text: The 2,3-Wittig rearrangement is a useful carbon-carbon bond forming reaction that transforms an allyl ether into a homoallylic alcohol.  However, a significant limitation of this reaction is the difficulty in forming the alkoxy-substituted carbanion which initiates the 2,3-sigmatropic rearrangement.  Traditionally, this required the use of a strong base and the presence of a stabilizing group to enable deprotonation of the ether substrate.  In 1978, Still dramatically increased the scope of the 2,3-Wittig rearrangement by introducing an organotin-substituted ether as a precursor to the alkoxy-substituted carbanion.  In this procedure, transmetalation of the organotin group can be achieved chemoselectively at low temperature using n-butyllithium to form an alpha-lithiated ether that undergoes the desired 2,3-sigmatropic rearrangement.  This variation of the traditional 2,3-Wittig reaction is known as the Wittig-Still rearrangement. <br />
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  An application of the Wittig-Still rearrangement in the context of natural product synthesis can be seen in Stills 1979 formal synthesis of the Cecropia juvenile hormone.  The organotin-substituted ether that serves as the substrate for the rearrangement was readily prepared by bis-alkylation of the corresponding diol with iodomethyltributyltin.  Treatment of this compound with two equivalents of n-butyllithium at low temperature resulted in transmetalation, initiating a double 2,3-sigmatropic rearrangement to give the bis-homoallylic alcohol product in 79% yield.  Notably, this rearrangement is highly stereoselective with regard to the geometry of the trisubstituted olefins, giving exclusively the (Z,Z')-isomer shown.<br />
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