Benzofurans rearrangement

Isotubaic acid — see Benzofuran-5-carboxylic acid, 4-hydroxy-2-isopropyl-Isouramil occurrence, 3, 144 5-Isoxalones potentiometry, 6, 11 Isoxanthopterin, 6-acetonyl-structure, 3, 276 Isoxanthopterin, 3,8-dimethyl-rearrangements, 3, 309 Isoxanthopterin, 6-methoxy-3,8-dimethyl-synthesis, 3, 297 Isoxanthopterin, 6-methyl-bromination, 3, 301 Isoxanthopterin, 8-methyl-synthesis, 3, 319 Isoxanthopterin, 6-phenacyl-structure, 3, 276... 

Scheme 2 demonstrates that [3,3] rearrangements of hydroxylamines, oximes or hydroxamic acids are a versatile synthetic tool to prepare different heterocyclic rings such as indoles, imidazoles, benzofurans and oxazolidines which are useful synthetic precursors of natural products. 

The [3,3]-sigmatropic rearrangement of O-aryl oximes was first described by Sheradsky and it has been used as an adequate route to benzofuran ring, a structural unit largely present in natural products. 

One potential approach for the preparation of benzofuran derivatives involves the acid-catalyzed cyclization of O-aryl oximes 1193. 39 (equation 36). The transformation is analogous to well-known Fisher indole synthesis. The reaction proceeds through a [3,3]-sigmatropic rearrangement of enehydroxylamine 120 to compound 121 followed... 

A new synthetic method of benzofuran was reported (equation 39). The [3,31-sigma-tropic rearrangement of Af-trifluoroacetyl enehydroxylamines 136 obtained in situ by acylation of oxime ethers 135 in the presence of trifluoroacetic anhydride lead to the synthesis of cyclic or acyclic dihydrobenzofurans 138. The effects of base and temperature on the reaction products were studied. A similar pathway to that of Fisher indolization was proposed. The acylimine formed by the [3,3]-sigmatropic rearrangement of the V-trifluoroacetyl enehydroxylamine 136 gave the dihydrobenzofuran 137 by an intramolecular cyclization or the benzofuran 138 after elimination. 

A similar synthesis of mechanistic interest rather than preparative value involves the thermal reaction of dimethyl 2,5-bisdiazo-3,4-diketoadipate (89, Scheme 23) with benzofuran (91)." The presumed intermediate is the pyrone cation 90 produced from the adipate 89 by the Wolff rearrangement, cyclization, and loss of nitrogen. Electrophilic substitution then affords the benzofuran 92, which can be isolated. Ring opening and cyclization of the resultant ketene 93 then affords the dibenzofuran 94 in poor (0.4%) yield. 

Foo, L.Y. et al., Proanthocyanidins from Lotuspedunculatus, Phytochemistry, 45, 1689, 1997. Steynberg, P.J. et al.. Acid-catalyzed rearrangements of flavan-4-phloroglucinol derivatives to novel 6-hydroxyphenyl-6a,llb-dihydro-677-[l]benzofuro[2,3-c]chromenes and hydroxyphenyl-3,2 -spirobi[dihydro[l]benzofurans], J. Chem. Soc., Perkin Trans. 1, 2395, 1997. 

Perkin Rearrangement (Coumarin-Benzofuran Ring Contraction)... 

Cyclodehydration of l-[4-chloro(or bromo)phenoxy]-l-phenylacetones (87) gives benzofurans (83)256 with rearrangement, both with chemical... 

The carbonyl compound (161), obtained by hydrolysis of 3-benzoyl-2(3//)-benzofuranone (160),407 gives the benzofuran derivative (150) (R1 = H, R3 = COOH, Ar = Ph). The same occurs with the product of the abnormal Chapman rearrangement 395 of iV-phenylbenzimino o-(carbomethoxymethyl) phenylether. 

The same intermediate occurs in the Lehmann method463 for converting the methiodides of phenolic Mannich bases into 2,3-dihydrobenzo-furans, through reaction with dimethyloxosulfonium methylide. Thus, o-benzoquinone methide (207) leads to 2,3-dihydrobenzofuran according to Scheme 4. Similarly, 2-naphtholgives l,2-dihydronaphtho[2,l-6]furan through 208 and 209, formed and rearranged in situ.i6i The reaction has been applied to the synthesis of polycyclic benzofurans.483,485... 

The Niki reaction474 (alkylation of dimedone by l,4-dibromo-2-methyl-2-butene, with formation of compound 218), applied to phloro-acetophenone and to 3-isopentyl-2,4,6-trihydroxyisobutyrophenone, leads respectively to 2,3-dihydrobenzofurans 219a and 219b. Furan 219b with toluene- >-sulfonic acid rearranges to the benzofuran 220b.475... 

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). 

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