Furans other simple furanic compounds

In the many recorded examples of the reaction, and because of its very nature, reports have tended to concentrate on compounds derived from a range of simple carbonyl compounds and a single (or at most two) dialkydiaryP or diheteroaryP hydrogenphosphonate or, alternatively, on combinations of a selection of hydrogenphosphonates with a relatively few carbonyl compounds, including propanal, benzenoid aldehydes " furan and thiophene aldehydes 3-formylindole ° 2- and 3-formyl-chromones " , diethyl oxomalonate and others . It is worthy of comment that... 

More than 70 degradation products have been identified. In addition to simple compounds, such as hydrogen sulfide, ammonia, formaldehyde, acetaldehyde, formic acid and acetic acid, dozens of other minor sulfur compounds are formed aliphatic sulfides, aliphatic carbonyl compounds with a thiol group, furans containing sulfur in the molecule (including the previously mentioned meaty aroma character imparting compound 2-methyl-3-furanthiol), thiophenes, thiazoles and alicycHc and heterocycHc bicycHc sulfur... 

Several modifications of the Feist-Benary furan synthesis have been reported and fall into two general classes 1) reactions that yield furan products 2) reactions that yield dihydrofuran products. One variant that furnishes dihydrofiirans uses substrates identical to the traditional Feist-Benary furan synthesis with a slight modification of the reaction conditions. The other transformations covered in this section involve the combination of P-dicarbonyls with reagents that are not simple a-halocarbonyls. Several reactions incorporate a-halocarbonyl derivatives while others rely on completely different compounds. 

Aside from alcohols, other oxygen nucleophiles have also participated in hydroalkoxylation reactions with alkynes. The most common of these are 1,3-dicarbonyl compounds, whose enol oxygens are readily available to add to alkynes. Cyclization reactions of this type have been carried out under Pd(0) catalysis with various aryl or vinyl iodides or triflates, often in the presence of CO, affording the corresponding furan derivatives (Equation (95)).337-340 A similar approach employing cyclic 1,3-diketones has also been reported to prepare THFs and dihydropyrans under Pd, Pt, or W catalysis.341 Simple l-alkyn-5-ones have also been isomerized to furans under the influence of Hg(OTf)2.342... 

Other 2-substituted cyclopropylideneacetates of type 3-X also entered this cycloaddition (Scheme 15) [19]. The endolexo selectivity is low but usually still higher than that of simple acrylic acid esters. The relative Diels-Alder reactivities of dienophiles 1-Me and 3-X as determined by competition experiments (Scheme 15) suggest a mechanism involving either diradicals or zwitterions as intermediates [19]. Surprisingly, the 2-fluoro derivative 3-F is less reactive than the parent compound 3-H. The 2-chloro and 2-bromo derivatives 1-Me and 3-Br have similar reactivities and cycloadd to furan (57) about 16 times faster than methyl acrylate. 

Some other natural compounds have been transformed for their use in the synthesis of polymers via olefin metathesis processes. As mentioned in the introduction, furans, which are obtained from carbohydrates, are perfect precursors of monomers for ROMP via simple Diels-Alder cycloadditions (n) (Scheme 25) [26]. In this regard, the first example of the ROMP of 7-oxabicyclo[2.2.1]hept-5-ene derivatives was reported by Novak and Grubbs in 1988 using ruthenium- and osmium-based catalysts [186]. The number of examples of ROMP with monomers with this generic structure is vast, and it is out of the scope of this chapter to cover all of them. However, it is worth mentioning here the great potential of a renewable platform chemical like furan (and derived compounds), which gives access to such a variety of monomers. 

This behavior may be due to different reasons. First, the different aromaticity of the compounds could play an important role to define the reactivity of the compounds. Furan is the lowest aromatic pentaatomic heterocyclic compound known, while the other compounds show higher aromaticity. However, this type of explanation cannot justify why thiophene does not react while simple dimethylthienyl derivatives react and why some dimethylthienyl derivatives react while some others do not show any reactivity. 

Systems where multiple heteroatom substitution is present are handled by the same general approach as used for bicychc systems we find the largest ring system that has a simple name and then specify the point of attachment of other rings. We observe in one example that follows, a fusion of furan at its 3,4- position with the parent cinnoline. As before, the numbers outside the rings are the final numberings for all members of the compound. 

Since heteroaromatic compounds sometimes exhibit interesting physical properties and biological activities, construction of substituted heteroaromatics has drawn some attention. Heteroaromatics can be divided into two major categories. One is the tt-electron-sufhcient heteroaromatics, such as pyrrole, indole, furan, and thiophene those easily react with electrophiles. The other is the 7r-electron-deficient heteroaromatics, such as pyridine, quinoline, and isoquinoline those have the tendency to accept the nucleophilic attack on the aromatic ring. Reflecting the electronic nature of heteroaromatics, the TT-electron-deflcient ones are usually used as the electrophiles.t The rr-electron-sufficient heteroaromatics having simple structures, such as 2-iodofuran and 2-iodothio-phene, have also been utilized as the electrophiles. Not only the electronic nature of the heteroaromatics but also coordination of the heteroatom to the palladium complexes influence catalytic activity. This is another reason why the couphng reaction did not proceed efficiently in some cases. 

---