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Sigmatropic)
A Sigmatropic reaction is a pericyclic reaction where the net result is one sigma bond being changed to another sigma bond.
Sigmatropic rearragements are classified by the substituent which moves and the backbone number of the destination substituent, for example a (1, 5) hydride shift.
The most well known of the sigmatropic rearrangements are the Cope rearrangement and Claisen rearrangement .
The hydride and methyl shifts are also sigmatropic reactions. Hydride shifts occur at 4n+1 positions in a suprafacial fashion, and methyl shifts occur at 4n+3 positions in a suprafacial fashion.
Most notably, Cyclopentadiene undergoes a (1, 5) hydride shift. (1, 3) hydride shifts would be antarafacial and are not observed. Antarafacial (1, 7) hydride shifts are observed in the conversion of lumisterol to vitamin D.