Science Concentrates

High complexity with dendralenes

A practical synthesis of [4]dendralene by chemists at Australian National University, in Canberra, has enabled them to explore the use of dendralenes in rapid construction of complex structures (J. Am. Chem. Soc. 2005, 127, 12188). Dendralenes are acyclic cross-conjugated polyenes, a simple example of which is [4]dendralene (shown in red). When used in a Diels-Alder reaction, [4]dendralene may combine with up to three dienophiles; however, such reactivity has not been fully explored because of the dendralene's scarcity. Now, Alan D. Payne, Anthony C. Willis, and Michael S. Sherburn have a method that routinely furnishes 10-g batches of the compound. They show that it reacts with methylmaleimides to yield products incorporating one, two, or three molecules of dienophile, depending on reaction conditions. The reaction shown, which occurs at room temperature, demonstrates the natural-product-like complexity that can be achieved directly with dendralenes.

Impurities thwart crystal growth

Impurities affect the growth of colloidal crystals not necessarily by their mere presence but by their curvature. The finding adds to the literature describing crystal growth of different materials under different conditions, a body of information useful for developers of pharmaceuticals, semiconductors, and polymers. Henk N. W. Lekkerkerker and colleagues at the University of Utrecht, the Netherlands, show that large, hard, spherical poly(methyl methacrylate) impurities with diameters ranging from 7.5 to 30 m interfere with the crystallization of colloidal poly(methyl methacrylate) particles with diameters of 1.5 m (Science 2005, 309, 1231). The authors followed the process with a laser scanning confocal microscope, observing that crystal growth is inhibited as it approaches the impurity. In fact, there is no crystal growth close to the impurity. The effect increases with decreasing impurity size, as the greater impurity curvature "frustrates" the crystal's attempts to order itself.

Extremely short CC bonds

Tetrahedrane, a pyramid-shaped C₄H₄ molecule, is the most strained cage compound known and has very high s orbital character in its hybridized orbitals, leading to short C-C single bonds. Coupling two tetrahedrane molecules together has been predicted to yield a molecule where the linking C-C bond is even shorter--the shortest noncyclic C-C single bond in a saturated hydrocarbon system. But because tetrahedrane is highly reactive, no one has been able to make this tetrahedranyltetrahedrane. Masanobu Tanaka and Akira Sekiguchi of the University of Tsukuba, in Japan, have worked around the problem by coupling more stable trimethylsilyl-substituted tetrahedrane molecules together (shown, R = trimethylsilyl) via a copper-mediated reaction (Angew. Chem. Int. Ed., published online July 25, dx.doi.org/10.1002/anie.200501605). The bond linking the tetrahedrane units is 1.436 long, significantly shorter than the usual 1.54- bond length between sp³ carbon atoms and even shorter than the 1.444 predicted for tetrahedranyltetrahedrane.

Electronic skin for robots

Electronic artificial skin made by integrating thin-film nets of pressure and temperature sensors holds the promise of providing robots with a nearly human sense of touch. Last year, Takao Someya and his colleagues at the University of Tokyo reported on a flexible pressure-sensor matrix made by fabricating an array of field-effect transistors in a polymer film; punching holes in the film area between the transistors to form a net; and then laminating the net with a net-shaped, pressure-sensitive rubber sheet. The team has now added the ability to detect temperature by preparing a thermal-sensor net by a similar process (Proc. Natl. Acad. Sci. USA, published online Aug. 17, dx.doi.org/10.1073/pnas.0502392102). The thermal-sensor net includes a transistor net laminated with an organic semiconductor diode net. The team tested the sensor nets by integrating them on a flat board and simultaneously measuring and mapping applied pressure and temperature. The nets are flexible enough to conform to 3-D surfaces, the researchers note, including robot fingers. With further refinement, they suggest the "E-skin" could also sense light, humidity, and sound.

Progress on prostate cancer treatment

A developmental compound intended to fight prostate cancer has been shown to halt tumor growth in mice. John M. Essigmann, Robert G. Croy, and coworkers at MIT designed and synthesized the compound (shown), which links a steroid to an aniline derivative through a 15-atom-long tether (R) (Chem. Biol. 2005, 12, 779). The compound damages DNA in cells in advanced prostate tumors. The damaged DNA attracts androgen receptors, which are overexpressed in these cells' nuclei. The researchers believe that the receptors block enzymes that would otherwise fix the damaged DNA. At the same time, because the receptors are localized at damage sites, they are unable to carry out their normal role in the tumor cells' growth and survival. The combination of irreparable DNA damage and disruption of survival pathways kills the tumor cells. The dual action makes the compound more effective than previous therapeutics that rely solely on DNA damage, Croy says. Cells can resist those therapeutics by repairing the damage or producing proteins that inhibit cell death.