Oxygen containing heterocycles furans

In addition to Maillard reactions, caramelization reactions, involving the dehydration and decomposition of sugars, can form a variety of volatile compounds found in roasted tree nuts such as heterocyclic oxygen-containing furan derivatives and maltol. 

Pyrrole, the simplest five-membered unsaturated heterocyclic amine, is obtained commercially by treatment of furan with ammonia over an alumina catalyst at 400 °C. Furan, the oxygen-containing analog of pyrrole, is obtained by acid-catalyzed dehydration of the five-carbon sugars found in oat hulls and... 

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... 

The formation of oxygen-containing heterocyclic compounds is also a consequence of the Maillard reaction. Amines and amino acids have a catalytic effect upon the formation of 2-furaldehyde (5), 5-(hydroxy-methyl)-2-furaldehyde (11),2-(2-hydroxyacetyl)furan (44),2 and 4-hy-droxy-5-methyl-3(2//)-furanone (111) (see Ref. 214). This catalytic effect can be observed with several other non-nitrogenous products, including maltol. The amino acid or amine catalysis has been attributed to the transient formation of enamines or immonium ions, or the 1,2-2,3 eno-lization of carbohydrates. Of interest is the detection of A -(2-furoyl-... 

The < a/o-peroxides of aromatic oxygen-containing five-membered heterocycles such as furan and oxazole are actually ozonides (1,2,4-trioxolanes), and by a reverse dipolar [3+2] cycloaddition they can be a source of carbonyl oxides. 

Heating of carbohydrates produces a number of aromatic compounds Including aldehydes, ketones, and dicarbonyls as well as oxygen containing heterocyclic compounds such as furans, dihydrofuranones, and pyrans through caramelizatlon and dehydration reactions. 

Oxygen-containing heterocycles are always less aromatic than their sulfur and nitrogen counterparts, e.g., imidazole thiazole >> oxazole and pyrazole > isothiazole > isoxazole. These trends follow those of pyrrole, thiophene and furan (Section 2.3.4.2). 1,2,3-Oxadiazole is unknown and all attempts to synthesize this compound have been unsuccessful. Although it is not the least aromatic of the oxadiazoles based on the HOMA index (cf. 1,3,4-oxadiazole), its instability can be attributed to easy isomerization to the acyclic valence tautomer (i.e., 85 - 86). 

Furfural (1), derived from annually renewable agricultural byproducts, is an important industrial chemical manufactured and used throughout the world. It is the feedstock for a number of derivative chemicals generically known as furans -the structural characteristic of which is the five-membered oxygen-containing heterocyclic ring. Furfuryl alcohol (2) is the most important derivative of commerce, where it is used primarily in synthesis of adhesive polymers. 

Furan A five-membered heterocyclic ring containing two carbon/carbon double bonds and an oxygen. 

Hydroalkoxylation of alkynes, or the addition of alcohol to alkynes, is a fundamental reaction in organic chemistry that allows the preparation of enol ethers and a variety of oxygen-containing heterocycles such as furan, pyran, and benzofuran derivatives. Bergbreiter et al. found that a Mnear poly-(A-isopropylacrylamide) (PNIPAM) polymer exhibited inverse temperature solubility in water (i.e., soluble in cold water but insoluble in hot water). A recoverable homogeneous palladium catalyst was prepared based on the polymer. The PNIPAM-bound Pd(0) catalyst was effective for the reaction of 2-iodophenol with phenylacetylene in aqueous THE media to give the target product... 

The higher Au for furanyl is consistent with a greater spin density on the 3-carbon, which would arise through the reduced delocalization in the oxygen-containing heterocycle of furan compared with the sulphur in thiophene. Differences in molecular geometry may also have an effect. 

Angeli was one of the most prolific chemists of his time his research focused on nitrogen chemistry, particularly the oxygen-containing acids of the diazo-compounds, and in the field of natural products. Prescient were his studies of constitution and smell, where he related olfactory activity to oxidation capacity, highlighting the chemistry and physiology of smell. His research in aromatic compounds was original and fundamental. To his work on benzene derivatives, he added research on heterocycles such as pyrrole, furan and indole. 

The most common oxygen-containing heterocyclic alcohol is furfuryl alcohol, which is a degradation product of sugars. Furfuryl alcohol results primarily from furan-2-carbaldehyde by reduction or a Cannizzaro reaction. A large number of other alcohols derived from furan, pyran and other heterocyclic compounds are products of the Maillard reaction. 

Various oxygen containing heterocycles, particularly furan derivatives, have been obtained from various precursors such as diols, allyl, homoallyl and progargyl ethers. Typical catalysts are Pd(II), TiC, or cobalox-ime(III). 

Five- and six-membered oxygen-containing heterocycles include furan, benzofuran, pyran, flavanone, flavonol, and cyclic carbonates, etc. The synthesis of these heterocycles by heterogeneous supported metal catalysts will be discussed here. 

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