Polycyclic molecules

Fused polycyclic naphthenes have much more compact structures than the substituted paraffins above and have even lower viscosity indexes although some can be redeemed by having n-decyl straight chains attached as in Table 3.7. 

Incorporation of several aromatic rings within the structure of these polycyclic aromatics to give flatter structures reduces VFs even further into the negative regime (Table 3.8). We ll see later in some examples that components with even more negative Vi s can be isolated from lube fractions. 

Therefore from the study of pure compounds, the Vis of base stock components decrease in the order isoparaffins with few branches multiply branched iso-paraffins = mononaphthenes with long chains attached = monoaromatics with 

Source Properties of Hydrocarbons of High Molecular Weight, Research Project 42,1940-1966, American Petroleum Institute, New York. With permission. 

Vis for Fused Polycyclic Aromatic and Naphthene Ring Structures 

Draw the more stable chair conformation of the following molecules, and estimate the amount of strain in each  

Identify each substituent in the following compound as axial or equatorial, and tell whether the conformation shown is the more stable or less stable chair form (green = Cl)  

The final point we ll consider about cycloalkane stereochemistry is to see what happens when two or more cycloalkane rings are fused together along a common bond to constmct a polycyclic molecule— for example, decalin. 

Polycyclic compounds are common in nature, and many valuable substances have fused-ring stmctures. For example, steroids, such as the male hormone testosterone, have 3 six-membered rings and 1 five-membered ring fused together. Although steroids look complicated compared with cyclohexane or decalin, the same principles that apply to the conformational analysis of simple cyclohexane rings apply equally well (and often better) to steroids. 

Another common ring system is the norbornane, or bicyclo [2.2.1] heptane, stmcture. Like decalin, norbornane is a bicydoalkane, so called because two rings would have to be broken open to generate an acyclic structure. Its systematic name, bicyclo [2.2.1] heptane, reflects the fact that the molecule has seven 

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The readily prepared 1-ethynylcyclohexanol (Chapter 14) can be isomerized with acidic catalysts into I -acetylcyclohexene, a compound of considerable use in the building up of polycyclic molecules. To illustrate the fact that a variety of conditions can be employed to carry out a given process, three methods are presented. 

Carbonyl condensation reactions are perhaps the most versatile methods available for synthesizing complex molecules. By putting a few fundamental reactions together in the proper sequence, some remarkably useful transformations can be carried out. One such example is the Robinson annulation reaction for tire synthesis of polycyclic molecules. The word annulation comes from the Latin annulus, meaning "ring," so an annulation reaction builds a new ring onto a molecule. 

Bridgehead atom (Section 4.9) An atom that is shared by more than one ring in a polycyclic molecule. 

Radical Diels-Alder reactions have been used mainly to synthesize polycyclic molecules. These reactions, like those that involve cations and anions as components, proceed quickly but generally do not give high yields. Thus, the tricyclic enone 14 is the result of an intramolecular Diels-Alder reaction of quenched vinyl radical intermediate 13 obtained by treating the iododienynone 12 with n-tributyltin hydride/2,2 -azobisisobutyronitrile (AIBN) [28] (Equation 1.11). 

Inner-outer-ring dienes are very useful in the synthesis of polycyclic molecules. Their reactivity in the Diels-Alder reaction depends on the type of ring (carbo-cyclic, heterocyclic, aromatic) that bears the ethenyl group or on the electronic effects of substituents at the diene moiety [30]. 

Diels-Alder reactions of conjugated cycloalkenones provide a very important method for rapidly constructing complex polycyclic molecules. Since cycloalkenones are very poorly reactive dienophiles, acceleration by special physical and catalytic methods is required in order to avoid high reaction temperatures and long reaction times which often lead to low product yields [8],... 

There are many examples of polycyclic molecules and bridged molecules that have one or more double bonds. There is flattening of the ring containing the C==C... 

Malacria, M. (1996) Selective Preparation of Complex Polycyclic Molecules from Acyclic Precursors via Radical Mediated or Transition Metal-Catalyzed Cascade Reactions. Chemical Reviews, 96, 289-306. 

Fig. 5. Iterative construction of polycyclic molecules in a nonskid- chain-like manner...

Figure 6. Polycyclic molecules for which the tautomeric equilibrium in aqueous solution has been studied using free energy calculations including explicit solvent. Tautomerism occurs by exchanging a proton between the labeled atoms.

Extensively developed by Ojima and co-workers, SiCaT and carbonylative silylcarbocyclization (CO-SiCaC) represent a rapid entry into polycyclic molecules of interest.271 For instance, the rhodium-catalyzed intramolecular SiCaT of triyne 441 afforded tricyclic compound 442 in high yield, accompanied by a small amount of cycloadduct 443 (Scheme 111).270... 

Owing to experimental difficulties, knowledge of aggregation effects in alkaline dithionite solutions of leuco vat dyes is sporadic [20,21]. Investigations based on absorption spectra have shown that, depending on concentration and temperature, planar polycyclic molecules such as the violanthrone derivatives Cl Vat Blues 19, 20 and 22 and the perylene tetracarboxydiimide derivatives Cl Vat Reds 23 and 32 are mainly present as monomers or dimers in leuco vat solutions. Violanthrones that do not have a coplanar structure because of the presence of... 

Since then, the photocycloaddition reaction has been extensively studied and has become a powerful tool for the construction of complex polycyclic molecules. High stereoselectivities are observed in some cases. The configuration of the diradical intermediate determines the stereochemistry of the adduct [33], Typical examples... 

Therefore, a fundamental axiom in the synthesis of complex organic compounds bearing chiral centres is the necessity of resorting to rigid structures whether in the starting materials (monocyclic or polycyclic molecules) or in the transition states (as in pericyclic reactions, for instance) in order to ensure an... 

ArmentroutPB (1999) Gas-Phase Organometallic Chemistry. 4 1-45 Astruc D, Daniel M-C, Ruiz J (2006) Metallodendritic Exo-Receptors for the Redox Recognition of Oxo-Anions and Halides. 20 121-148 Aubert C, Fensterbank L, Gandon V, MalacriaM (2006) Complex Polycyclic Molecules from Acyclic Precursors via Transition Metal-Catalyzed Cascade Reactions. 19 259-294... 

Monocyclic or polycyclic molecules containing binding sites for one or more so-called guest molecules or ions. Examples include the crown ethers as well as numerous macrocycles containing other electronegative atoms besides oxygen. See Inclusion Complexes Host-Guest Interactions Cydodextrins... 

When the percentage of aromaticity is to be calculated, benzene is the 100% reference structure (AJV = 0). With the aid of the AN index, the aromaticity of a separate ring in a polycyclic molecule may be estimated (85KGS867). 

This section is about alkylcyclohexanes and related polycyclic molecules consisting of chair six-membered rings. Our work must thus accommodate conformational features such as those commonly described as hutanc-gauche interactions. 

Electron transfer from polycyclic aromatic radical anions in polar solvents can also initiate propagation.120 168 169173 One of the early and best understood systems is naphthalene-sodium, a green solution of stable, solvated naphthalene radical anion.176 177 The electron transfer from the radical anion to the monomer yields a new radical anion [Eq. (13.33)]. The dominant reaction of the latter is its head-to-head dimerization to the stabile dimeric dicarbanion [Eq. (13.34)], which is the driving force for the electron transfer even when electron affinity of the monomer is less than that of the polycyclic molecule. Propagation proceeds at both ends of the chain ... 

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