D. A. Evans Sigmatropic Rearrangements-2 Chem 206 SIGMATROPIC REACTIONS-FMO-Analysis [1, 5] Sigmatropic Rearrangements(C migration) [1s, 5s]alkyl shift RETENTION a[1, 5] Sigmatropic Rearrangements(H migration) [1a, 5a]alkyl shift= INVERSION disfavored 0090 11, 5](C migration): Stereochemical Evaluation RETENTION nonbonding 230280°C 99 I. Ih 15s Dewar-Zimmerman Analysis: Retention 00000 thermal photochemical View as cycloadditon between following species suprafacial preferred 0 phase inversions Huckel toplogy 6 electrons therefore allowed thermally either or the transiton structureR R R R R H H R R R H H Me Me H H R H H H H R Me Me H H H R H H R H H Me Me D. A. Evans Sigmatropic Rearrangements-2 Chem 206 ■ [1,5] Sigmatropic Rearrangements (C migration) [1s,5s] alkyl shift Þ RETENTION SIGMATROPIC REACTIONS - FMO-Analysis 1 2 3 D/hn R = H, CR3 4 5 1 2 3 4 5 ■ [1,5] Sigmatropic Rearrangements (H migration) [1a,5a] alkyl shift Þ INVERSION disfavored ■ [1,5] (C migration): Stereochemical Evaluation 230-280°C RETENTION [1,5s]H- shift [1,5s]C- shift nonbonding thermal hn photochemical the transiton structure View as cycloadditon between following species: pentadienyl radical + either, or suprafacial preferred Dewar–Zimmerman Analysis: Retention 0 phase inversions Þ Huckel toplogy 6 electrons therefore, allowed thermally 1 3 5 1 5