Oxygen-containing pyrans

Table 3.20 lists examples of the preparation of oxygen-containing heterocycles by RCM. Further examples, including lactones [895], pyrans [896,897], chromenes [839], tetrahydrofurans [838], phosphonates [898], and oxepines [856,899-902], have been reported. For references to macrocyclizations see Scope and Limitations in this section. 

The cyclic form of glucose is termed glucopyra-nose, since the new ring system is a reduced form of the oxygen heterocycle pyran. Nucleophilic attack onto the planar carbonyl may occur from either of its two faces, generating two different stereochemistries at this new chiral centre, designated as a or p. This new chiral centre is termed the anomeric centre. Since there are other chiral centres in the molecule, the mixture of a- and -anomeric forms is not a racemate, but a mixture of diastereoisomers (see Section 3.4.4). The mixture does not contain 50% of each anomer (see below). Although both forms are produced, the form with the equatorial hydroxyl is thermodynamically favoured (see Section 3.3.2). 

Amino-4H-pyrans represent a significant class of oxygen-containing heterocycles, whose high biological activity provides broad prospects for their practical applications. 

Zirconium(IV) isopropoxide, 352 Containing one oxygen—tetrahydro-pyrans... 

Examples of photochemically induced im electrocyclizations in oxygen-containing systems are relatively rare. 2//-Phenanthro[9,10-h]-pyran-4-carboxamides (16), for example, have been obtained in this way by irradiation of dienones (17).17 Similarly, the a-pyran (18) is formed on irradiation of ( )-//-ionone (19)18 a triplet excited state is believed to be involved and the reaction proceeds via the Z-isomer (20). The reverse process involving ring opening is more common and can lead to a wide variety of photochemically derived products. Thus, the products of irradiation of 2,2-dimethylchromene... 

Other oxygen-containing systems that have been studied include 2,2,6,6-tetramethyltetrahydropyran-3-one,225 2-alkoxytetrahydropyran-3-ones,226 2-acyl-2,3-dihydro-4W-pyrans,227 cyclic acetals,228 and a wide variety of carbonyl-containing pyranoside derivatives.229... 

The comparison of some oxygen-containing cycles presented in Table 7.11 allows one to reveal a significant effect of the double bond location with respect to the heteroatom. Results with 2,3-dihydrofuran (entry 1) and 3,4-dihydro-2H-pyran (entry 3) show that the double bonds having the nearest location to the oxygen exhibit strong... 

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. 

The abundance of six-membered oxygen containing heterocycles in bioactive natural products continues to encourage the development of new and improved syntheses. There is a vast amount of new literature dedicated to the synthesis of pyrans and their benzo derivatives however, many traditional approaches are still of great value and this chapter should be read in conjunction with the corresponding chapters in the first and second editions of Comprehensive Heterocyclic Chemistry <1984CHEC, 1996CHEC-II>. 

The cyclic hemiacetal that has a five-membered ring is called a furanose. This name is derived from that of tire five-membered, oxygen-containing heterocyclic compound fu-ran. The hemiacetal with a six-membered ring is known as a pyranose after the heterocycle pyran. 

Pyrans constitute another class of oxygen-containing heterocycles that have been prepared from Ni-catalyzed cycloaddition reactions. The coupling of diynes and aldehydes could be mediated by the combination of a Ni(0)... 

The naming of these compounds is a bit odd. Pyran refers to the six-membered oxygen-containing heterocyclic ring system with two double bonds. It is not aromatic though compounds like pyrones are. The compound with only one double bond is therefore dihydropyran, and the saturated ring system istetrahydropyran. 

Possibly the sequence of reaction steps first involves reaction between the aniline and the benzaldehyde to form an imine, which adds- as carbenium ion- to the dihydrofuran or- pyran to form an oxocarbenium ion that alkylates the aromatic ring. In view of the size of the product molecules, the reaction is assumed to occur at the outer surface of the zeolite. In the product the aromatic ring is positioned trans or cis with respect to the oxygen-containing ring. The trans cis ratio varies from 74 26 to 92 8... 

Oxygen heterocycles 10 was reacted with different oxygen-containing condensing reagents to yield 2-amino-3-cyanopyran derivatives. Reaction of arylidenemalonaldehyde 55 gave 2-amino-3-cyano-4 -pyran-5-carboxaldehyde 56. ... 

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... 

Pyran is a six-membered oxygen-containing heterocyclic ring system with two double bonds. 

Figure 11.17. Examples of unsaturated fatty acids. Large numbers of such alkenes both as ( )- and (Z)-isomers exist. The intermediate in the oxidation of arachidonic acid to (only one shown member of the) prostaglandins is part of a cascade of reactions to an entire family of compounds that vary in oxidation state. Leukotrienes (three conjugated double bonds and no five-membered ring) and thromboxanes with six-membered oxygen-containing rings (oxa-cyclohexane, pyran, oxane) are also derived from C20 polyalkenes.

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. 

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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