Benzofurans propargyl ethers

Intramolecular [4+2]-cycloaddition reactions, which involve base-induced isomerization of a propargyl ether to an allenyl ether, have been extensively studied. Treatment of 157 with a base caused an intramolecular Diels-Alder reaction of the intermediate allenyl ether to give tricyclic compounds 158 [131]. An asymmetric synthesis of benzofuran lactone 159 was achieved by an analogous procedure [132],... 

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 contrast to the allyl ethers (87) the analogous propargyl ethers were cyclized in poor yield ( 30%) to the corresponding benzofuran using the same conditions as in Eq. (52) [245]. 

When a nucleophilic functionality is present at ano-position with respect to the leaving group, cyclized products (e.g., benzofurans) are obtained. Another variant is that starting from alkynes represented by o-hydroxyphenyl propargyl ether, whereby the formation of fZ)-2-arylidene-l,4-benzodioxanes is accessible. ... 

Benzofurans. 2-Chlorophenyl propargyl ether stirred and heated 3 hrs. at 230° with K-carbonate in sulfolane -> 7-chloro-2-methylbenzofuran. Y 67%. F. e., also chromenes, and without bases, s. N. Sarcevic, J. Zsindely, and H. Schmid, Helv. 56, 1457 (1973) in the presence of Ag-fluoroborate s. ibid. 56, 2981. 

On the other hand, an allyl ether can also be considered a protected alcohol that is liberated by Pd(0)-catalyzed allylation for further catalytic transformation. As an excellent example for sequential homobimetallic catalysis, Gabriele and coworkers [85] just recently disclosed a Pd(0)-catalyzed deprotection-Pd(II)-catalyzed heterocyclization of allyloxyaryl propargylic alcohols 97 to provide benzofurans 98 in excellent yields (Scheme 33). 

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. 

Propargyl aryl ethers undergo a Claisen rearrangement and then ring closure to produce 2-methyl-benzofurans directly. ... 

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