Alkylfuran synthesis, furan

Bamford-Stevens reaction of the tosylhydrazones of the readily available tetrahydrofuran-3-ones provides a useful synthesis of 2,3-dihydrofurans which may be dehydrogenated to furans with 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (66CJC1083). Tetrahydrofuran-2-ones (y-butyrolactones) may be alkylated in the 3-position with LDA and an alkyl halide. The products on reaction with phenyl selenylchloride and LDA, and subsequent oxidation, yield 3-alkylfuran-2(5//)-ones reducible with DIBAL to furans (75JOC542). 

Alkylfurans are readily prepared by ring synthesis. The cyclization of substituted 1,4-diketones in the presence of acid to yield alkylfurans is a particularly useful reaction (see Section 3.12.2.2). Appropriately substituted oxiranes behave as masked 1,4-diketones and on acid catalysis cyclize to yield the substituted furan. 

The hydrolysis (or alcoholysis) of furans involves nucleophilic addition of water (or alcohol) to an initially formed cation, giving rise to open-chain 1,4-dicarbonyl compounds or derivatives thereof This is in effect the reverse of one of the general methods for the construetion of furan rings (section 15.14.1.1). Succindialdehyde cannot be obtained from furan itself, presumably because this dialdehyde is too reactive under conditions for hydrolysis, but some alkylfurans can be converted into 1,4-dicarbonyl products quite efficiently, and this can be viewed as a good method for their synthesis, of cyclopentenones derived from them. Other routes from furans to 1,4-dicarbonyl compounds are the hydrolysis of 2,5-dialkoxytetrahydrofurans (section 15.1.4) and by various oxidative procedures (section 15.2). 

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