Furans 2 molecules

Badger and coworkers devised a sequential synthesis of [ 18]annulene-l,4 7,10 l 3,16-trioxide which is formally the condensation product of three furan molecules and three ethylenes . The synthesis is illustrated below in Eq. (3.25). The [18]annulene trioxide was obtained as a red solid (mp 215—216 °d) whose proton nmr spectrum showed two peaks of equal area at 8.66 and 8.68 ppm. 

Activation of pyrrole, thiophene, and furan molecules with pentaammineos-mium(n) 97CRV1953. 

The propagation involves the attack of this species onto the C-2 position of another furan molecule leading to the growth of a chain with the following structure ... 

Furfuryl alcohol in an acid medium gives rise to reactions of polycondensation reactions of successive electrophilic substitutions involving furan molecules. This reaction is identical to the reaction described for benzyl alcohol on p.256 and represents the same dangers. It is carried out under the same conditions, ie in a sulphuric medium. The electrophilic species that comes into play is very similar to the benzyl cation. 

Protonation is the simplest form of electrophilic attack in furan it leads to polymerization since the ring can open to polyfunctional products or the protonated species can act as an electrophile and attack another furan molecule, so most work has to be on substituted furans. Even so, the naturally occurring trialkyl furan, petasalbin, is cleaved by so mild a reagent as silica164 ... 

Condensed benzo[i>]furan molecules can be prepared by inter- or intra-molecular Diels-Alder reactions from furo[3,4-b]benzofurans, and some interesting intermolecular examples are listed below. As can be seen, the furo[3,4-i>]benzofuran 60 underegoes Diels-Alder reactions with naphtho-l,4-quinone in the presence of Znl2 as a Lewis acid to form the aromatized cycloadduct. When the diene precursor reacts with benzo-l,4-quinone in the absence of Znl2, the product is obtained as an endo-exo mixture <00JCS(P1)1387>. 

Somewhat distorted furan molecules are also seen in the clathrates [C29H23AsF304W] [PF6] -CaHaO <1999JOM120> and (Ci2Hi2Mn6024) 2 C4H40 (prepared from manganese(ll) formate and furan) <2004CC416>. 

The convenient precursors of unsaturated sugars, dihydrop)Tan or dihydrofuran derivatives, can be also obtained from an appropriately functionalized furan molecule. Two main approaches are used. The first is based on the oxidative rearrangement of furfuryl alcohols (Achma-towicz reaction) [22], the second one involves an acid-catalyzed reaction of aldehydes with 2-trimethyl-silyloxyfuran (Casiraghi reaction) [23] (O Scheme 4). 

A description of the electronic structure of the furan molecule is based on the assumption that all ring atonis are sp -hybridized (see Fig. 5.2a). The overlap of the five 2pz atomic orbitals yields delocalized r-MOs, three of which are bonding and two antibonding. 

Benzynes generated by the fluoride-induced elimination of TMS and OTf groups from o-trimethylsilyl [6]- and [7]-heUcenol triflates selectively undergo an intramolecular Diels-Alder reaction with one of the benzene rings within the hehcenes, rather than an external furan molecule, giving rise to bridged derivatives of coro-nene (Equation 12.42) [86). 

The signals in the region typical of benzenoid compounds indicate that a diamagnetic ring current is induced in the furan molecule. Thus, furan fulfills an important experimental criterion for aromaticity in cyclic conjugated systems. 

A1 1 cage complex of tetragonal coordination capsule 581 with an encapsulated furane molecule (Scheme 5.83) is described in [83] to slowly dissociate in aqueous solution, whereas in the presence of benzene as concurrent guest, it releases furane into bulk solution during 1 h. Addition of maleimide resulted in its Diels-Alder reaction with furane in this solution [83]. 

Aqueous mineral acids react with BF to yield the hydrates of BF or the hydroxyfluoroboric acids, fluoroboric acid, or boric acid. Solution in aqueous alkali gives the soluble salts of the hydroxyfluoroboric acids, fluoroboric acids, or boric acid. Boron trifluoride, slightly soluble in many organic solvents including saturated hydrocarbons (qv), halogenated hydrocarbons, and aromatic compounds, easily polymerizes unsaturated compounds such as butylenes (qv), styrene (qv), or vinyl esters, as well as easily cleaved cycHc molecules such as tetrahydrofuran (see Furan derivatives). Other molecules containing electron-donating atoms such as O, S, N, P, etc, eg, alcohols, acids, amines, phosphines, and ethers, may dissolve BF to produce soluble adducts. 

Manufacture of thiophene on the commercial scale involves reactions of the two component method type wherein a 4-carbon chain molecule reacts with a source of sulfur over a catalyst which also effects cyclization and aromatization. A range of suitable feedstocks has included butane, / -butanol, -butyraldehyde, crotonaldehyde, and furan the source of sulfur has included sulfur itself, hydrogen sulfide, and carbon disulfide (29—32). 

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