Rings containing More than One Heteroatom

Boyd and B. J. Walker, in Aliphatic, Alicyclic, and Saturated Heterocyclic Chemistry , ed. W. Parker (Specialist Periodical Reports), The Chemical Society, London, 1973, Vol. 1, Part II, p. 474. 

Wieringa, J. Strating, H. Wynberg, and W. Adam, Tetrahedron Letters, 1972, 169. 

A dioxetan intermediate (123) has been isolated from the ozonolysis of ethylidenecyclohexane in pinacolone and appears to be formed via reduction of the molozonide (124) by solvent. On heating, (123) gave cyclohexanone and acetaldehyde, which are the normal ozonolysis products. 

The diazetidine (125) is slowly formed in the reaction of tm j -2,3-dimethyl-1-methylenecyclopropane with 4-phenyl-l,2,4-triazoline-3,5-dione. Retention of the cyclopropane ring in (125) contrasts with the products of the analogous reaction of the allene (126). A variety of ring systems, including 

A stepwise mechanism is also suggested for the reaction of 4-phenyl-l,2,4-triazoline-3,5-dione with vinyl esters The proposed intermediate (136) 

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Five-membered rings containing more than one heteroatom behave in a more complex way as regards aminomethylation. Imidazoles and thiazoles 85 react at 4 and 5 posi-tions, " - although reactions of thiazole with arylamines occurring at position 2 are reported. ... 

This chapter provides an update of Chapter 8.33 in CHEC-II(1996) <1996CHEC-II(8)863>. The work carried out on the bicyclic ring systems with ring junction P, As, Sb, or Bi has focused primarily on phosphorus. Very little work has been done on the other heteroatoms and as such the synthesis and reactivity of these compounds have been reviewed as one section, Section 12.12.7. Most of the compounds in this class contain more than one heteroatom, the additional atoms usually being oxygen and nitrogen. 

Two-ring heterocycles containing more than one heteroatom Purines... 

The organometallic compounds, obtained by aromatic lithiation reaction have been used for the synthesis of various heterocyclic compounds containing more than one heteroatom in a ring or metal atoms in the ring. These types of compounds available by the method are included in a recent review... 

Ring Systems containing more than one Heteroatom... 

Large systems contain ring atoms enough to allow more than one way of bonding between a given sequence of atoms, and a wealth of valence isomerizations becomes possible. Already for seven ring members, two types of bicyclic isomer are observed, namely the [4.1.0] and the [3.2.0] systems, each with positional ring isomers if more than one heteroatom is present. 

Heterocyclic compounds, or heterocycles, are molecules with rings that contain more than one type of atom they are to be distinguished from carbocycles, which are molecules with rings that contain only carbon atoms. The heterocyclic compounds of greatest interest to medicinal chemists have carbon rings containing one or two heteroatoms— atoms other than carbon. 

Space restrictions mean that the reactivity of multiheteroatom systems or fused systems where both rings are heterocyclic cannot be covered in this section - the reader is referred to the relevant CHEC volumes. Space again dictates that the chemistries of oxygen- and sulfur-containing six-membered heterocycles, and the chemistry of monocyclic six-membered heterocycles with more than one heteroatom, are only briefly indicated alongside the description of pyridine/quinoline/isoquinoline chemistry, but especially where these are not shown by the pyridine prototypes, but again the reader should study the CHEC volumes for a full discussion. The inclusion of an extra heteroatom in a six-membered system exaggerates the effect of the first and so often it is possible to predict properties by extrapolation however, the same is not true for the five-membered systems, so these heterocycles with more than one heteroatom are considered in detail and separately. 

Many heterocycles containing two or more heteroatoms can exist in more than one tautomeric form. Both external (polarity of the solvent, intramolecular hydrogen bonding) and internal (relative resonance stabilization of the conjugated system, stabilities of the functionalities) factors influence such tautomeric equilibria. For five- and six-membered rings, frequently more than one tautomer preserves the aromatic character. Comparisons of tautomeric equi-... 

Heterocyclic compounds make up the third and largest class of molecular frameworks for organic compounds. In heterocyclic compounds, at least one atom in the ring must be a heteroatom, an atom that is not carbon. The most common heteroatoms are oxygen, nitrogen, and sulfur, but heterocyclics with other elements are also known. More than one heteroatom may be present and, if so, the heteroatoms may be alike or different. Heterocyclic rings come in many sizes, may contain multiple bonds, may have... 

Topics in Heterocyclic Chemistry presents critical accounts of heterocyclic compounds (cyclic compounds containing at least one heteroatom other than carbon in the ring) ranging from three members to supramolecules. More than half of the more than 10000 compounds hsted in Chemical Abstracts are hete-rocychc compounds. The branch of chemistry deahng with these heterocyclic compounds is called heterocyclic chemistry, which is the largest branch of chemistry and as such the chemical hterature appearing every year as research papers and review articles is vast and can not be covered in a single volume. 

We have looked at the five-membered aromatic heterocycles pyrrole, furan and thiophene in Section 11.5. Introduction of a second heteroatom creates azoles. This name immediately suggests that nitrogen is one of the heteroatoms. As soon as we consider valencies, we discover that in order to draw a five-membered aromatic heterocycle with two heteroatoms, it must contain nitrogen A neutral oxygen or sulfm atom can have only two bonds, and we cannot, therefore, have more than one of these atoms in any aromatic heterocycle. On the other hand, there is potential for having as many nitrogens as we like in an aromatic ring. 

Heterocycle A compound which contains a closed ring system in which the atoms are of more than one kind. Sulfur, nitrogen, and oxygen are common heteroatoms found in petroleum heterocycles. 

Tetralin-type structures containing attached aromatic and aliphatic rings with very short aliphatic side chains seem to fit the structural data best. Except for MD-2 and MD-3, which average slightly more than one half oxygen atom per molecule, the molecules do not contain significant amounts of heteroatoms (i.e., N, S or 0), moisture or inorganics. However, compared to petroleum-derived fuels, they do contain substantial amounts of N and 0. 

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