Oxygen-nitrogen heterocycles

Substances that contain one or more atoms other than carbon as part of a nng are called heterocyclic compounds Rings m which the heteroatom IS oxygen nitrogen or sulfur rank as both the most common and the most important... 

Addition compounds form with those organics that contain a donor atom, eg, ketonic oxygen, nitrogen, and sulfur. Thus, adducts form with amides, amines, and A/-heterocycles, as well as acid chlorides and ethers. Addition compounds also form with a number of inorganic compounds, eg, POCl (6,120). In many cases, the addition compounds are dimeric, eg, with ethyl acetate, in titanium tetrachloride-rich systems. By using ammonia, a series of amidodichlorides, Ti(NH2) Cl4, is formed (133). 

The heteroaromatic compounds can be divided into two broad groups, called n-excessive and n-deficient, depending on whether the heteroatom acts as an electron donor or an electron acceptor. Furan, pyrrole, thiophene, and other heterocyclics incorporating an oxygen, nitrogen, or sulfur atom that contributes two n electrons are in the rr-exeessive group. This classification is suggested by resonance structures and confirmed by various MO methods. ... 

Ruonne atoms in aromatic nitrogen heterocycles are readily replaced by oxygen nucleophiles [77] Bistnfluoromethyl hydroxylarmne anion is an mterest-ing nucleophile for the mtroduction of oxygen mto perfluoropyndine Rearrangement of the product occurs at 125 °C [18] (equation 12)... 

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... 

The majority of sequential radical reactions deal with cyclizations as the key steps. The constructions of carbocycles, oxygen, and nitrogen heterocycles using (TMSlsSiH as a mediator are many and represents the expansion and importance of these synthetic approaches. For example, Nicolaou and coworkers found that (TMSlsSiH serves as a superior reagent in the radical-based approach toward the synthesis of azadirachtin, an antifeedant agent currently used as an insecticide, and in other related systems. ° ° Here below we collected a number of reactions mostly from the recent work in the area of intramolecular reactions. 

The interstellar dust was shown to contain quinone derivatives as well as oxygen-rich condensed aromatic compounds the quinones were present in both hydrated and carboxylated form. Very little nitrogen was present in the compounds detected. The cometary material, however, contained condensed nitrogen heterocycles. Hardly any oxygen was detected in the solid phase of the cometary dust it possibly evaporates from the tail of the comet in the form of water or oxidized carbon compounds. The authors assume that these analytical results could lead to a reconsideration of the current biogenesis models (Kissel et al 2004 Brownlee, 2004). 

An extensive study of the conformational behavior of perhydroazepine 66 was carried out by Espinosa and coworkers105. This pure force field based analysis, with no reference to experiment or other theoretical methods, was part of a series of studies on cycloheptane, cycloheptene and some of their oxygen, sulfur and nitrogen heterocyclic analogs. MM2... 

The electrodecarboxylation of heterocycles having a carboxylic group in a-position to an heteroatom (oxygen, nitrogen) can lead to the corresponding methoxylated product when the electrolysis is performed in methanol [143,184-189], as shown in Scheme 101. 

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-... 

In oximes 305 with a nucleophile anti to the oxime oxygen atom, the cyclization is endo and a nitrogen heterocycle 306 is formed. These cyclizations are extremely useful in natural product syntheses. 

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