Allyl rearrangement prototropic

This reaction, for which the term prototropic rearrangement is sometimes used, is an example of electrophilic substitution with accompanying allylic rearrangement. The mechanism involves abstraction by the base to give a resonance-stabilized carbanion, which then combines with a proton at the position that will give the more stable olefin 56... 

A prototropic allylic rearrangement took place during the Dimroth rearrangement of 8-allyl-5-benzyl-7-methyl-l, 2,4-triazolo[4,3-c]pyrimidine... 

On treatment with camphorsulfonic acid/pyridine, methyl ( )-1-[(.S )-4-mcthylphcnylsuirinyl]-2-alkenoates undergo, enantioselectively, a sequential prototropic shift and allylic sulfoxide/ sulfenate rearrangement to produce methyl (R,/f)-4-hydroxy-2-alkcnoates in 64- 72% optical purity63a. 

In systems such as (241) which are capable of prototropic change, basic nucleophiles could cause preliminary rearrangement to (242). The migration of the double bond places the leaving group in the reactive allylic position, and its replacement becomes very facile. The primary substitution product (243) is the allylic one, but further prototropy could form the vinylic substitution product (244) formally derived directly from (241) (equation 24). This route, which requires highly... 

As mentioned before, allenes can be formed by prototropic rearrangement of alkynes or, if an appropriate hydrogen is present in the allylic position, by direct elimination. The bromovinyl ether yields, in a trans elimination, the alkyne ether (Scheme 39), but the other isomer, where trans elimination is not possible, gives both the alkynyl ether and the allenyl ether (Scheme 40). This corresponds to the problem of Hofmann and Zaitsev orientation in alkene synthesis. 

Double bond migration. Price and Snyder found that by virtue of its special solvent effect, DMSO greatly accelerates the prototropic rearrangement of allyl ethers to c/s-propenyl ethers under catalysis by potassium /-butoxide. Cunningham... 

The attempt to synthesize pyrroles via prototropic isomerization of O-allyl ketoximes to 0-(0-l-propenyl) ketoximes, followed by their [3,3]-sigmat-ropic rearrangement, failed [316,317]. Instead, in the case of O-allyl cyclohexanone oxime (KOH/DMSO or Bu OK/DMSO, 50°C-140 C, 1-6 h), a mixture of 2-methylene-l-cyclohexanol, 5,6,7,8-tetrahydroquinoline, and cyclohexanone in... 

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