Furans, original

The activation of allenes is a rather new, but particularly promising area of gold catalysis.381,400 The first example for such a transformation is the cycloisomerization of allenic ketones 480 to furans 482 which probably occurs via intermediate 481 (Scheme 147). Hashmi et /.401,401a showed that this reaction proceeds much faster when gold(m) chloride in acetonitrile is employed as the precatalyst instead of the traditionally used silver salts (cf. Section 9.12.3.2). The products are usually contaminated by substituted furans originating from a Michael addition of aurated 482 to the substrates 480, thereby indicating that the gold catalyst is also capable to activate C-H bonds of furans. 

The name coumarone, which was previously used for benzo[6]furan, originates from this synthesis. The reaction of the intermediate 3,4-dibromO 3,4-dihydrocoumarin with KOH leading to benzofuran is known as Perkin rearrangement. 

The isomerization of vinyl- or ethynyl-oxiranes provides a frequently exploited source of dihydrofurans or furans, but analogous conversions of vinylaziridines have not been applied so often. While most of the examples in Scheme 87 entail cleavage of the carbon-heteroatom bond of the original heterocycle, the last two cases exemplify a growing number of such rearrangements in which initial carbon-carbon bond cleavage occurs. 

Butin et al. reported that the indole derivative 29 was prepared by treatment of 2-tosylaminobenzylfuran 25 with ethanolic HCl in 78% yield. The furan ring served as the origin of a carbonyl group in this modification of the Reissert procedure. ... 

The spontaneous polymerization of furan adsorbed on carbon black with or without SnCl4 vapours35 has been explained by a similar cationic mechanism. Also, the polymerization of gaseous furan on liquid acidic surfaces35 has the same origin, but in these systems the polymers suffer an acid-catalyzed hydrolysis of their tetrahydrofuran rings which produces a considerable proportion of hydroxyl and carbonyl groups. 

In this section some systems are briefly described which make use of furan derivatives as starting materials for chemical transformations which eventually lead to polymeric products. However, with the exception of Section (c), these final products bear little resemblance to the original furan compounds used and are, therefore, only marginally relevant to the context of the present review. 

The origin of these complications, not common in other polymers, at least on such a short time scale, is connected to the presence of the furan ring or of struc-... 

Maleic anhydride is an important industrial fine chemical (see original citations in [43]). The oxidation of C4-hydrocarbons in air is a highly exothermic process, therefore carried out at low hydrocarbon concentration (about 1.5%) and high conversion. The selectivity of 1-butene to maleic anhydride so far is low. The reaction is composed of a series of elementary reactions via intermediates such as furan and can proceed to carbon dioxide with even larger heat release. As a consequence, hot spots form in conventional fixed-bed reactors, decrease selectivity and favor other parallel reactions. 

Sometimes, hydrogen sulfide converts an oxygen-containing heterocycle into a sulfur-containing one, and as furans and polycyclic furans are very common in nature such reactions may be the origin of the polycyclic thiophenes mentioned above. A typical example is the formation of the isothiazolin-3-thione (6) from the isoxazolin-3-thione (7) on treatment with hydrogen sulfide and hydrogen bromide.8... 

Recently, researchers have detected 2,5-dimethylfuran and 2-methylfuran and normal alkanes in kerogen of the 2.7 x 109 year old Belingwe, Rhodesia stromatolites, by the method of pyrolysis/ GC/MS [26]. They concluded that although furans could probably be derived from many compounds, their probable origin is in bacterial and algal sugars, and that the alkanes are either products of decarboxylation of fatty acids or unaltered constituents of ancient organisms. 

Furthermore, intercepting the furylpalladium(II) species 130 with an electrophile would result in a carbodepalladation in place of protodepalladation. Therefore, a tandem intramolecular alkoxylation of p.y-acetylenic ketone 127 was realized to afford trisubstituted furan 131 when allyl chloride was added to the original recipe [103]. 2,2-Dimethyloxirane was used as a proton scavenger, ensuring exclusive formation of 3-allylated 2,5-disubstituted furan 131 without contamination by protonated furans. 

The student is recommended to carry out the Diels and Alder diene synthesis when making preparations from the original literature. For example, he should condense cyclo-hexadiene with quinone (Annalen. 1933, 507, 288) or furane with maleic anhydride (Ber., 1929, 62, 554). 

Previous studies on various aliphatic and aromatic alkynes have evidenced that this type of furan-2-ones could also be produced in the presence of the rhodium carbonyl cluster [Rh4(CO)i2] [39-41]. Labeling experiments using D2O allowed the authors to assign the origin of the hydrogen atoms (Scheme 6). Noteworthy, dicobalt octacarbonyl follows a different catalytic... 

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