may help plants warn themselves and their neighbors when things get rough. But, he adds, a response to airborne NO could have unintended consequences in plants exposed to the NO released by burning fossil fuels.

Diphosphines probe active sites in zeolites

A new NMR-based procedure for characterizing active centers in solid catalysts has been demonstrated by chemists at the State University of New York, Stony Brook. Clare P. Grey, Luming Peng, and Peter J. Chupas have shown that a family of diphosphine molecules (shown), in which the distance between phosphorus atoms is controlled by adjusting the length of an alkyl spacer (n = 1, 3, or 6), can be used as probes to estimate distances between acid sites in zeolites. By exposing an acidic zeolite to the dibasic probe molecules of various lengths and then analyzing the protonated and adsorbed species with solid-state NMR methods, the group determined the number and relative strength of acid sites per unit cell within known distances of other acid centers in the catalyst [J. Am. Chem. Soc., 126, 12254 (2004)]. The team notes that the technique is now being applied to a variety of materials featuring much lower concentrations of acid sites. Similar methods may be especially useful in studies of bifunctional catalysts, such as the palladium-zeolite materials used for hydrogenation of aromatic compounds.

Dendritic hydrogels go easy on the eyes

About 11 million cataract removal surgeries are performed worldwide each year. During the procedure, the surgeon makes an incision in the cornea, which is then left to heal or is closed with nylon sutures. Both techniques have drawbacks, such as increasing the risk of either infection or inflammation. Now, Boston University chemistry and biomedical engineering professor Mark W. Grinstaff and coworkers at Boston University and Duke University Medical Center have created a transparent hydrogel sealant that works like a bandage for the incision [J. Am. Chem. Soc., published online Sept. 21, http://dx.doi.org/10.1021/ja045870l]. The hydrogels are made via in situ polymerization of peptide dendrons and poly(ethylene glycol) macromers. In vitro, eyes that have been cut and then sealed with Grinstaff's hydrogel are able to withstand higher intraocular pressures than either conventionally sutured or self-sealed wounds. Grinstaff also reports that using the hydrogel is easier and faster than suturing.