Alkanes tricyclic—

It was mentioned above that even alkanes undergo Wagner-Meerwein rearrangements if treated with Lewis acids and a small amount of initiator. An interesting application of this reaction is the conversion of tricyclic molecules to adamantane and its derivatives. It has been found that all tricyclic alkanes containing 10 carbons are converted to adamantane by treatment with a Lewis acid such as AICI3. If the substrate contains more than 10 carbons, alkyl-substituted adamantanes are produced. The lUPAC name for these reactions is Schleyer adamantization. Two examples are... 

Additional mechanistic problems are encountered upon extension of the discussion to the rearrangements of larger tricyclic alkanes such as the per-hydroaromatics discussed earlier (Scheme 1 and Eq. (4—6))13-ls) and the rearrangement of homoadamantane to 1-methyladamantane (Scheme 4) 23>. As illustrated in Scheme 4, the first formed alkyl adamantanes in all of these re-... 

Another compound which gave identical mass spectra in two of the extracts, WYOl and ALBl, appears to be one of the tricyclic alkanes, m/e = 276. An intense peak at m/e 247 corresponds to M-29. The mass spectrum was an excellent match with that presented by Philip, et al. (10) for which the structure shown below was proposed. An intense peak at m/e 123 was noted and is probably due to the fragmentation shown. 

At a slightly longer retention time the component m/e 234 appears to be another tricyclic alkane, C17H30. Several similar tricyclic alkanes were reported by Jones et al. (4). 

Several groups of hydrocarbon biological markers were identified in the coal extracts by mass spectrometry using selected ion scans. n-Alkanes, pristane, sesquiterpenes, several tricyclic alkanes and pentacyclic triterpanes with molecular weights from 398 to 454 were detected. 

S.M. Weinreb and co-workers were surprised to find that the convergent stereoselective synthesis of marine alkaloid lepadiformine resulted in a product that gave a totally different NMR spectra than the natural product. This finding led to the revision of the proposed structure of lepadiformine. In the final stages of the synthesis, they exposed a tricyclic piperidone intermediate to Ciemmensen conditions to remove the ketone functionality. Under these conditions the otherwise minor elimination product (alkene) was formed predominantly however, it was possible to hydrogenate the double bond to give the desired alkane. 

Transition metal catalyzed decomposition of unsaturated a-diazo ketones or a-diazo esters is a powerful method for the synthesis of certain 2-oxobicyclo[n.l.0]alkanes. In contrast to the thermal (see Section 1.2.1.2.4.2.6.1.) and photochemical (see Section I.2.I.2.4.2.6.2.) methods, which have only been applied successfully in a few cases, the carbenoid version has been extensively utilized for the construction of simple or highly substituted bicyclic, tricyclic or higher systems of predictable stereochemistry (for reviews, see refs 2, 82, 320). Several of the cyclopropanes so obtained have been transformed further into natural products with diverse molecular skeletons. As examples and procedures have already been presented in Houben-Weyl, Vol. E19b, ppl088ffand 1271 ff, only some important aspects concerning the scope and limitation of the method as well as recent developments concerning its stereochemistry will be discussed here. 

Extended tricyclic alkanes with isoprenoidal side chains (C19-C30 Fig. 4.25), which can be named after the C25 member 13p,140C-cheilanthane (a sesterterpane), are common in oils and ancient sediments and they may be microbial in origin (Aquino Neto et al. 1983). The hexaprenol in Fig. 5.26, a common eubacterial and archaebacterial component, seems a likely precursor (Ourisson et al. 1982). Occasionally, the cheilanthane series has been found to extend to C54 (de Grande et al. 1993), suggesting that hexaprenol either undergoes further polymerization reactions or is not the sole precursor. 

What kind of structure might we predict for benzene if we knew only what the early chemists knew The molecular formula (CgHg) tells us that benzene has eight fewer hydrogens than an acyclic (noncyclic) alkane with six carbons (C H2 +2 = C6H14). Benzene, therefore, has a degree of unsaturation of four (Section 3.1). This means that benzene is either an acyclic compound with four tt bonds, a cyclic compound with three tt bonds, a bicyclic compound with two tt bonds, a tricyclic compound with one TT bond, or a tetracyclic compound. 

Fig. 6. Acyclic alkane, hopane and tetracyclic alkane cross plots of end member oils A and B and mixtures of each. Red squares are selected end member oils red circles represent calculated mixtures of oils A and B in 10% increments. Definition of ratios %35ab= 100 x S R C35 homohopane/(S R C34 homohopane + S R C35 homohopane) %24/4= 100 x C24 tetracyclic terpane/(C24 tetracyclic terpane + C23 tricyclic and C25 tricyclic terpane). Note that for the acyclic alkanes which have similar concentration ranges the mixing curve is essentially linear, whereas for the cyclic biomarkers with very different end member concentrations, the mixing curve is distinctively nonlinear. In this illustrative case of course the actual variations in %24/4 are not sensibly interpretable.

Analysis of tricyclic alkanes has been less reported in the literature, in comparison with mono-, di-, tetra- and penta-cyclics. However, since we intend to review also separation of fossil fuels, we have to mention compounds such as fichtelite (C19H34) and other diterpenoid tricyclics derived from abietic acid. Abietic acid and levopimaric acid belong to the resin acids, which exude from incisions of bark or trunk of high plants. The derived hydrocarbons, such as fichtelite, were found in the saturated fraction of peat bed extractions. The identification and structure elucidation of fichtelite was based on m.p. 46.5 °C and optical activity [a]D = 18°, as well as on its resistance to chemical attack... 

TABLE 8. CNMR chemical shifts of tricyclic and polycyclic alkanes... 

Thiols and thioethers are the most commonly encountered sulphur derivatives which can be directly reduced to alkanes. For this purpose, Raney Ni has been traditionally used. An example of its use is the synthesis of let- and 7j5-eremophilanes from a tricyclic thiophene precursor (equation 29) However, more recently nickel complex reducing agents ( NiCRA ) have been profitably and effectively used for this purpose, as indicated by the reduction of dodecanethiol to dodecane (equation 30)". Similarly, nickelocene in the presence of LAH has been used to effect desulphurization of thiols . A combination of NaEtaBH and FeCl2 also achieves the thiol-to-alkane transformation quite satisfactorily The increasing utility of nickel boride has also been exploited for the transformation of sulphides to alkanes as indicated in equation 31. ... 

The oxazolinone 107, a new recoverable chiral auxiliary that gives a high level of asymmetric induction in model aldol reactions and Diels-Alder cycloadditions (in which the reactant is connected to the auxiliary by -acylation), has been synthesized from L-gulonic acid. The key step in its synthesis was the thermal intramolecular nitiene insertion shown in Scheme 21. The formation of oxazolinethiones on hydrolysis of isothiocyanato-sugar derivatives is covered in detail in Chapter 9, section 3.3. Base-catalysed reaction of StS-O-isopropylidene-D-xylofuranosylamine with P-isothiocyanato-alkanals led to tricyclic derivatives such as 108, considered as cyclonucleoside analogues. ... 

IMth enynes, mixtures of addition products resulting from 1,2-and 1,4-addition have been observed. Radical addition with tricyclic alkanes has also been observed. ... 

Other than rt-alkanes, pristane, phytane, and other isoprenoid alkanes that are present in abundance in unaltered oils and easily recognizable by GC and GC-MS analyses, most of the biomarker components are present in trace amounts. Among these, naphthenic hydrocarbons, including tricyclic terpanes, pentacyclic triterpanes, and steranes, have been studied most extensively because they carry more specific information on source and depositional environment than normal and isoprenoid alkanes [21]. 

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