[3 + 4] Annulation of α,β-Unsaturated Acylsilanes with Enolates of α,β-Unsaturated Methyl Ketones: Scope and Mechanism

Reactions of the E and Z isomers of (β-(trimethylsilyl)acryloyl)( tert -butyl)dimethylsilanes with lithium enolate of α,β-unsaturated methyl ketones at −80 to −30 °C afford cis-5,6- and trans-5,6-disubstituted 3-cyclohepetenones, respectively. The same [3 + 4] annulation is observed in the reaction of (β-(tri- n -butylstannyl)acryloyl)silanes. The annulation products are readily transformed into 4-cycloheptene-1,3-dione by treatment with NBS or mCPBA. The observed stereospecificity in the annulation is explained by the reaction pathway that involves an anionic oxy-Cope rearrangement of 1,2-divinylcyclopropanediol intermediate generated via Brook rearrangement of the 1,2-adduct of a lithium enolate. Isolation of vinylcyclopropanol derivative from the reaction of (β-(tri- n -butylstannyl)acryloyl)silanes with lithium enolate of 2‘-bromoacetophenone and its transformation into cycloheptenone derivative with LDA provide strong support for the proposed mechanism. Further support is obtained from the reactions of 1,2-divinylcyclopropyl acetates with 2 equiv of MeLi affording cycloheptenones stereospecifically. Also, β-alkyl-substituted acryloylsilanes and cycloalkenylcarbonylsilanes are found to participate in the [3 + 4] annulation.