Nitrogen-containing Rings

Rings containing Nitrogen. - The reactions of aziridines with alkylidene-phosphoranes and with phosphorus(III) nucleophiles and the reactions of 

3-amino-2// -azirines with NH-acidic compounds have been reviewed.  

Rings Containing Nitrogen.— The a-position of saturated nitrogen heterocycles may be activated to subsequent C—C bond formation through anodic oxidation, and this method makes tropone alkaloid precursors (217) readily accessible. The synthesis of various 8- and 9-membered lactams is now possible from 

Macrocyclic polyazaheterocycles include the well known spermidine alkaloids, the synthesis of which has attracted a great deal of attention over the past few years. An efficient 6-step total synthesis of the dihydroperiphylline system (218), which has recently been isolated from plant sources, has been reported. 

The method involves successive ring expansion of smaller heterocyclic units. Another paper describes boron-templated cyclizations of small units for the construction of macrocyclic lactams [e.g. (219)]. The fact that the reactions occur 

The first report of an intermolecular amino-alkylation has been published with the synthesis of 3-aryl-4-azidohomo-adamantane (220). The reaction occurs in yields in excess of 90%, presumably via an imine intermediate, and does not proceed without AICI3. 

The novel reaction of 2,5-dimethylbenzoxazole with DMAD for two weeks in the absence of light gives an azepine derivative among a mixture of products.  

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As carbazoles, where a ring containing nitrogen is condensed with one or more aromatic rings, forming neutral compounds. 

Number of ring members Rings containing nitrogen Rings containing no nitrogen ... 

Tertiary amines have been shown to react with isocyanates ia an analogous fashion to form ureas (41—43). Similarly, a2iridines (three-membered rings containing nitrogen) are found to react with isocyanates to yield cycHc ureas. Tertiary amines have also been shown to form labile dipolar 1 1 adducts with isocyanates reminiscent of salt formation. In contrast, formaldehyde acetal aminals form iasertion products with sulfonyl isocyanates (44,45). 

The separate stems for three-, four- and five-membered rings containing nitrogen are used for historical reasons. Those for the different groups of six-membered rings are... 

FIGURE 13-1 Chemical structure of 5-hydroxytryptamine (5-HT serotonin) and related indolealkylamines. The indole ring structure consists of the benzene ring and the attached five-member ring containing nitrogen. 

Pyrimidine a heterocycle, the core of which consists of a single six-membered ring containing nitrogen atoms at positions 1 and 3. 

Imine formation is an important reaction. It generates a C-N bond, and it is probably the most common way of forming heterocyclic rings containing nitrogen (see Section 11.10). Thns, cycliza-tion of 5-aminopentanal to A -piperideine is merely intramolecular imine formation. A further property of imines that is shared with carbonyl groups is their susceptibility to reduction via complex metal hydrides (see Section 7.5). This allows imines to be... 

Now let us now consider pyrrole, where we have a five-membered ring containing nitrogen. Pyrrole is also aromatic. This is somewhat unexpected how can we get six n electrons from just five atoms The answer is that each carbon contributes one electron as before, but nitrogen now contributes... 

Rings Containing Nitrogen with Oxygen and/or Sulfur... 

Phenylcyclopropan recently prepared from cinnamic ester and hydrazine hydrate1. These two substances react to give a compound having a five membered ring containing nitrogen which probably hae this formula OeftsCH - CH - OHg. The pyrazolene. 

Electrochemical reduction of aromatic halides and subsequent intramolecular reaction of the resulting aromatic cr-radical with another aromatic ring have also been shown to be applicable to formation of a six membered hetero-ring containing nitrogen, though the products are not always directly related with natural alkaloids 33). 

The intermediate is an aziridinium ion (aziridines are three-membered rings containing nitrogen—the nitrogen analogues of epoxides). The hydroxide ion chooses to attack only the less hindered terminal carbon 1, and a rearrangement results—the amine has migrated from carbon 1 to carbon 2. 

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