[3-UNSATURATED ACYLSILANES WITH ENOLATES OF ALPHA,BETA-UNSATURATED METHYL KETONES - SCOPE AND MECHANISM(4] ANNULATION OF ALPHA,BETA)

Reactions of the E and Z isomers of (beta-(trimethylsilyl)acryloyl)(te rt-butyl) with lithium enolate of alpha,beta-unsaturated methyl ketone s at -80 to -30 degrees C afford cis-5,6- and trans-5,6-disubstituted 3-cyclohepetenones, respectively. The same [3 + 4] annulation is obser ved in the reaction of (beta-(tri-n-butylstannyl)acryloyl)silanes. The annulation products are readily transformed into 4-cycloheptene-1,3-d ione by treatment with NBS pr mCPBA. The observed stereospecificity in the annulation is explained by the reaction pathway that involves an anionic oxy-Cope rearrangement of 1,2-divinylcyclopropanediol intermed iate generated via Brook rearrangement of the 1,2-adduct of a lithium enolate. Isolation of vinylcyclopropanol derivative from the reaction of (beta-(tri-n-butylstannyl)acryloyl)silanes with lithium enolate of 2'-bromoacetophenone and its transformation into cycloheptenone deriva tive with LDA provide strong support for the proposed mechanism. Furth er support is obtained from the reactions of 1,2-divinylcyclopropyl ac etates with 2 equiv of MeLi affording cycloheptenones stereospecifical ly. Also, beta-alkyl-substituted acryloylsilanes and cycloalkenylcarbo nylsilanes are found to Participate in the [3 + 4] annulation.