Tricyclic hydrocarbons, structure

Treatment of the alcohol whose structure is shown here with sulfuric acid gave as the major organic product a tricyclic hydrocarbon of molecular formula CigHig. Suggest a reasonable structure for this hydrocarbon. 

This alkynone thermolysis has served as the key step in several natural product syntheses. The considerations that lead to regio- and stereochemical control are illustrated by an elegant synthesis of clovene 725. Thermolysis of the symmetrical alkynone 4 could lead to three different enones, two of which (5 and 6) arc illustrated. In fact, only 8-methyltricyclo[8.3.1.01 5]dodec-3-en-2-one (5) is isolated, in 80% yield. The structure of 5 was confirmed by straightforward conversion to the tricyclic hydrocarbon clovene. The outcome of the thermolysis was anticipated, as it had previously been observed that C-H insertion is most facile when the intermediate alkylidene can achieve coplanarity with the target C-H bond. In this system, such coplanarity is achievable only via the boat conformation of one of the two symmetrical rings. 

A new tricyclic hydrocarbon, senoxydene, which has been isolated from Senecio oxyodontus, has been assigned the structure (304) on the basis of its n.m.r. spectrum together with that of the corresponding epoxide.It is suggested that once again humulene could act as the precursor (Scheme 48) see, however, p. 104. 

A few specific examples of cyclopropanation using the above methods are shown in Scheme 2.134. The naturally occurring insecticide /ra -chrisanthemic acid 396 served as an obvious target to check the viability of carbene addition as a preparative method. This compound was first synthesized (in the mixture with the cis isomer) by the monocyclopropanation of 2,5-dimethyl-2,4-hexadiene. Since then, numerous analogs of 396 were prepared by similar reactions. Some of the analogs are now widely used as efficient and ecologically safe pesticides. The formation of the tricyclic hydrocarbon 397 from 1,5-hexadiene proceeds as a sequence of inter- and intramolecular carbene transfer reactions. An initial carbene precursor, CHBr3, is actually employed here as an equivalent of a unique tetradentate Ci synthon The preparation of 398 via intramolecular [2 + 1] photocycloaddition is a typical example of the efficiency of this route for the construction of the polycyclic framework frequently encountered in the structures of natural compounds. 

Bicyclo[3.3.0]oct-l(5)-ene 178 (Scheme 4.55) is a stable compound with a flattened alkene fragment and exhibits a regular pattern of reactivity. Computational studies revealed, however, that installation of a short 3,7-bridge should lead to noticeable pyramidalization of the double bond. Compounds like 179-181 were synthesized to check this prediction. Tricyclic hydrocarbon 179, with the smallest possible bridge, was generated as a transient species from diiodide 182. The formation of 179 is implicated by the isolation of its cyclodimer 183 (or respective Diels-Alder adduct if the reaction is carried out in the presence of a 1,3-diene). The next member of this series, 180, is more stable. In fact, the formation of 180 was ascertained not only from the structure of the final products (as was done for 179), but also by its matrix isolation and analysis of spectral data. The selenium derivative 181 was found to be stable at ambient temperature in the absence of oxygen. X-ray data confirmed a noticeable pyramidalization of the double bond in 181 but the distortion was different [Pg.372]

C12 hydrocarbon (271)135 by a similar route but this compound is not identical to a tricyclic hydrocarbon isolated from East Indian sandalwood oil and reported to have this structure. (+)-Copacamphor (272) and (+)-copaborneol (273) have been isolated from Espeltiopsis guacharaca.136 The interesting sinularene derivative (274) and the acetoxycyclosinularane (275) have been isolated from the marine source, Clavularia inflata.137 These compounds, together with an aromadendrane derivative (see p. 181) are the first sesquiterpenoids from Octocorallia of the order Stolonifera. 

Kinetic data for isomerizations of cyclobutenes and bicyclic and tricyclic hydrocarbons having cyclobutene rings are summarized in Table 8. A number of conclusions concerning the effects of structure on reactivity can be drawn from these data. Entropies of activation for isomerization of most monocyclic cyclobutenes fall in the range —2 to+ 3 eu. Alkyl substituents at C-1 increase the energy of activation by about 2-3 kcal.mole . Two alkyl substituents at C-1 and C-2 increase the energy of activation by about 4 kcal.mole , while... 

Of the fundamental nonalternant hydrocarbons, only two prototypes were known about fifteen years ago azulene (XI, Fig. 5), the molecular structure of which was determined by Pfau and Plattner and fulvene (XIX) synthesized by Thiec and Wiemann. Early in the 1960 s many other interesting prototypes have come to be synthesized. Doering succeeded in synthesizing heptafulvene (XX) fulvalene (XXI) and heptafulvalene (XXIII). Prinzbach and Rosswog reported the synthesis of sesquifulvalene (XXII). Preparation of a condensed bicyclic nonalternant hydrocarbon, heptalene (VII), was reported by Dauben and Bertelli . On the other hand, its 5-membered analogue, pentalene (I), has remained, up to the present, unvanquished to many attempts made by synthetic chemists. Very recently, de Mayo and his associates have succeeded in synthesizing its closest derivative, 1-methylpentalene. It is added in this connection that dimethyl derivatives of condensed tricyclic nonaltemant hydrocarbons composed of 5- and 7-membered rings (XIV and XV), known as Hafner s hydrocarbons, were synthesized by Hafner and Schneider already in 1958. 

Longifolene is a tricyclic sesquiterpene. It is a typical terpene hydrocarbon in terms of the structural complexity. The synthetic challenge lies in construction of the bicyclic ring system. Schemes 13.24 through 13.33 describe nine separate syntheses of longifolene. We wish to particularly emphasize the methods for carbon-carbon bond formation used in these syntheses. There are four stereogenic centers in longifolene,... 

Novel biomarkers, i.e. tracer derivatives from unknown natural products, are sometimes encountered in geological or environmental samples, typically as hydrocarbons. The detection and determination of these compounds are usually based on the interpretation of mass spectra in GC-MS analyses. The proofs of chemical structures are based on the proposed interpretation of the MS data, separation and purification of the unknown compounds, exact structure determination by NMR methods or X-ray crystallography (if the compound is a solid that can be crystallized), and finally, comparison with a synthetic standard. The next question concerns the biological source of the biomarker precursor compound. Many biomarkers still have no proven natural product precursors nor known biological sources (e.g. perylene, tricyclic terpanes). " ... 

The structure of this series, as in the previous cases, was established from mass spectral and SIR-GC/MS data, Figure 10, and the known structure of the series of tricyclic terpenoid hydrocarbons, present in the saturate fraction of the oil, the most abundant of which is the C23 member (21.22 ... 

However, tricyclic pyrrolotropylium ion 647 [Scheme 173 94JCS(P1)2579] absorbs at a shorter wavelength than the corresponding hydrocarbon, the cycloheptazulenium ion, but with similar fine structure. 

PCDDs, PCDFs, and PCBs are chemically classified as halogenated aromatic hydrocarbons. The chlorinated and brominated dibenzodioxins and dibenzofurans are classified as tricyclic aromatic compounds with similar physical and chemical properties, and both classes are similar structurally. Certain PCBs (the so-called coplanar or mono-ortho coplanar congeners) are also structurally and conforma-tionally similar. The most widely studied of these compounds is TCDD this compound, often called simply dioxin, represents the reference compound for this class. 

Granting that a tricyclic ring system takes precedence over a tetracyclic ring in simplicity, the other structural modifications, namely introduction of oxygen functions and iinsaturation, are high order deviations from the simple hydrocarbon. However, we are concerned only with compounds containing the steroid skeleton. 

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