From Polyenes To Polycycles

A new protocol for preparing polycyclic compounds enantioselectively promises to make it easier to build these pharmaceutically relevant scaffolds (J. Am. Chem. Soc., DOI: 10.1021/ja310386m). The reaction, developed by Erick M. Carreira and coworkers at the Swiss Federal Institute of Technology, Zurich, uses an iridium-based catalyst and a zinc triflate promoter to coax the cascade reaction of polyenes (example shown). To begin the cascade, a racemic allylic alcohol within the polyene structure reacts with an iridium catalyst. This interaction appears to dictate the stereochemistry of the first cyclization. Subsequent cyclizations are effectively encoded in the first ring, the researchers believe. Using the protocol, Carreira and colleagues prepared nine polycyclic compounds—including heterocycles—in good yields, with enantiomeric excesses reaching 99%. “It is important to note that all the reactions are conducted at ambient temperature with commercial-grade solvents and reagents, making this cyclization protocol convenient to perform,” the chemists point out.