Allylic vinylic ethers

Claisen Rearrangements - allyl vinyl ether to an y,5-unsaturated carbonyl... 

CLAISEN - IRELAND Rearrangment Rearrangement ol allyl phenyl ethers to o (or p-)allylphenols or of allyl vinyl ethers to y.S-unsaturated aldehydes or ketones (Claisen) Rearrangement ol allyl esters as enolale anions to y.S-unsaturated acids (Ireland)... 

Unimolecular reactions that take place by way of cyclic transition states typically have negative entropies of activation because of the loss of rotational degrees of freedom associated with the highly ordered transition state. For example, thermal isomerization of allyl vinyl ether to 4-pentenal has AS = —8eu. ... 

Table 12.3 Claisen rearangement of allyl vinyl ether to form 5-hexenal ...

Rearrangement of allyl vinyl ethers or allyl aryl ethers... 

The Claisen rearrangemenC is a thermal rearrangement of allyl aryl ethers and allyl vinyl ethers respectively. It may be regarded as the oxa-version of the closely related Cope rearrangement. Claisen has discovered this reaction first on allyl vinyl ethers 1, and then extended to the rearrangement of allyl aryl ethers 2 to yield o-allylphenols 3 ... 

Two other important sigmatropic reactions are the Claisen rearrangement of an allyl aryl ether discussed in Section 18.4 and the Cope rearrangement of a 1,5-hexadiene. These two, along with the Diels-Alder reaction, are the most useful pericyclic reactions for organic synthesis many thousands of examples of all three are known. Note that the Claisen rearrangement occurs with both allylic aryl ethers and allylic vinylic ethers. 

Allyl vinyl ethers have been prepared using the ylide (101) but only from non-enolizable carbonyl compounds. The ethers rearrange on heating to give a-allyl aldehydes, e.g. (102). 

The reactants can be made from allylic alcohols by mercuric ion-catalyzed exchange with ethyl vinyl ether.220 The allyl vinyl ether need not be isolated and is often prepared under conditions that lead to its rearrangement. The simplest of all Claisen rearrangements, the conversion of allyl vinyl ether to 4-pentenal, typifies this process. 

Scheme 6.14. Claisen Rearrangements of Allyl Vinyl Ethers and Related... 

Synthetically valuable [2,3]-sigmatropic rearrangements include those of allyl sulfonium and ammonium ylides and a -carbanions of allyl vinyl ethers. 

Grieco observed a facile [3,3]-sigmatropic rearrangement of an allyl vinyl ether in water, giving rise to an aldehyde (Eq. 12.71). 

The corresponding methyl ester similarly underwent the facile rearrangement. A solvent polarity study on the rearrangement rate of the allyl vinyl ether was conducted in solvent systems ranging from pure methanol to water at 60°C.156 The first-order rate constant for the rearrangement of the allyl vinyl ether in water was 18 x 10-5 s 1, compared with 0.79 x 10-5 s 1 in pure methanol. 

An earlier example of this type of domino reaction was reported by Greeves and coworkers (Scheme 2.194) [443]. Treatment of either the ( )- or (Z)- allyl vinyl ether 2-870 with NaH initiates the [2,3]-Wittig rearrangement to afford 2-872 via 2-871. The subsequent oxy-Cope rearrangement led to the aldehyde 2-873, which was reduced with NaBH4 to give the alcohols 2-874. Both isomers of 2-870 predominantly generated the (/ )-xyu-product 2-874 in comparable ratios as the main product. 

Cu-catalyzed domino reactions have been used for the synthesis of carbocycles, as well as for heterocycles such as indoles, benzoxazoles, and quinoxalines. A very useful process is also the combination of the formation of allyl vinyl ethers, followed by a Claisen rearrangement. 

As shown earlier in many examples, the Claisen rearrangement of allyl vinyl ethers also provides a very powerful method for carbon-carbon bond formation in domino processes. Usually, the necessary ethers are formed in a separate step. However, both steps can be combined in a novel domino reaction developed by Buchwald and Nordmann [306]. This starts from an allylic alcohol 6/4-102 and a vinyl iodide 6/4-103, using copper iodide in the presence of the ligand 6/4-104 at 120 °C to give 6/4-105 (Scheme 6/4.25). The reaction even allows the stereoselective formation of two adjacent quaternary stereogenic centers in high yield. 

---