Propargyl-Claisen rearrangement ethers

Scheme 10.24 Propargyl vinyl ether Claisen rearrangement.

Another abnormal Claisen rearrangement is the product formed on heating phenyl propargyl ether. The normal product O-allenyl phenol rearranges by a [1, 5] hydrogen shift and then there is an electrocylic ring closure to give chromene which is the observed product. 

The Claisen rearrangement of propargyl vinyl ethers directly delivers the allene no equilibrium is observed. This reaction was also successful with complex substrates in order to show this, of numerous examples [375, 513-536], the compounds 159 [537] and 161 [538] are depicted (Scheme 1.71). 

Thermal propargyl rearrangement-Claisen rearrangement of propargyl vinyl ethers and Cope-type rearrangements of propargyl malonates [93]. 

The pyrolytic reactions of pentafluorophenyl and heptafluoro-2-naphthyl propargyl ethers give product mixtures containing benzo- and naphtho[6]furan derivatives, resulting from initial Claisen rearrangement followed by further intra- and intermolecular reactions.31 34... 

The propargyl ether 1 was heated in refluxing dimethylaniline for 5 hours in the expectation that Claisen rearrangement followed by cyclisation would lead to the linear tricyclic system. The only product - isolated in only 12.5% yield - was, however, the angular tricyclic ketone 2. 

Rearrangement of aryl propargyl ethers.1 Claisen rearrangement of the naphthyl propargyl ether 1 at 215° results in the benzopyran 2 as the only isolable product (40% yield). Addition of 1 equiv. of CsF results in the benzofuran 3 as the major product, presumably formed via an a-allenyl ketone (a). Related salts such as KF, RbF, or BaF2 are completely ineffective. This modified Claisen rearrangement provides a route to o-hydroxy aldehydes such as 4 from a phenol. 

In another related and well-known [3,3]-sigmatropic shift usually performed under thermal conditions, the propargyl-Claisen rearrangement,62 silver salts were also able to catalyze the reaction. Silver tetrafluoroborate and hexafluoroantimonate proved to be the best catalysts for this reaction, leading quantitatively to allenic p-ketoesters when starting from propargyl ethers derived from p-ketoesters (Scheme 3.41).63 64... 

The silver-catalyzed, propargyl Claisen rearrangement has also been combined with cyclization reactions. Propargyl ethers derived from [3-ketoesters were rearranged by silver hexafluoroantimonate and the resulting allenic [3-ketoesters cyclized on treatment with base, leading to the formation of 2/7-pyrans in moderate to excellent yields (Scheme 3.48).75 The cyclization process was described as a base-catalyzed... 

Dihydro-2-hydroxypyrans are formed from propargyl vinyl ethers in a Au(I)-catalysed Claisen rearrangement - heterocyclisation sequence in wet dioxane. Stereochemical features in the substrate are retained in the product. Incorporation of an alkanol function adjacent to the O atom results in the formation of spiroketals (Scheme 3) <06JA8132>. [3-Hydroxyallenes... 

Propargyl naphthyl ethers can be rearranged to to naphthofurans in shorter reaction times when exposed to microwave irradiation (Scheme 89) <1996JCM338>. These alkynes first undergo Claisen rearrangement to form an allene intermediate which in turn spontaneously cycloisomerizes to the benzofuran product. 

---