Silicon‐Directed Nazarov Reactions III. Stereochemical and Mechanistic Considerations

The influence of remote substituents on the stereochemical outcome of electrocyclization in the silicon‐directed Nazarov reaction has been examined. While the degree of stereocontrol was modest (ca. 3:1) the substituent in the major isomer (4,5 or 7‐substituted cis‐hexahydroind‐2‐en‐1‐ones) is always cis to the protons on the ring fusion. Thorough spectroscopic and conformational analysis revealed that divergent senses of electrocyclization are responsible for the observed products. Additional experiments suggest that steric, rather than stereoelectronic forces control the sense of cyclization. A qualitative description of the nature of reactive intermediates in the silicon‐directed Nazarov reaction is proposed as well as an explanation for the remarkable efficacy of FeCl₃ for inducing the reaction.