Ring Opening Reactions of Furans

Ring cleavage in the furan system is usually obtained by catalytic hydrogenolysis. Examples are given in Section IV of addition reactions, and in Section VII of conversion reactions. Oxidative ring opening has already been described in a part of Section IV. 

Nonoxidative Ring Opening 1. Acid-Catalyzed Ring Fission 

The acid fission of furans, to yield 1,4-dicarbonyl compounds, is not satisfactory from the preparative point of view. This fission reaction has recently been restudied.169, 287-292 It is preferable to use the route involving 2,5-dialkoxy-2,5-dihydrofurans, which on hydrogenation and acid hydrolysis give the desired 1,4-dicarbonyl compounds. 

Clauson-Kaas and J. T. Nielsen, Acta Chem. Scand. 9, 475 (1955). 

Luke and J. Srogl, Collection Czech. Chem. Comrnun. 26, 2238 (1961). 

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The chemical consequences of /3-protonation are illustrated further by the ring-opening reactions of furans with methanolic hydrogen chloride and of (V-substituted pyrroles with hydroxylamine hydrochloride (Scheme 11) (82CC800). 

Ring-opening reactions of furans can be initiated (a) by means of Bronsted acids in aqueous medium or (b) by means of oxidative transformations [17]. 

Ring opening reaction of alkylidenecyclopropanone acetals readily proceeds in the presence of Lewis or Bransted acids to produce l-alkylidene-2-oxyallyl cation, which is provided for the reaction with nucleophiles such as chloride, alcohols, siloxyalkenes, and furans. The reaction of this cation with the carbon nucleophiles gives products of [4 + 3] and [3 + 2] cycloaddition as well as those of nucleophilic addition. The modes of addition reactions are controlled by the oxy group of the cation and by the reaction conditions including solvent. 

A series of a,a-dimethoxysilyl enol ethers has been prepared and shown to undergo diastereoselective [4+3]-cycloaddition with furan and cyclopentadiene in the presence of catalytic amounts of trimethylsilyl triflate <99SL213>. Furo[3,4-d]oxazoles and furo[3,4-d]thiazo-les react with 13-dimethyloxyallyl to give [4+3]-cycloadducts. The ring opening reaction of these compounds with H2S04Mt20 yields armulated hydroxytropones <99H(51)1225>. 

The ring-opening reactions of lithiated derivatives have been reviewed comprehensively. A well-known example of this latter possibility is the ring-opening of 3-lithiobenzo[ ]furan (401) to the lithium salt of 2-ethynylphenol (402) (78CHE353). 

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