Heterocyclics with 3 hetero atoms

The first step is to select a constitutional repeat unit (CRU), which may contain one or more subunits. The name of the polymer is then the name of the CRU, in parentheses prefixed by poly. Before naming the CRU, it must be orientated correctly. This involves placing the constituent parts in order of seniority with the highest to the left. In descending order, this would be heterocyclic rings, chains with hetero atoms, carbocyclic rings, and chains with only carbon atoms, if such an order is possible chemically. 

There is much in common between catalysts for cationic polymerization and the Friedel-Crafts and its analogous reactions. Although the field of Friedel-Crafts reactions has a long history, exploring new catalysts is still a vital area. The purpose of those studies is primarily to realize acylation and/or reactions of substrates with hetero atoms, such as heterocyclic compounds, using a catal)rtic amount of a Lewis acid. Since a conventional add, such as AICI3, forms the complex with a product and/or a substrate, an equimolar amount of the acid to a substrate is required. Consequently, the main product is often accompanied by significant by-produrts in industrially used Friedel-Crafts processes. Thus, reactions with various Lewis adds and solid adds have been reported recently. 

For such purposes as locating substituents, monocyclic heterocycles having only one non-carbon atom are numbered starting with the hetero atom as 1. If there is more than one hetero atom of the same kind, the more saturated hetero atom takes precedence (cf. imidazole in Table 1). In isoxazole and isothiazole, the divalent atom takes precedence, in conformity with the pnaciples of the Hantzsch-Widman system (Section 1.02.2.2). 

Heterocyclic rings may be named by considering them to be derived from hydrocarbons by replacing one or more carbon atoms by heteroatoms. The hydrocarbon corresponding to a particular heterocyclic system is found by formally replacing each hetero atom with... 

Chapters 11 and 12 both deal with C-N-E (E = S, Se, Te) heterocycles. The coverage is limited to ring systems in which the number of hetero atoms exceeds the number of carbon atoms. The topics are organized according to ring size. After a brief section on four-... 

Very few reactions of carbenes with heterocyclic systems containing more than one hetero atom have been studied. They are confined to variants of the Reimer-Tiemann formylation of thiazoles, pyra-zoles, iminazoles, and indolizines/ and ring expansion does not appear to have been observed. 

Prior to 1960 little work had been done on reactions of heterocyclic compounds involving hetarynes, i.e. intermediates with a triple bond in the nucleus containing the hetero atom. Since then interest in hetarynes has grown and investigations in this area are developing rapidly using information available from carbocyclic aryne chemistry. Therefore, a short survey of the chemistry of arynes is presented before summarizing typical problems encountered in hetaryne chemistry. 

Selenazole is the selenium-containing compound in the series of heterocyclic 5-membered ring azoles with two different hetero atoms, of which the first two members are oxazole and thiazole. The numbering of the ring system is according to the scheme given (1). 

In the reactions of electrophilic substitution, isoxazole is far less active than the five-membered heterocycles with one hetero atom and pyrazole. It is closer to pyridine, but more reactive. 

It is of interest that these reactions exhibit stronger activation of the substituent at C-5 by a neighboring electron-acceptor group compared with the corresponding ortho-substituted benzene. This is possibly owing either to the effect of the nitrogen hetero atom or to a weaker delocalization of multiple bonds in the heterocyclic nucleus. 

Interestingly, when the chloro analog was transmetallated and treated with 3-ethoxy cyclohexen-l-one, the expected enone (XI) was not observed, but an enone with a mass of 34 units greater than (XI) was noticed. It also indicated the enone carried the chloro analog. It was presumed that the hetero atoms in the heterocycle present in the starting material (VIII) had performed a directed metallated lithiation providing a different enone bearing the chloro moiety. 

As far as the polymerisation of heterocyclic compounds with one hetero-atom is concerned (cyclic ethers and their analogues) there seems little doubt at present that the propagation involves a displacement at the positive propagating centre. The ring which is part of this -onium ion is opened between the charged atom and a carbon atom next to it, and this becomes attached to the hetero-atom of the monomer ... 

Nitration Instead of a mixture of nitric acid and sulphuric acid, nitration of these three heterocycles is carried out with acetyl nitrate (formed from nitric acid and acetic anhydride). Nitration is in place mainly at one of the carbon atoms next to the hetero-atom. 

