Rings fused

We may also encounter aromatic hydrocarbons that feature fused rings. Thus, naphthalene is effectively 

Molecular orbital calculations snggest that the jr electrons in naphthalene are delocalized over the two rings and this results in substantial stabilization. These molecules are planar, and all p orbitals are suitably aligned for overlap to form n bonding molecular orbitals. Although we can draw Kekule structures for these compounds, it is strictly incorrect to use the circle in hexagon notation since the circle represents six jr electrons. Naphthalene has 10 carbons, and therefore 10 jr electrons, and anthracene has 14 jr electrons. The circle notation suggests 12 or 

The gas-phase structure of trans-2-decalone has been investigated by combined electron diffraction, conformational energy, and vibrational analyses. Initially the minimum-energy conformations were calculated by molecular mechanics  

Reagents i, m-CPBA-pentane ii, 2 equiv. LiMejCu iii, LiAlH4 iv, Mel. 

In a redetermination of the 5a- and 5P-cholestan-4-one equilibrium it was found that 87 + 1% of the 5a-species is present at 25°C in ethanol. For 9-methyl-l-decalone the cis-isomer is favoured at equilibrium, and for 10-methyl-1-decalone the trans-isomer is favoured. These results are supported by molecular mechanics calculations. The amount of cis-isomer contained in equilibrium,mixtures of 1-decalones is larger than in the steroidal counterparts, and this was rationalized in terms of the two low-energy a s-conformers which are available in the cis-decalones. 

The conformational situation in levopimaric acid and related steroidal dienes has been investigated.  

The four isomeric [l,l,4,4- H4]3-dimethylamino-trans-2-decalols have been synthesized in order to investigate their conformations. Where the amino-function is axial and the hydroxy equatorial, /h-2—h-3 = spectral measurements 

Either ring of an alkyl-substituted naphthalene may be reduced and the rate ratio depends both on the position of R and its size 2S). Naphthalenes 

The above generalities apply particularly to palladium. Hydrogenation over platinum or rhodium are far less sensitive to the influence of steric crowding. Reduction of 1-t-butylnaphthalene over platinum, rhodium, and palladium resulted in values of /ci//c2 of 0.42, 0.71, and 0.024, respectively. Also, unlike mononuclear aromatics, palladium reduces substituted naphthalenes at substantially higher rates than does either platinum or rhodium. For example, the rate constants, k x 10 in mol sec g catalyst , in acetic acid at 50 C and 1 atm, were (for 1,8-diisopropylnaphthalene) Pd (142), Pt(l8.4), and Rh(7.1)(25). 

A great deal of control can nowadays be exerted over the products obtained on partial hydrogenation of fused aromatic systems. Selectivity depends on 

Hydrogenation of 4,5-dihydropyrene (2g) over 10% Pd-on-C (500 mg) in 20 ml of EtOAc at 50 psig afforded tetrahydropyrene quantitatively 17). These and many other highly regioselective results (22) contrast with the complex mixture reported earlier when more strenuous conditions were employed. 

Cowen and E. J. Eisenbraun, Unpublished Ph.D. Thesis of K. D. Cowen, Oklahoma State Univ., Stillwater, 1977. 

Examine the geometry of norcarane. What is the conformation of the cyclohexane ring Choose a name (chair, twist boat, half-chair, etc. see Chapter 5, Problem 4) that accurately describes its shape. The bridgehead hydrogens in norcarane are cis. Do you think a trans stereoisomer is possible Explain. 

Examine the structures of trans and cis-decalin. What is the conformation of the two cyclohexane rings in each isomer Obtain the energy of each decalin isomer. Which is more stable and why  

Make a sketch of each decalin isomer, and label the orientation of the bridgehead hydrogens with respect to each ring (equatorial or axial). Build a plastic model of each isomer and determine its conformational flexibility (a flexible molecule can undergo a ring flip, but a locked molecule cannot). Is flexibility responsible for stabihty  

Examine the structure of cyclodecane, a molecule which contains the same number of carbons as decalin, but only has one ring (a model of the most stable conformation is provided). Compare it to cis and trans decalin. Make a plastic model of cyclodecane. Is it flexible or locked What conformational properties of cyclodecane can be anticipated from the properties of decalins What properties cannot be anticipated How do you account for this  

