And fused ring compounds

Hiickel s rule (1931) for planar species states that if the number of ir electrons is equal to 2 + 4n, where n equals zero or a whole number, the species is aromatic. The rule was first applied to carbon-containing monocyclics in which each C is capable of being 5p -hybridized to provide a p orbital for extended v bonding it has been extended to unsaturated heterocyclic compounds and fused-ring compounds. Note that benzene corresponds to n = 1. 

At the NBS, in addition to his polymer activities, he was also part of a large effort for the preparation and characterization of hydrocarbons within a molecular-weight range of 170 to 351, which comprised normal paraffins, cycloparaffins, aromatics, and fused ring compounds. The characterization included the pressure and temperature variation of density, viscosity, refractive index, and so on [Schliesser et al., 1956]. 

Simple examples of WLN are C2H5OH is Q2 CH3C0 0CH3 is IVOl For branch chain and fused ring structures rules determine the order of notation. It is claimed that over 50% of all organic structures can be represented by less than 25 characters, witherite, BaCOj. The white mineral form of barium carbonate. Used as a source of Ba compounds and in the brick and ceramic industries. 

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). 

This reaction is most often carried out with R = aryl, so the net result is the same as in 14-17, though the reagent is different. It is used less often than 14-17, but the scope is similar. When R = alkyl, the scope is more limited. Only certain aromatic compounds, particularly benzene rings with two or more nitro groups, and fused ring systems, can be alkylated by this procedure. 1,4-Quinones can be alkylated with diacyl peroxides or with lead tetraacetate (methylation occurs with this reagent). 

Dimethylaminolysis of the novel fused ring compound (42), whose crystal structure has just been reported (see Section 7), gave a derivative of probable structure (43) on the basis of and P n.m.r. data. On the... 

Method 1 is used for the Table Names of compounds and their derivatives in the list above. Method 2 is used for other fused-ring compounds. Several examples of Method 1 and Method 2 names for the same compounds are ... 

The classes of compounds that must contain an aromatic group include fused ring compounds, substituted benzenes and phenols. 

Compounds with cyclic structures. This includes homocyclic, heterocyclic and fused ring systems. 

It is difficult to conceive of these o-jt type transfers in compounds such as (Ci2Fg)2Si, as shown in Scheme 6. These fused-ring compounds are analogous to the intermediates proposed for the boron compounds (Scheme 5) and for the mass spectra of diarylphosphinic acids in which... 

The possible conformations for these fused ring compounds are illustrated in Fig. 9. Any discussion of the relative stability of boat and chair forms depends on the axial/equatorial positions of the phosphorus substituents. It seems likely that ring inversion will be fast on the n.m.r. time-scale, so that the structures within a given group (i), (ii) or (iii) could all contribute (to varying degrees) to an observed spectrum. 

The reactions of the fused ring compound (5S) with secondary amines, giving (69) as a mixture of isomers and of (67) (X = Br) with t-butanol (and phenol) have been studied. With primary amines, cage compounds such as (68) are the major products Cage-forming reactions of this general type were first... 

Fig. 9. Possible conformations of fused-ring compounds (e.g. (58)). Nitrogen atoms and ring substituents are omitted. The first two letters for each structure refer to the ring conformations (b = boat, c = chair) and the second two letters refer to the phosphorus-exo-substituent (a = axial, b = bowsprit, e = equatorial, f = flagpole)...

Since the publication of CHEC-II(1996), there have been very few examples related to the reactivity of substituents attached to ring carbon atoms. One case involves the reaction of 3-benzylidene-2,3-dihydro-2-methyl-l,2-benzothiazin-4-one 1,1-dioxide 163 with the alkylidenephosphorane derived from salt 164 forming the tricyclic-fused ring compound 165 (Scheme 20) <1996J(P1)2541>. This material 165 was oxidized with 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) affording the biphenyl 166. Ring-opened product 167 was produced from 165 upon exposure to />-toluene-sulfonic acid and heat. 

Rather simpler structures have been described for this series of fused ring compounds. Thiophenes 424, lacking substituents at positions 2 and 5, follow the same pathway. Hence 424 has been cyclized to 425 in the presence of base (Equation 159) <1999BMC297>. 

Although the Hiickel An+ 2 rule is rigorously derived for monocyclic systems, it is also applied in an approximate way to fused-ring compounds. Since two fused rings must share a pair of ir electrons, the aromaticity and the delocalization energy per ring is less than that of benzene itself. Decreased aromaticity of polynuclear aromatics is also revealed by the different C—C bond lengths. 

While the synthesis of benzofuran derivatives from suitable furan derivatives has been comparatively little investigated, the synthesis of fused two-ring and three-ring compounds containing a pyridine nucleus from furan derivatives has been more successful furo[3,2-c]pyridines (345),752>753 2,3-dihydrofuro[2,3-a ]quinolines (from 4,5-dihydro-3-furoic acid),754 acrophyllin (346), and 7-hydroxydictamnine (347) (from ethyl 2-ethoxy-4-oxo-4,5-dihydrofuran-3-carboxylate),755 are examples of compounds in this class which have been prepared. 

Orf/io-fused and ortho- and pen-fused ring compounds containing hetero atoms are named according to the fusion principle described in Rule A-21 for hydrocarbons. The components are named according to Rules A-21, B-l and B-2. When the name of a component in a fusion name contains locants (numerals or letters) that do not apply also to the numbering of the fused system, these locants are placed in square brackets (as are also the locants for fusion positions required by Rule A-21.5). The base component should be a heterocyclic system. If there is a choice, the base component should be, by order of preference ... 

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