Chemistry

Building a Staircase

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Science  03 Mar 2006:
Vol. 311, Issue 5765, pp. 1215
DOI: 10.1126/science.311.5765.1215b

Despite carbon's propensity to adopt a tetrahedral bonding geometry, chemists have managed over the years to squeeze it into a wide range of strained shapes, such as cubes and dodecahedra. However, it was remarkable to find that anaerobic Candidatus “Brocadia anammoxidans” bacteria, which are presumably more concerned with function than geometry, produce a fatty acid derivative in which the acyl chain is tethered to five cyclobutane rings, fused through shared edges as in a staircase. Despite an estimated strain energy of 75 kcal/mol, this molecule is a primary component of the intracellular membrane in which ammonia is metabolized.

Mascitti and Corey previously synthesized this compound in racemic form and have now achieved an efficient asymmetric synthesis, in which the C8 carboxylate chain is bound to one specific external corner of the staircase motif. The authors achieved enantioselection through the use of a bulky dimethylphenylsilyl group, which directed cyclopentenone orientation in the photoinduced [2+2] cycloaddition that formed the fourth cyclobutane ring. In general, the synthesis relied heavily on cyclizations and rearrangements induced by ultraviolet irradiation. How the bacteria make this molecule (presumably in the dark) remains a mystery. — JSY

J. Am. Chem. Soc. 128, 10.1021/ja058370g (2006).

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