Oxygen containing heterocycles

It was possible to obtain the desired canthine 51 exclusively, simply by increasing the MW reaction time from 5 to 60 min at 180 °C. The ratio of 50 to 51 could be increased from 9 1 to 1 2. Further increasing the temperature to 220 °C, which is 100 °C above the boiling point of HOAc, for 40 min improved the selectivity to 1 19 in favor of canthine 51. 

In previous sections, MW-accelerated MCRs were shown to be efficiently catalyzed by several insoluble inorganic supports. Similar results were obtained when mild 

Another multicomponent procedure used to construct oxygen containing heterocycles under MW conditions is the synthesis of pyrano[2,3-d]pyrimidine 56 [55]. 

This mixture was irradiated for 5 min at 75 °C to furnish the desired pyrano[2,3-d]pyrimidines 56 in good yield (75-85%) [55]. 

The reaction was performed in an unmodified domestic MW oven with an irradiation time of 10-14 min at 560 W and furnished good yields (73-86%) of the final product 58. Diastereoselectivity strongly favored the syn isomers ( 96 4). 

Substituted tetrahydropyrans are key structural fragments in numerous natural products, and this has prompted much research into their synthesis. Among the many known methods, we will focus on the Maitland-Japp reaction and its modernization 

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Heterocyclic systems, such as the substituted indoline illustrated below,13 may also be constructed via cyclization of unsaturated organolithiums and a recent review of the preparation of nitrogen- and oxygen-containing heterocycles via this approach is available.14... 

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 key step in Biichi s total synthesis of loganin (5i)23 made use of the expansion of a four-membered ring to an oxygen-containing heterocycle by retro-aldol reaction. This strategy was later exploited by Tietze in his total synthesis of hydroxyloganin (54) as well as hydroxyloganic acid (55) 24). 

Fig. 31.2 Oxygen-containing heterocyclic compounds found in AG-secretions (A) and/or in colostrum (C)...

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. 

Table 3.20. Formation of oxygen-containing heterocycles by ring-closing metathesis.

Table 4.7. Preparation of five-membered, oxygen-containing heterocycles by intramolecular carbene 1,5-C-H insertion.

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-... 

Conformational Analysis of Saturated Six-Membered Oxygen-Containing Heterocyclic Rings... 

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

A significant portion of the sulfur- and nitrogen-containing species in crude oil can be found in heterocyclic form within the asphaltene, maltene, and resin compounds. Oxygen-containing heterocycles may also be present. Examples of high-molecular-weight aromatic, resinous, and polar compounds found in crude oil are provided in TABLE 3-1. 

The competition between attack by solvent at C-6 (intermediate 78) and attack at C-5 by the hydroxyl group on C-2 (intermediate 77) explains these results, which show the extent to which closure of an oxolane ring is favored. In principle, a third (unobserved) possibility exists, namely, closure of a six-membered, oxygen-containing heterocycle as a result of opening of the epoxide at C-6 by the hydroxyl group on C-2. 

Among the heterocyclic compounds, there are aromatic, e.g. pyridine, as well as nonaromatic, e.g. tetrahydrofuran, compounds. Similarly, there are saturated (e.g. tetrahydrofuran) and unsaturated (e.g. pyridine) heterocyclic compounds. Heterocycles also differ in their ring sizes, e.g. pyridine has a six-membered ring, whereas tetrahydrofuran is a five-membered oxygen-containing heterocyclic compound. 

Data on l70 NMR spectra of five-membered oxygen-containing heterocycles are scarce. 170 chemical shifts, 80 (with respect to H2nO), near 350 ppm have been observed for isoxazoles and 280 and 303 ppm for 1,2-dioxolane (124) and its bridged analogue (125) (85JOC4484). 

TT-Electron delocalization in isoxazole seems to be more effective than in oxazole however, isothiazole is less aromatic than thiazole thus it is not a general rule that 1,2-diazoles possess higher aromaticity in comparison with 1,3-diazoles. Oxygen-containing heterocycles are always less aromatic than their sulfur and nitrogen counterparts, e.g. thiazole > imidazole > > oxazole. At the same time, the relative aromaticity of S- and N-containing heterocycles can interchange (pyrazole > isothiazole > isoxazole). 

Advances in the Synthesis of Oxygen-containing Heterocycles K. Fukui, Yuki Gosei Kagaku Kyokai Shi, 1966, 24, 741-754. 

Paths of Biogenesis of Natural Oxygen-Containing Heterocyclic Compounds N. S. Vul fson and L. S. Chetverikova, Tr. Botan. Inst., Akad. Nauk SSSR, Ser. 5, Rast. Syr e, 1965, no. 12, 31-43. 

General Properties of Oxygen-Containing Heterocyclic Compounds R. Rambaud, Traite Chim. Org., 1945, 18, 36. 

A variety of pathways give rise to the nitrogen-and oxygen-containing heterocyclic rings of nature. 

Examples of the formation of oxygen-containing heterocycles by processes involving [6 + 2]383 and [8 + 2]384 cycloadditions have also been described. 

Unlike carbocyclic and oxygen-containing heterocyclic systems, catalytic enantioselective synthesis of eight-membered ring amines proceeds efficiently and with excellent enantioselectivity. These catalytic ARCM reactions can be carried out in the absence of solvent as well. Representative data regarding cat-... 

Here is the generation of a 7-membered oxygen-containing heterocycle. 

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