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Science Concentrates

December 6, 2004 | A version of this story appeared in Volume 82, Issue 49

Immobilized Ti-calixarenes are active and selective catalysts

Grafting titanium-calixarene complexes onto silica produces effective epoxidation catalysts, according to a study from the University of California, Berkeley. Chemical engineering professors Alexander Katz and Enrique Iglesia and graduate student Justin M. Notestein have shown that the immobilized form of the catalyst (shown) is more than 20 times more active and far more selective than the solution-phase compound in olefin epoxidation reactions using organic hydroperoxides as oxidizing agents [J. Am. Chem. Soc., published online Nov. 24,]. The researchers propose that the multidentate and bulky structure of the calixarene ligand keeps the metal centers isolated from one another during reaction with alkenes. Separating the titanium centers prevents oligomerization, which would lead to formation of unreactive and unselective Ti–O–Ti structures. The team notes that the catalysts are robust and reusable and exhibit long-term stability under ambient storage conditions.

New route to chiral carbon centers

A catalytic method to make chiral carbon centers via enantioselective alkynylation of prochiral CH2 groups has now been put to synthetic use. Zhiping Li and Chao-Jun Li of McGill University, in Montreal, recently showed that a simple copper catalyst can be used to alkynylate sp3 C–H bonds adjacent to a nitrogen atom, yielding a new chiral carbon center. The CuBr catalyst activates both an sp C–H bond of a terminal alkyne and a prochiral sp3 C–H bond of an amine, then enantioselectively couples the two activated bonds together to give a propargylamine. Now Li and Li have demonstrated their method's synthetic utility by using it to make a number of biologically important chiral tetrahydroisoquinoline alkaloids (shown) [Org. Lett., published online Nov. 18,]. They say the new method is more direct and simpler than previous methods for making optically active C-1-substituted tetrahydroisoquinoline derivatives.

Light makes cells stick

Using a photochemical reaction, researchers have devised a practical way to control cell adhesion on a substrate, a key process in cell cultivation for tissue engineering. At the center of the method is 1-(2-nitrophenyl)ethyl 5-trichlorosilylpentanoate (NPE-TCSP). When exposed to light from a standard fluorescence microscope, this compound splits, releasing a 2-nitrobenzyl group. Tohru Takarada, Mizuo Maeda, and coworkers at the Japanese research institute RIKEN and at Kanagawa University use NPE-TCSP to chemically modify a glass substrate. When coated with bovine serum albumin (BSA), the modified substrate forms a surface that does not promote cell adhesion. To make that surface cell adhesive, the researchers irradiate it, causing the 2-nitrobenzyl group to break off, carrying with it the non-cell-adhesive BSA. What remains of the NPE-TCSP is then covered with fibronectin, a cell-adhesion-promoting protein. Various types of cells attach to the resulting substrate and grow [J. Am. Chem. Soc., 126, 16314 (2004)]. By using photomasks, the researchers can pick and choose which parts of the substrate to photoactivate for cell adhesion.

Elders delay bee maturation

In a honeybee colony, older worker bees forage while the younger adults concentrate on tasks within the nest. If foragers are removed from a colony in an experiment that mimics predation, the younger bees mature more rapidly than normal so they can take up the missing bees' task. But if foragers are confined to the hive by simulated rain, their younger hive mates mature more slowly. The older worker bees slow the maturation of younger members of the colony by producing ethyl oleate, according to a research team led by Gene E. Robinson of the University of Illinois, Urbana-Champaign [Proc. Natl. Acad. Sci. USA, 101, 17559 (2004)]. The compound may be circulated in the colony by trophallaxis, "a form of food exchange that also serves as a prominent communication channel in insect societies," the authors note. Ethyl oleate is one of just a few "primer pheromones" that have been identified. Unlike the better known "releaser pheromones," which lead to rapid but transient changes in animal behavior, primer pheromones result in longer lasting changes in behavior and physiology, the researchers write.

'Staircase' fatty acid synthesized

A fascinating new chapter in natural products chemistry was opened two years ago when researchers announced the isolation of a remarkable lipid that contains a chain of five cis-fused cyclobutane rings known as a ladderane (C& EN, Oct. 21, 2002, page 41). That compound, pentacycloanammoxic acid methyl ester (shown), is found in anaerobic ammonium-oxidizing bacteria and has now been synthesized as the racemate by postdoc Vincent Mascitti and emeritus professor of chemistry Elias J. Corey at Harvard University [J. Am. Chem. Soc., 126, 15664 (2004)]. Mascitti tells C&EN that it took 16 steps--and nine months of hard work--to transform cyclooctatetraene into the racemic fatty acid ester. (The natural product's absolute configuration is still unknown.) The route the two chemists took is completely different from previous ladderane syntheses, according to Mascitti, and it could still be improved. For example, a step in which N2 is photochemically extruded from an azo intermediate proceeds in very low yield. "We are trying to figure out how to avoid this step," Mascitti says.


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