1,5-Sigmatropic rearrangements carbanion-accelerated

Blechert has developed an interesting synthesis of 2-substituted indoles which involves the conjugate addition of V-phenylhydroxylamine salts (or V-phenylnitrones) to electron-deficient allenes, followed by carbanion-accelerated hetero-Cc rearrangement of the Michael adduct. For exanple, addition of the hydroxylamine salt (46) to the allenyl sulfone (47) produces the anion (48), which undergoes rapid 3.3-sigmatropic rearrangement to afford the -keto sulfone (49). Cyclization to the indole proceeds smoothly upon exposure to formic acid (Scheme 3). 

Finally, no examples of carbanion-accelerated 1,3-sigmatropic rearrangements appear to have been observed to date, although Okamura and coworkers have reported a transformation which may involve a sulfinyl carbanion accelerated 1,5-hydrogen shift. ... 

Two other examples of carbanion -accelerated rearrangements have been described in the literature. The first one leads from the anion of the 60-allylic guanine 211 in two consecutive [3.3] sigmatropic shifts (a combined Claisen-Cope rearrangement) via 212 and 213 to the 8-allylic guanine 214 173). 

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See also in sourсe #XX -- [ ]

See also in sourсe #XX -- [ ]

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