Furans alkylfurans

Oxidative cleavage of furanes. Alkylfuranes of type 1 are oxidatively cleaved by PCC to cis-1,4-enediones (a), which isomerize in the acidic medium to the more stable trans-isomers (2). These products are converted into 4-methoxycyclo-pentenones (3) in methanolic sodium hydroxide. The yields are lower (50-60%) in this sequence when R1 = H. 

Alkylfurans, halofurans, alkoxyfurans, furfuryl ester and ethers, and furfural diacetate [613-75-2] behave similarly. Furans containing electron withdrawing constituents, for example, furfural, 2-furoic acid, and nitrofurans, fail as dienes even with very strong dienophiles. 

An important extension of these reactions is the Mannich reaction, in which aminomethyl-ation is achieved by the combination of formaldehyde, a secondary amine and acetic acid (Scheme 24). The intermediate immonium ion generated from formaldehyde, dimethyl-amine and acetic acid is not sufficiently reactive to aminomethylate furan, but it will form substitution products with alkylfurans. The Mannich reaction appears to be still more limited in its application to thiophene chemistry, although 2-aminomethylthiophene has been prepared by reaction of thiophene with formaldehyde and ammonium chloride. The use of A,iV-dimethyf (methylene) ammonium chloride (Me2N=CH2 CF) has been recommended for the iV,iV-dimethylaminomethylation of thiophenes (83S73). 

The furan ring can be made to polymerize through one or both of its double bonds and the polymers obtained will therefore have dihydro- and tetrahydrofuran rings in their backbone. This situation occurs when furan, the alkylfurans, benzofuran and some dihydrofurans are treated with suitable initiators and is discussed in the first section of this chapter. 

In conclusion, furan and 2-alkylfurans can be polymerized only by acidic initiators or by y-radiation because the other standard methods of polyaddition fail to induce a chain-propagation reaction. 

Furan derivatives with an aromatic system fused on one of the ring s double bonds, such as benzofuran, naphthofuran etc., can be polymerized cationically through the other ring s double bond. In these polymerizations the complications encountered with furan and alkylfurans [see Section III-A-l-c] are absent because only one unsaturation is available for propagation, the other being tied up in the benzene system... 

A comparison of the cationic polymerization of 2,3-dihydrofurans with that of furan and 2-alkylfurans shows that the complications of the latters two, arising from the dienic character of the monomers, obviously vanish when the monomer is a simple cyclic vinyl ether with just one reactive site, viz. the carbon-carbon double bond. However, it also points out that ring opening in the polymerization of furans by acidic catalysts in the absence of water is unlikely, because otherwise it would also occur to some degree in the polymerization of dihydrofurans. 

The effect of acids on 2-alkylfurans was discussed in Section III-A-l-c). It was remarked that 2,5-disubstituted furans are less reactive in this context than mono-substituted derivatives. This observation again points to the important role played by a free C-5 position in resinification processes. 

Furyllithium reacts rapidly with trialkylborons though the furylborons 91 believed to be formed are not actually isolated. Treated with an electrophile (iodine or A-chlorosuccinimide), they transfer one alkyl residue to the furan ring and eject the boron residue in a reaction well known in other series (Scheme 46). The 2-alkylfurans are indeed produced in excellent yields and the method is better than many other syntheses.236... 

Vitexilactone, obtained from Vi tex cannabifolia (Verbenaceae), has been shown to have the structure (16). It has been correlated with rotundifuran. An interesting iron(ii)-catalysed decomposition of unsaturated cyclic peroxides derived from butadienes leads to 3-alkylfurans. This procedure has been used to convert the peroxide (17) from ds-biformene into the furan (18). Some diterpenoid furans are amongst the constituents of Austroeupatorium inulaefolium (Compositae). These include the diketone austrofolin (19), the corresponding 12-alcohol, and the 15-alcohol (20). A triol, austroinulin (21), was also identified. 

Disubstituted furans can be prepared by the 3-lithiation of 2-phenylthio-5-alkylfurans, followed by reaction with an electrophile and then desulfurization with Raney nickel (Section 3.3.3.8.4). 3-Furylmercuri acetate can be obtained from furan-2-carboxylic acid (CHEC 3.11.3.9) and transformed to other 3-substituted furans via the lithio compound. 

Furan exhibits a single band of medium intensity at 208 nm. Alkylation tends to effect a bathochromic shift of this band to ca. 220 nm as in the many natural products, e.g. difurospinulosin (7), that are /3-alkylfurans. The slight bathochromic shift in the spectrum of [8](2,5)furanocyclophane (69T(25)5357), as compared to 2,5-dimethylfuran, has been ascribed to strain. From the limited data available auxochromic substituents produce little effect on the furan spectrum. 

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. 

Alkylfurans may be prepared by decarboxylation of alkylfurancarboxylic acids or by Wolf-Kishner reduction of furan aldehydes and ketones. The direct alkylation of furan is also useful in obtaining alkylfurans. 

Oxidation of alkylfuranes.5 Furanes substituted by alkyl groups at C2 and C5 arc oxidatively cleaved by PCC to /ran.v-enediones, which are useful precursors to cyclopcnlcnoncs. The ra-isomers are formed originally and are then isomerized by the acidic reagent. 2-Alkylluranes are also oxidized in this manner. 

The distribution of the singlet oxygen between the external solvent and the liposomes can be further assessed by measuring the quenching rates of substrates whose reaction rate is very little solvent dependent. Alkylfuranes fulfill this requirement. Data obtained for several furanes are included in Table 4. 

Since the a-directing power is more pronounced in furan than in thiophene (see Section III,A, 2), the preference for substitution at C-5 is here reinforced. Substitution in 2-alkylfurans actually occurs at this position in all cases and no detectable formation of other isomers is observed.1... 

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