Six membered heterocyclics can be synthesized in various ways. Nitrogen analogues of stilbene, like the anilides of benzaldehyde or acetophenone sometimes form azaphenanthrenes 19). Similarly the plasma reaction of azobenzene yields a small amount of diazaphenanthrene 19>. Better results are observed in the cyclization of hydroxy, amino or nitro compounds. Suitable starting materials are molecules with two phenyl groups linked by a carbon or hetero atom. 

Five-Membered Ring Heterocycles with Two Hetero Atoms Imidazoles, Benzimidazoles, 1H-pyrazoles, Oxazoles, Isoxazoles, Thiazoles, and Indazoles... 

The reaction was applied to the synthesis of compounds with prostaglandin-like properties 269). It was also employed for the production of (o,to -dialkoxycarboxylates and -carboxamides271). Dow 272 has extended the reaction principle to a number of 5-ring heterocyclic compounds containing 2 hetero atoms ... 

A short time ago numerous triorganyl-cyclotriphosphanes have become accessible as pure substances on various synthetic routes (2). Moreover, three-membered phosphorus heterocycles with carbon, silicon, germanium, arsenic, boron, and sulfur as hetero ring atoms could be synthesized (2). Recently, we have found that such compounds with elements of the fifth periodtare als stable enough fop existence. Through the reaction of K(Bu )P-P(Bu )K with Bu S C1 or Bu -SnC at -78°C, the diphosphastib rane ... 

We are familiar with heterocyclic ring systems with various elements, ring sizes, and numbers of hetero atoms. Here only systems with Si-Si bonds are discussed. The number of possible systems is very great in this field with the frequent possibility that different isomers are present. Only a small number of such ring systems have so far been synthesized. 

The first heterocyclic silane with boron as a hetero atom was recently prepared280- 281> ... 

The optically active 1,3-oxathiane derivative 81, synthesized as a chiral auxiliary (cf. Scheme 30), has been reported. The isomers were assignend by X-ray diffraction in the solid state trans) and by NOE experiments in solution cis) (97CPB778). Rules for the specification of the absolute configuration of the enantiomers R or S) for 1,3-oxathianes and other heterocycles with at least two hetero atoms were proposed and proved... 

A number of catalytic reactions studied by Russian authors are necessarily left out of this review. These include nitration and sulfona-tion (138). As an example the use of alumina in the synthesis of heterocyclic compounds may be mentioned. Ethylene oxide with ammonia gave pyridine with hydrogen sulfide about 20 % thiophene was formed at 300-450° (211). At 200°C. ethylene oxide and hydrogen sulfide gave thioxane and dithiane (212,429). Yur ev developed conditions under which the hetero atom of furan, thiophene, and pyrrole could be interchanged (425,426,427,428). 

Most importantly, the scope of the Diels-Alder reaction is very high - not only allowing the synthesis of cyclohexenes and 1,4-cyclohexadienes using 1,3-butadienes and alkenes and alkynes, respectively, but also giving access to a multitude of different heterocycles by exchanging the atoms a-d in the butadiene as well as the atoms e and f in the alkene by hetero atoms such as oxygen, nitrogen and sulfur. However, also dienes and dienophiles with several other atoms as phosphorous, boron, silicone, and selenium have been described. Thus, many different heterodienes and heterodienophiles have been developed over the years (Tables 1-1 and 1-2). 

This section is concerned with methods for the ring formation of heterocyclic systems containing one, two or more hetero atoms, with the enamine nitrogen being included or not in the new heterocycle created. 

Five-membered heterocycles with two vicinal chalcogen atoms in the ring system can be used as stable precursors for sulfur as well as for selenium-containing hetero-1,3-dienes in cycloaddition reactions. Consequently, 3//-1,2,4-thiaselenazoles have been used for the in situ formation of 4,4-bis(trifluoromethyl)-l-thia-3-azabuta-1,3-dienes, which exist at room temperature only as 4,4-bis(trifluoromethyl)-2//-l,3-thiazetes. This strategy was applied to the synthesis of the first stable selenophosphorane from bis(trifluoromethyl)-substituted 3//-diselenazol and 2-methoxy-1,3,2-dioxaphospholan [78AG(E)774] (Scheme 83). 

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