The conformational situation in 9-methyl c/s-decalin-l,3-dione (342) and other 9-methyl ds-decalins has been investigated. In CDClj at 25 °C at higher concentrations (saturated solution is 0.42 mol 1 ), (342) exists as the keto-form to the extent of 80% and the enol form to the extent of 20% at 50 °C the corresponding figures are 90% and 10%. In dilute solution the proportion of keto form rises to 95 %. This behaviour parallels that of earlier cited results with cyclohexane-1,3-dione. It is considered that the keto-form consists of approximately equivalent proportions of (342a) and (342b). For (343) the axial P-proton at 5 3.28 p.p.m. with = 1 Hz was unaltered down to — 59 °C this, and an examination of models, supports the sole existence of the conformation shown in (343) under the conditions examined. In (344) the la-proton of 

Wylde and Cerveau have examined the n.m.r. spectra of the four 3-dimethylamino-trans-decal-2-ols (345)—(348) and the derived quaternary ammonium iodides (350)—(353) with a view to uncovering conformational distortion. For (345) and (346) the C-2 methine proton has = 7 Hz and 20—22 Hz, respectively these values are unexceptional although in the case of 

For (347) the C-2 methine proton takes the form of a well-resolved quadruplet with = 20 Hz on account of an angular deformation which leads to an almost eclipsed disposition of the C-2 and C-3 methine protons. 

For the trimethylammonio-derivatives (350), (351), and (353) the conformations appear virtually identical with those of the dimethylamino-derivatives. However, for (352), the substituted ring is considered to adopt a twist-chair conformation. 

Reaction of the diaxially substituted A -benzoyl 0-methanesulphonate (354) in anhydrous ethanol in the presence of potassium acetate leads to formation of oxazoline (355) characterized by AH = 22.3 kcal moland AS = — 1.6 e.u. With the diequatorial isomer (356a) cyclization is considered to take place via a small equilibrium concentration K = 0.0026) of the boat-chair conformer (356b) to give (357). This reaction has AH = 26.2kcalmol and AS = —1.6 e.u. In the corresponding cis-isomers no oxazoline formation was observed. 

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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. 

Stork-Eschenmoser Hypothesis- Olefin Geometry is preserved in the cyclization reaction, i.e. trans olefin leads to a trans fused ring jucntion A. Eschenmoser HCA 1955, 38, 1890 G. Stork JAGS 1955, 77, 5068... 

Only relatively few examples of interesting target molecules containing rings are known. These include caryophyllene (E.J. Corey, 1963 A, 1964) and cubane (J.C. Barborak, 1966). The photochemical [2 + 2]-cycloaddition applied by Corey yielded mainly the /ranr-fused isomer, but isomerization with base leads via enolate to formation of the more stable civ-fused ring system. 

Intramolecular reactions between donor and acceptor centres in fused ring systems provide a general route to bridged polycyclic systems. The cts-decalone mesylate given below contains two d -centres adjacent to the carbonyl function and one a -centre. Treatment of this compound with base leads to reversible enolate formation, and the C-3 carbanion substitutes the mesylate on C-7 (J. Gauthier, 1967 A. Belanger, 1968). 

Proton-catalyzed olefin cyclizations of open-chain educts may give tri- or tetracyclic products but low yields are typical (E.E. van Tamelen, 1968, 1977 see p. 91). More useful are cyclizations of monocyclic educts with appropriate side-chains. The chiral centre to which the chain is attached may direct the steric course of the cyclization, and several asymmetric centres may be formed stereoselectively since the cyclizations usually lead to traas-fused rings. 

Buckminster Fuller was the inventor of self-supporting polygon frameworks, e.g., of pavilions. Synonyms (root)ballene, buckminsterfullerenc( BF ), carbosoccer, soo r(ball)ene, spherene. The proposed bridged-fused ring system and von-Baeyer names and numberings vary. 

The reaction of the allylic acetate with a diene system 784 affords the poly-fused ring system 785 by three repeated alkene insertions[487]. An even more strained molecule of the [5.5.5.5] fenestrane 788 has been constructed by a one-pot reaction in a satisfactory yield by the Pd-catalyzed carbonylation-cycliza-tion of 786 without undergoing elimination of /3-hydrogen in the cr-alkylpalla-dium intermediate 787 owing to unfavorable stereochemistry for syn elimination[488]. 

In aprotic conditions acetic anhydride sodium acetate induces formation of a fused ring through an intra molecular condensation. It results in a pyrrolo[2,l-fc]thiazole (39), which constitutes an interesting intermediate for the synthesis of dyes (Scheme 18) (40). 

The interesting reactions where a free mercapto group is linked to the nitrogen atom of the thiazole (63), after the cleavage of a fused ring, is another illustration of the additive properties of the carbocation (Scheme 40). 

