Aromatic 6-Membered Compounds

There are two distinct classes of compounds that fit the criteria mentioned above alkene-functionalized chalcone derivatives (Fig. IB) and enone-functionalized chalcone derivatives (Fig. 1C). Within each class, both aromatic and non-aromatic compounds exist. Those compounds functionalized at the alkene include i) 3-membered heterocycles, e.g., epoxide and aziri-dine compounds, ii) 5-membered aromatic derivatives including fused and non-fused compounds, and iii) 6-membered aromatic pyrazine compounds. The enone-functionalized compounds include i) 5-membered aromatics such as pyrazole and isoxazole compounds, ii) 5-membered non-aromatic compounds for example pyrazolines and isoxazolines, and iii) 6-membered non-aromatics where a discussion of heterocyclic and non-heterocyclic compounds will be given for completeness. 

Membered Non-Aromatic Ring Compounds a) Pyrazoline Compounds... 

On the basis of topological criteria, Balaban predicted in 1965 the aromaticity of compounds with 10-jr-electron systems in eight-membered rings with two heteroatoms (analogous to the cyclooctatetraene dianion).250 Schroth and co-workers were among the first to investigate the systems experimentally, and they synthesized compounds 210—213 (Scheme... 

The preparative importance of the acyloins depends on the fact that they are intermediate products, from which many 1 2-diketones can be obtained. The simplest aromatic member of this group is benzil (anisil and furil are analogous) like its aliphatic prototype diacetyl CH3.CO.CO.CH3 (and like anhydrous glyoxal) it is yellow in colour. Diacetyl is obtained from methyl-ethyl ketone via the monoxime of the former compound (von Pechmann). It is remarkable that diacetyl condenses to p-xyloquinone. (Formulate). 

Pyrimidine is a six-membered aromatic heterocyclic compound that contains two nitrogen atoms, separated by a carbon atom, in the ring. Nucleic acids, DNA and RNA, contain substituted purines and pyrimidines. Cytosine, uracil, thymine and alloxan are just a few of the biologically significant modified pyrimidine compounds, the first three being the components of the nucleic acids. 

Although the number of Diels-Alder cycloadditions with open-chain and alicyclic dienes is very large, the number of examples with aromatic heterocyclic compounds is relatively small. The introduction of a vinyl group as a substituent onto a heterocycle increases the number of possibilities of reaction. This new possibility, however attractive for synthetic purposes, is successful, with a few exceptions, only with 7r-excessive five-membered heterocyclic derivatives. As is usual in this kind of reaction, Michael additions, ene reactions, [2 + 2]-cycloadditions, and polymerization compete with the Diels-Alder cycloaddition. 

It is also well known from the classic work of L. Wohler and Wenzelberg [7] that the sensitivity to impact of aromatic nitro compounds increases with increase in the number of substituents for a given member of the nitro groups. 

The naming of these compounds is a bit odd. Pyran refers to the six-membered oxygen-containing heterocyclic ring system with two double bonds. It is not aromatic though compounds like pyrones are. The compound with only one double bond is therefore dihydropyran, and the saturated ring system istetrahydropyran. 

Hydrogenative cyclization of aromatic nitro compounds at the amino group has been described in a number of examples and widely utilized in the synthesis of five- and six-membered /V-heterocyclic compounds. Walker synthesized 5,6-dimethoxyoxin-dole and 5,6-dimethoxyindole by hydrogenative cyclization of ethyl 4,5-dimethoxy-... 

In Section 3.4.1.1 the reactivities of the major types of azole aromatic rings are briefly considered in comparison with those which would be expected on the basis of electronic theory, and the reactions of these heteroaromatic systems are compared among themselves and with similar reactions of aliphatic and benzenoid compounds. Later, in Sections 3.4.1.23.4.1.10 these reactions are reconsidered in more detail. Reactions of nonaromatic five-membered compounds with more than one heteroatom are considered in Section 3.4.2 and reactions of substituents attached to aromatic azoles are covered in Section 3.4.3. The reactions of azoles can only be rationalized and understood with reference to the complex tautomeric and acidbase equilibria shown by these systems. Tautomeric equilibria are discussed in Chapter 2.4. Acidbase equilibria are considered in Section 3.4.1.3 of the present chapter. 

Bases, Neutral Salts.— As a base it forms salts, in which form the diazo compound is obtained by diazotization, and which though also unstable has been isolated in small quantities and the composition and properties determined. Of the three salts, the sulphate, chloride and nitrate, the first is the most stable and the last is the least stable. They are colorless crystalline neutral compounds soluble in water, difficultly soluble in alcohol and insoluble in ether. After being prepared by the ordinary diazo reaction, with sodium nitrite in cold acid water solution, they may be precipitated in crystalline form by the addition of alcohol and ether. If the diazotization is effected in alcohol solution by means of amyl nitrite or ethyl nitrite the crystals of the diazonium salt separate at once. These salts of diazo benzene all show true salt characteristics, e.g., they lower the freezing point of solutions. The diazo radical, (CeHs—N2—) is thus basic toward strong acids, and the hydroxide, the non-isolated hypothetical diazo benzene, CeHs—N2—OH, is the free base. It may be considered as the simplest aromatic diazo compound and the mother substance of all other members of the class. 

In Britain during 1932-1933, a major discovery took place at what would soon be an ICI acquisition, Scottish Dyes. By chance, a blue colorant was obtained during preparation of phthalimide from phthalic anhydride and ammonia. Though not an aromatic amino compound, the stable product is important in the history of dye discovery. It was a phthalocyanine compound, of the type first prepared in 1907. ICI manufactured the copper analogue, known as Monastral fast blue. Introduced in 1934, it represented the first member of the only new structural class of synthetic dye in the 20th century83. 

There is thus obtained from the higher members (compounds having five or more carbon atoms) of the amides, first the primary amine, and secondly, the nitrile of the next lower acid. In the aromatic series, the reaction for the preparation of primary amines, which contain the amido group in the benzene ring, is not of general importance, since these... 

The first fact indicates that benzene is an aromatic member of a series of compounds. Another example is the following ... 

The improved electrochemical synthesis (7) of poly pyrrole has led to its use as coating for the protection of n-type semiconductors against photocorrosion in photoelectrochemical cells. (8,9) Recently, it was announced that pyrrole was not the only five-membered heterocyclic aromatic ring compound to undergo simultaneous oxidation and polymerization. Thiophene, furan, indole, and azulene all undergo electrochemical polymerization and oxidation to yield oxidized polymers of varying conductivities (5 x 10 3 to 102 cm- ). (10-13) The purpose... 

---