New synthetic approach to cyclopenta-fused heterocycles based upon a mild nazarov reaction. 2. Further studies on the torquoselectivity

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 alpha-alkoxydienylboronates. These compounds undergo a 4pi 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 delta-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.