Benzofurans Claisen rearrangements

The Claisen rearrangement of 2-propynyl aryl ethers yielding benzofurans is well known. In a straightforward extension, on heating 2-propynyl (3-pyridyl) ether (45) in either DMF or decane both furopyridines (46,47) and a pyranopyridine (48) were formed. The product distribution depends on the solvent used. A mechanism which accommodates these results is given in Scheme 9 (78HCA2542). 

The Carroll rearrangement, a variation on the ester Claisen rearrangement, is a useful method for the preparation of y, 8-unsaturated ketones from allyl acetoacetates, and has been adapted to provide a method for the synthesis of a number of specific arylacetones. Thus, treatment of the p-quinol 1 with diketene and a catalytic amount of DMAP at room temperature gave a 72% yield of the arylacetone 2 together with 5% of the benzofuran 3. 

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. 

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. 

Method a Hydroxylated allyl compounds (which can be obtained from allyloxy compounds of type A by Claisen rearrangement form benzofurans or 2,3-dihydroxybenzofurans by ring closiure. 

In the same way, Brdnsted acid catalysts such as trifluoroacetic acid substantially accelerate the Claisen rearrangement of allylphenyl ether. However, the initially formed allylphenols generally react further under the acidic reaction conditions. For instance, crotyl p-tolyl ether (219) in trifluoroacetic acid affords benzofuran 220 as the main reaction product derived from cyclization of the Claisen rearrangement product 221 (equation 151) . ... 

The benzofuran derivatives 215 and 217 were obtained also by Claisen rearrangement of 2-phenylsulfinyl-2-propenyl phenyl ethers 214 (refluxing in mesitylene in the presence of SiOi, 180 °C, 22 h) (equation 97) as well as of aryl /3-chloroallyl ethers 216 (equation 98). These aryl ethers act here as the synthetic equivalents of aryl propar-gyl ethers. 

Palladium-induced cyclization of o-allylphenols (at about 65 °C) usually leads to a mixture of chromenes, benzofurans and their dihydro derivatives, but by judicious choice of carboxylic acid sodium salt, moderately good yields of the predominating 2//-benzopyran may be obtained [2864]. Such reactions have been reviewed [B-41, 3069, 3501, 3505]. 3-Chloropropynyloxybenzene undergoes a Claisen rearrangement and subsequent cyclization when the ether is heated in AT,iV-diethylaniline, but when the chlorine atom is replaced by a... 

Claisen rearrangement of allyl phenolic ethers, followed by oxidation of the alkene generates ortho-hydroxy-arylacetaldehydes which close to give benzofurans under acid catalysis, the example showing the synthesis of 8-methoxypsoralen (33.8.1)7 The formation of 2-substituted benzofurans from 2-(ortho-hydroxyaryl)-ketones is also very easy. ... 

The employment of aryl 2-chloroprop-2-enyl sulfides (or ethers) as thio-Claisen rearrangement substrates neatly eliminates the necessity for an oxidative step, thus providing a route to 2-methyl-benzothiophenes (-benzofurans). " ... 

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

Oxidative degradation of ancistrocladine (1) with KMn04 to the naphthalene carboxylic ester 2, which was independently synthesized, accounted for more than half of the carbon atoms of the molecule and simultaneously demonstrated the site of the linkage to the other molecular moiety. The structure and substitution pattern of the tetrahydro isoquinoline part of 1 became evident mainly from degradation products like the 7-lactone 7, the fully aromatic isoquinoline 4, as well as the benzofurane 9 (see Scheme 2), obtained by a Claisen rearrangement... 

An important plant protection agent based on catechol is carbofuran, manufactured by Bayer diViA FMC, It is obtained by the reaction of catechol with methallyl chloride, followed by Claisen rearrangement at around 200 °C and ring closure to the respective benzofuran, with subsequent reaction of the hydroxyl group with methyl isocyanate in the presence of triethanolamine. (An alternative method of synthesis is based on o-nitrophenol). 

Biichi et al. reported the total synthesis of atrovenetin via a base-promoted site-selective Claisen rearrangement followed by the formation of a benzofuran framework 111 [83]. 

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