Reactions with Five-Membered Ring Systems Containing One Heteroatom

Reactions with Five-Membered Ring Systems Containing One Heteroatom 

The very efficient reaction of furan with benzyne, discovered by Wittig and Pohmer was the earliest example of benzyne behaving as a dienophile.39 Formation of the oxygen-bridged adduct 39 and its acid-catalyzed isomerization into 1-naphthol are often used diagnostically for the detection of benzyne generated by new pathways. 

Tetrahalogenobenzynes have been trapped with furan to give adducts analogous to 39 containing F, Cl, Br, or I the tetrachloro derivative of 44 has also been prepared.50,51 The adducts obtained from furan and a number of other arynes are documented in Ref. 1. 

Wittig, E. Knauss, and K. Niethammer. Justus Liebigs Ann. Chem. 630, 10 (1960). 

Tetrahydrofuran is widely employed as a solvent for reactions involving benzyne, yet it has been shown to be attacked by benzyne (from anthranilic acid diazotized in situ) to give products derived via the betaine 47.52 In the presence of water 48 and 49 are obtained (23% and 8%, respectively), whereas under anhydrous conditions the main product is 50 (17%) by reaction of 47 

Benzyne adds to the furanophane 40 to give successively 1 1 and 2 1 adducts the latter (41) probably has the anti configuration. 1,3-Diphenyl-benzo[c]furan (43) is also an efficient trap for benzyne, and the adduct 44 gives 9,10-diphenylanthracene in diglyme at 162°C, or on reduction with zinc and acetic acid. - Some other instances of the loss of the bridging oxygen atom of furan adducts occur when the aryne in question is generated from an 

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V. Reactions with Five-Membered Ring Systems Containing One Heteroatom. . 194... 

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