Chemistry

A Guide to Achieving Closure

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Science  27 Jan 2006:
Vol. 311, Issue 5760, pp. 439
DOI: 10.1126/science.311.5760.439a

Epoxides are versatile intermediates in both enzymatic and laboratory syntheses of complex organic compounds. These three-membered rings, composed of an oxygen and two substituted carbon atoms, are strained and can be opened readily by scission of a C-O bond. Moreover, the liberated oxygen can attack another epoxide in the same molecule, forming a larger and more stable cyclic ether in the process.

In general, an epoxide can be opened via attack at either carbon, and substituents introduced to favor one path over the other can prove difficult to remove from the desired product. Simpson et al. have found that the tetracyclic core common to the ladder polyethers (marine natural products associated with red tides) can be prepared efficiently via a base-catalyzed epoxide-opening cascade that is guided by trimethylsilyl substituents. Attack by the oxygen is favored at the silyl-substituted carbon of the adjacent epoxide, yielding the naturally occurring six-membered rings over the kinetically favored five-membered ones. Furthermore, including a fluoride salt in the reaction mixture has the happy consequence of eliminating the pendant trimethylsilyl group after each ring closes. — JSY

J. Am. Chem. Soc., 10.1021/ja057973p (2006).

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