5-Substituted furfurals

Examination of the permanganate oxidation of furfural and 5-substituted furfurals, at pH 11.5-13.3 in a stopped-flow apparatus, viz. 

Bisulphite addition compounds are given by all the 5-substituted furfurals with the exception of 2,5-diformylfuran. 5-Ethoxymethyl-furfural forms a diethyl acetal25 and the diacetates of 5-methylfurfural and 5-acetoxymethylfurfural have been obtained.65 The 5-substituted furfurals all condense with amino compounds to give a series of crystalline hydrazones, phenylhydrazones and substituted phenylhydrazones and the semicarbazones, oximes and anilides are also well-characterised derivatives. A feature of the 2,4-dinitrophenylhydrazones is that some... 

The equilibria between the 00-cis- and 00-/ra t-conformers of 3-, 4-, and 5-substituted furfurals have been examined by MP2/6-31G(d,p) calculations. The geometry of the furan nucleus appears little disturbed by the substitution pattern <2003JCC429>. 

The selective e-functionalization of 5-substituted furfurals via trien-amine intermediates was reported. This methodology was successfully... 

Pyran-4-ones bear an obvious structural similarity to the all-carbon cyclohexadienones discussed above. However, the original studies of their photochemical behavior revealed only dimerization processes to produce a cage product resulting from two successive head-to-tail [2 + 2]-photocycloadditions (Scheme 29)54. Much later, small amounts of substituted furfural 121 were observed during the irradiation of 11955a. It was speculated that 121 could arise from bicyclic epoxide 120, an intermediate analogous to those formed in cyclohexadienone photochemistry. Subsequent reports noted that further irradiation of... 

Substituted furfurals do not react at a synthetically-useful rate when formic acid/hydrogen peroxide is used. This suggests that the reaction takes place in the water phase and that substituted furfurals enter this phase only with difficulty. 

Normal carbinol formation occurs when alkyl dihalides are reacted with substituted furfurals in THF [Eq. (3) 23] or when a-haloesters react with a simple aldehyde [Eq. (4) 24]. 

Fructose or other carbohydrates containing hexoses provide a cheap source for the two substituted furfurals 345,346 (Reaction scheme 237) [711]. Catalytic reduction of the acetate 348 cleaves at the heterobenzylic position to give 5-methylfurfural 347. Electrolytic oxidation affords a polyreactive intermediate 349 for future purposes. Weakly acidic thermal treatment causes rearrangement to a norpyrethrolone 350. 

Hydrorefining can substitute for extraction processes such as furfural where it integrates perfectly into the conventional process scheme. 

The oxidative coupling of thiophene, furan[338] and pyrrole[339,340] is also possible. The following order of reactivity was observed in the coupling of substituted furans[338] R = H > Me > CHO > CO Me > CH(OAc)i > CO2H. The cross-coupling of furans and thiophenes with arene is possible, and 4-phenylfurfural (397) is the main product of the cross-coupling of furfural and benzene[341]. 

Table 5 summarizes the reactions of isoprene with aromatic aldehydes and unsaturated aldehydes. Salicylaldehyde provides the expected product as a cyclic boric ester derivative and shows apparently lower stereoselectivity, giving a mixture of 1,3-anti and 1,3-syn isomers in a ratio of 6 1 (run 1, Table 5). 2-Furfural reacts as usual and provides a 1,3-anti isomer as a single diastereomer in good yield (run 2). Unsaturated aldehydes, irrespective of their substitution patterns, undergo homoallylation selectively with excellent 1,3-anti selectivity, the geometry of the double bond of the starting aldehydes remaining intact (runs 3-5). 1,2-Addition to unsaturated aldehyde takes place selectively and no 1,4-addition is observed.

Although highly reactive, 2/7-azirines are of considerable synthetic interest and serve as a source of the 3-fluoro-4//-l, 3-diazepines 86. Reaction of 80 with difluorocarbene in the presence of furfural gave 86, rather than the expected furfural-derived products 83. Rearrangement of the initial 1,3-dipolar intermediate 81 to 84 and then cycloaddition of 84 with 80 are proposed as key steps in the reaction the intermediate cycloadduct 85 gave 86 on base-induced elimination of HF. Nucleophilic displacement of the fluoro group in 86 provided access to further substituted 1,3-diazepines <06TL639>. 

Addition of carbon nucleophiles to furfural tosylhydrazone provides 5-substituted 2E,4E-pentadienyls in good yields <00TL2667>. The ab initio calculations at the RHF/3-21G level have been utilized to study the origins of diastereoselectivity of the vinylogous Mannich reaction of 2-methylfuran with pyrrolinium ion . A simple procedure for isomerization of 2-furylcarbinols to cyclopentenones under neutral condition was reported and a new mechanism was proposed <00H(52)185>. 

Substitution products react like benzaldehyde itself (anisaldehyde gives anisoin). Similarly furfural gives furoin. (Write out the equation.)... 

A recent extension of this methodology includes condensation of 44 with a variety of substituted benzaldehydes, furfurals, and thiophene carboxaldehydes afforded analogues of 48 in good to excellent yield as shown in Table 6.2 (Fig. 6.6). These products were completely characterized spectroscopically with an excellent discussion of the effects of substituents on the corresponding UV, IR, MS, and NMR spectra. Neutralization of 48 with sodium bicarbonate in aqueous ethanol produced the corresponding free bases uneventfully. These compounds were evaluated both as cyanine dyes and for biological activity. Condensation of 44 with ( )-3-(5-methylfuran-2-yl)-2-propenal affords the expected E, )-dienes 49a and 49b in 50 and 90% yield, respectively (Fig. 6.7). 

ButenoUdes. 4-Substituted 2-butenoIides can be prepared by reaction of organolithium reagents with 4-hydroxy-2-butenolide (1), easily prepared from furfural (equation I). The reaction of 1 with 2 equivalents of RLi or ArLi in-THF at... 

Early work on the experimentally established conformational preferences in solution for a variety of other 2-substituted heterocycles is summarized in Table 30. Most of these conclusions have been deduced either from dipole moment measurements in benzene or by the use of lanthanide induced shifts for chloroform solutions. The aforementioned MO studies correctly predict the preferred conformations, (63, R = H) or (64, R = H), of pyrrole-2-carbaldehyde, thiophene-2-carbaldehyde and furfural in the gas phase. 

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