Other Nitrogen Heterocyclics

Various endo-imino cyclic ethers (L) undergo cationic polymerization to yield poly(A-acylalkyleneimine)s (LI). The most widely studied monomers are the 2-substituted 2-oxazo-lines (m = 2) (also referred to as 2-substituted-l,3-oxazolin-2-enes) [Culbertson, 2002  

Goethals, 1989a,b Kobayashi and Saegusa, 1984, 1986 Kobayashi and Uyama, 2002 Kobayashi et al., 1990a,b Kubisa, 1996 Tomalia and Killat, 1985]. Propagation proceeds via nucleophilic attack of monomer on the C—O bond of an oxazolinium ion  

LI can be hydrolyzed to the corresponding polyamine LII. This is the only route to linear polyethyleneimine (m — 2) [Tanaka et al., 1983]. The polymerization of ethyleneimine yields a highly branched and cyclized product. 

An alternate approach to forming polymers by ring opening of 2-oxazolines involves the 2-mercaptoalkyl-2-oxazoline LIV, which polymerizes to LVII on heating [Gunatillake 

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Naphthalene and other fused ring compounds are so reactive that they react with the catalyst, and therefore tend to give poor yields in Friedel-Crafts alkylation. Heterocyclic rings are also tend to be poor substrates for the reaction. Although some furans and thiophenes have been alkylated, a true alkylation of a pyridine or a quinoline has never been described.However, alkylation of pyridine and other nitrogen heterocycles can be accomplished by a free radical (14-23) and by a nucleophilic method (13-15). 

Pyrrole derivatives substituted in positions 1-, 3-, or 4- have also been electrochemically polymerized (positions 2- and 5- must be free for polymerization). Besides homopolymers, copolymers can also be prepared in this way. Other nitrogen heterocycles that have been polymerized by anodic oxidation include carbazole, pyridazine, indole, and their various substitution derivatives. 

In common with many other nitrogen heterocycles azepin-2-ones and their benzo and dibenzo derivatives exist solely as the amide tautomers (76AHC(S1)554). 

Yields versus aryl halides are moderate to good. Other nitrogen heterocycles such as 2-chloro-5-trifluoromethylpyridine or 2-chloropyrimidine were also coupled with p-MeOCgHjZnBr in 33% and 54% yields, respectively. Under the same conditions, no coupling product was observed with 3-chloropyridine. This specific behavior of 2-chloropyridine as compared to aryl halides can be explained by the presence of the nitrogen atom close to the carbon—halogen bond. 

Illi130 extended the work of Bocchi et a/.128 to the N-sulfonylation of indole a method that could be used as well with other nitrogen heterocycles described in this section. 

N- Methylation and quaternization have been reported to occur in only a few other nitrogen heterocycles. For example, N- methylnicotine (56) and N- methylcotinine (57) are... 

This section and the section on alkaloids in the first survey (7) are artificially small since many halogenated alkaloids are presented in the sections on pyrroles, indoles, carbolines, tyrosines, and other nitrogen heterocycles. It might be noted that the very large number of brominated alkaloids that are obviously tyrosine-derived are now included in Sect. 3.22.3 (Tyrosines). 

The specific proposal is that the hydroxide attacks the 2- or the 4-position of the co-ordinated pyridine to form a hydroxy-substituted 1,2- or 1,4-dihydropyridine (Fig. 8-17). These hydroxy species are known as pseudo-bases. This behaviour is fully in accord with the known behaviour of A-alkylpyridinium cations. Although the pseudo-bases of simple A-alkylpyridinium cations are not dominant solution species under aqueous conditions, those derived from a variety of other nitrogen heterocycles are readily formed and are well-known. The suggestions had the advantage of linking the apparently divergent fields of heterocyclic and co-ordination chemistry by explaining some well-documented anomalies in the reactivity of pyridine complexes. 

Therefore, this review is mainly devoted to the application of electroorganic reactions to the synthesis of some key skeletons which are commonly found in natural alkaloids and some key intermediates which are useful materials for the formation of alkaloidal structures. The synthesis of some other nitrogen heterocycles which are usually not found in alkaloidal compounds will not be part of this review even though the structures are highly interesting as targets in organic synthesis. 

The following correlations developed from the sodium borohydride reductions of pyridinium ions can be extended to explain the products of reduction of the other nitrogen heterocycles ... 

Electron Density. Electron density calculations for 1,6-naphthyridine have been made for comparison with those of other nitrogenous heterocycles and for the rationalization of ionization phenomena and electronic spectra.676 840 1126 1173 Electron Spin Resonance. Radicals derived from 1,6-naphthyridine have been studied,1329 especially with respect to their ESR spectra.1083 Infrared/Raman Spectra. Assignments for the major bands in IR/Raman spectra of 1,6-naphthyridine and related compounds have been reported.44 1124 1251 Ionization. Theoretical calculations for the protonation of 1,6-naphthyridine have been carried out for comparison with those of the other naphthyridines and related heterocycles.806 813... 

There is little to report on the application of molecular orbital calculations to this class of heterocycles. Yamaguchi et al. <91AX(C)590> have observed that the semiempirical AMI method underestimates the double bond character of the bicyclic C—C bond in 2-phenyl-1,2,3-triazolo[4,5-e][l,2,3,4]tetrazine (24) (Table 1). The AMI method has been reported to overestimate bond lengths in other nitrogen heterocycles <92JCS(P1)2779>, and this is possibly attributable to an overestimation of core-repulsion energies for multinitrogen systems. 

A wide range of other nitrogenous, heterocyclic solvents has been used as solvents for the preparation of sucrose surfactants by ester exchange. These include N-methyl- and various N-acyl-morpholines, 272,328-330 jV-formyl- and N-acetyl-piperdine,272 N-formyl- and N-acetyl-... 

Of particular importance are oligopyridine ligands such as 2,2 6, 2",2" 6", 2""-pentapyridine. These molecules allow synthesis of helical, inter-finked and knotted molecular arrays.1 7 Similar species with interlocking rings are the catenands and torands.m Some examples of oligopyridines are (9-XVIII) and (9-XIX). Other nitrogen heterocycles are the following ... 

These compounds satisfy the criteria for aromaticity (planar, cyclic n system, and the Huckel number of 4n -I- 2 71 electrons) pyrrole, imidazole, indole, pyridine, 2-methylpyridine, pyrimidine, and purine. The systems with 6 7i electrons are pyrrole, imidazole, pyridine, 2-methylpyridine, and pyrimidine. The systems with 10 7i electrons are indole and purine. The other nitrogen heterocycles shown are not aromatic because they do not have cyclic 7i systems. 

Quinoline was chosen for the denitrogenation experiment with the assumption that its behavior is representative of many other nitrogen heterocyclic compounds present in syncrude from tar sand and oil shale. 

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