Spiro polymers are also sometimes classified as ladder polymers, and molecules in which the ladder structure is interrupted by periodic single bonds are called semiladders. Consisting entirely of fused ring structures, ladder polymers possess very rigid chains with excellent thermal stability. 

Fluoronaphthalenes and other fused-ring fluoroaromatics, Side-chain fluotinated aromatics,... 

Few appHcations for fluoronaphthalenes and related polycycHc stmctures have materialized. The fused-ring bicycHc, suHndac [38194-50-2] a monofluorinated indene-3-acetic acid, is used as an antiinflammatory agent. 

Eused-ring polycycHc fluoroaromatics can be made from the corresponding amino fused-ring polycycHc or from preformed fluoroaromatics, eg, 4-fluorophenyl-acetonitrile [459-22-3] (275). Direct fluorination techniques have been successfully appHed to polycycHc ring systems such as naphthalene, anthracene, benzanthracenes, phenanthrene, pyrene, fluorene, and quinoHnes with a variety of fluorinating agents xenon fluorides (10), acetyl hypofluorite (276), cesium fluoroxysulfate (277), and electrochemical fluorination (278,279). 

The aromatic core or framework of many aromatic compounds is relatively resistant to alkylperoxy radicals and inert under the usual autoxidation conditions (2). Consequentiy, even somewhat exotic aromatic acids are resistant to further oxidation this makes it possible to consider alkylaromatic LPO as a selective means of producing fine chemicals (206). Such products may include multifimctional aromatic acids, acids with fused rings, acids with rings linked by carbon—carbon bonds, or through ether, carbonyl, or other linkages (279—287). The products may even be phenoUc if the phenoUc hydroxyl is first esterified (288,289). 

The accepted configuration of naphthalene, ie, two fused benzene rings sharing two common carbon atoms in the ortho position, was estabUshed in 1869 and was based on its oxidation product, phthaUc acid (1). Based on its fused-ring configuration, naphthalene is the first member in a class of aromatic compounds with condensed nuclei. Naphthalene is a resonance hybrid ... 

The fused 3+ ring aromatics in petroleum include both cata- and peri-condensed stmctures (see Table 4, Fig. 8). The cata-condensed species are those stmctures where only one face is shared between rings, the peri-condensed molecules are those that share more than one face. The fused ring aromatics form the class of compounds known as polynuclear aromatic hydrocarbons (PAH) which includes a number of recognized carcinogens in the 4+ ring family (33). Because of the potential health and environmental impact of PAH, these compounds have been studied extensively in petroleum. 

Table 4. Fused-Ring Polynuclear Aromatic Hydrocarbons Found in Petroleum ...

Fig. 8. Stmctures of fused-ring polynuclear aromatic hydrocarbons. See Table 4.

Azonia substitution at a naphthalene bridgehead position gives the quinolizinium ion (16). Oxonia substitution, elsewhere, forms the 1- and 2-benzopyrylium ions (17) and (18). The two most well-known monoaza systems with three aromatie fused rings are aeridine (19), derived structurally from anthraeene, and phenanthridine (20), an azaphenanthrene. The better-known diaza systems inelude phenazine (21) and 1,10-phenanthroline (22), while systems with three linearly fused pyridine rings are ealled anthyridines, e.g. the 1,9,10-isomer (23). 

Pyrazoles, Indazoles, and their Derivatives as Starting Materials for the Syntheses of Fused Ring Systems... 

As discussed in the theoretical section (4.04.1.2.1), electrophilic attack on pyrazoles takes place at C-4 in accordance with localization energies and tt-electron densities. Attack in other positions is extremely rare. This fact, added to the deactivating effect of the substituent introduced in the 4-position, explains why further electrophilic substitution is generally never observed. Indazole reacts at C-3, and reactions taking place on the fused ring will be discussed in Section 4.04.2.3.2(i). Reaction on the phenyl ring of C- and A-phenyl-pyrazoles will be discussed in Sections 4.04.2.3.3(ii) and 4.04.2.3.10(i), respectively. The behaviour of pyrazolones is quite different owing to the existence of a non-aromatic tautomer. 

Fused ring systems containing a pyrazole unit can be prepared either from the heterocyclic moiety by formation of a pyrazole ring or from the reaction between a pyrazole derivative and a suitably functionalized reagent. The ring systems thus obtained are discussed in detail in other chapters (Chapters 4.05, 4.35, 4.36) but it is of interest to discuss here those methods which start from a pyrazole derivative as the reactions involved can be considered as examples of the reactivity of pyrazoles. The most widely studied fused ring systems are the [5.6] systems and the examples described in this section will be chosen from this group and, occasionally, from [5.5] and [5.7] systems. 

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