Heterocycles without a Functional Group

Structure of Five-membered Rings with One Heteroatom 

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Where a polymer has functional groups in the main chain, typical of polymerization by step-reaction chemistry or chain polymerization of heterocyclic monomers, the functional groups in the chain are often the weakest and may undergo reactions which do not produce radicals. Thermal degradation by random scission, without significant yield of volatile compounds is typical of such polymers. Examples include polyesters, polyamides and polyurethanes, all of which undergo thermolyis by reaction at the functional groups ... 

Diels-Alder reactions are one of the most fundamental and useful reactions in synthetic organic chemistry. Various dienes and dienophiles have been employed for this useful reaction.1 Nitroalkenes take part in a host of Diels-Alder reactions in various ways, as outlined in Scheme 8.1. Various substituted nitroalkenes and dienes have been employed for this reaction without any substantial improvement in the original discovery of Alder and coworkers.2 Nitrodienes can also serve as 4ti-components for reverse electron demand in Diels-Alder reactions. Because the nitro group is converted into various functional groups, as discussed in Chapters 6 and 7, the Diels-Alder reaction of nitroalkenes has been frequently used in synthesis of complex natural products. Recently, Denmark and coworkers have developed [4+2] cycloaddition using nitroalkenes as heterodienes it provides an excellent method for the preparation of heterocyclic compounds, including pyrrolizidine alkaloids. This is discussed in Section 8.3. 

Six-six condensed heterocyclic systems without a tautomeric functional group(s) [for instance, pteridine 135 (R = R2 = R4 = H)], do not exhibit prototropic tautomerism. The introduction of an oxo(thioxo)- and/or amino group(s) into the pteridine system, and the appearance of at least one NH group in the ring, leads to functional and/or annular tautomerism. Pteridine is formally the parent of three groups of compounds of particular interest because of their biological importance pterins (136), lumazines (137), and flavins (138). 

Ammonium salts with two different alkyl chains were prepared directly via subsequent alkylations of dimethylamine with primary bromides and crystallization. Commercial hexadecyl-methylamine can be conveniently applied in the same way in order to convey functionality to cationic synkinons. A recent example describes subsequent alkylations with a small functional and a long-chain primary bromide (Scheme 2.4). A-acylated / -phenylenediamine was also alkylated at the second nitrogen atom which had two different alkyl chains, with or without extra functionality . After deacylation, this head group can be diazotized or coupled oxidatively with various heterocycles in water (Scheme 2.4). Photoactive and coloured membrane surfaces are thus obtained. Phenylene-diamine, pyridine and in particular A-methyl-4,4-bipyridinium chloride are relatively weak nucleophiles. Substitution of bromides is slow and the more reactive iodides can rarely be obtained commercially, but the selection of nitromethanes as solvent for bromide substitution is of great help as well as the addition of sodium iodide to enforce a Finkelstein reaction or a combination of both. 

In cases other than the aforementioned heterocycle syntheses (see Section 1.17.9), the final step of a synthetic sequence employing azaallyl metal reagents derived from hydrazones is removal of the hydrazone group. This can be accomplished by several methods listed in Table 1 without affecting other functional groups in the molecule. The potential effects of oxidation, strong acid and racemization at any... 

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