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New Synthetic Approach to Cyclopenta-Fused Heterocycles Based upon a Mild Nazarov Reaction. 2. Further Studies on the Torquoselectivity

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posted on 2004-10-29, 00:00 authored by Cristina Prandi, Alessandro Ferrali, Antonio Guarna, Paolo Venturello, Ernesto G. Occhiato
Conjugated alkoxytrienes in which one of the double bonds is embedded in a heterocyclic moiety are obtained by the Pd-catalyzed coupling reaction of lactam- and lactone-derived vinyl triflates with α-alkoxydienylboronates. These compounds undergo a 4π electrocyclization process (Nazarov reaction) under acidic conditions and afford cyclopenta-fused heterocycles in good yields. As a continuation of a previous study, the torquoselectivity of this Nazarov reaction has been investigated using 2-alkyl-substituted pyrrolidinone and 2- and 4-substituted δ-valerolactone derivatives. High or complete stereoselectivity has only been observed with the 2-alkyl-substituted heterocycles. Both steric and stereoelectronic effects could contribute to determining the stereoselection of the ring closure.

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