Sigmatropic reanangement

Blechert has developed an interesting synthesis of 2-subsdtuted indoles which involves the conjugate addition of V-phenylhydroxylamine salts (or A(-phenylnitrones) to electron-deficient allenes, followed by carbanion-accelerated hetero-Cope rearrangement of the Michael adduct. For example, addition of the hydroxylamine salt (46) to the allenyl sulfone (47) produces the anion (48), which undergoes rapid 3,3-sigmatropic reanangement to afford the P-keto sulfone (49). Cyclization to the indole proceeds smoothly upon exposure to formic acid (Scheme 3). 

The [3,3] sigmatropic reanangement is a six-electron reaction. No matter how the six electrons are distributed among the two three-atom components, the dominant FMO interaction in the TS is between of one component and ip of the other component. The reaction proceeds suprafacially with respect to both components. 

Ohler, E., and Kotzinger, S., Synthesis of (3-amino-l-alkenyl)phosphonic acids from allylic a- and y-hydroxyphosphonates. Sigmatropic reanangement of dialkyl (l-azido-2-alkenyl)phosphonates, Liebigs Ann. Chem., 269, 1993. 

Sigmatropic reanangements (e.g. ene and Qaisen reactions) arc beyond the scope of this review however, there are specific examples of this transformation, promoted by organoaluminum reagents. 

Allylic sulfides can be transfonned by a 2,3-sigmatropic reanangement into allylic nitrogen derivatives (equation 26) via a sulfimide intermediate (Scheme 36). ° ° ... 

The 3,3-sigmatropic shift of ketene 0-acetals, first developed by Eschenmoser, - preceded studies of the ynamine-Claisen reanangement, where the reactive intermediate is formed at ambient tempera-... 

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