Bicyclic hydrocarbons

Among the most important of the bicyclic hydrocarbons are the two stereoisomeric bicyclo[4.4.0]decanes, called cis- and rrani-decalin. The hydrogen atoms at the ring junctions are on the same side in c/5-decalin and on opposite sides in tmns-AecdXm. Both rings adopt the chair confonnation in each stereoisomer. 

Selective oxidations are possible for certain bicyclic hydrocarbons.285 Here, the bridgehead position is the preferred site of initial attack because of the order of reactivity of C—H bonds, which is 3° > 2° > 1°. The tertiary alcohols that are the initial oxidation products are not easily further oxidized. The geometry of the bicyclic rings (Bredt s rule) prevents both dehydration of the tertiary bridgehead alcohols and further oxidation to ketones. Therefore, oxidation that begins at a bridgehead position... 

Bulo et al. <2004AG732> have studied the degenerate rearrangements that occur among bicyclic hydrocarbons with three-membered rings. Extending the work to include phosphorus resulted in the formation of compound 197. Compound 198 was heated to 50 °C and the Woodward-Hoffmann-allowed product for a [ l,5]-shift (compound 199) was produced (Scheme 17). 

Some useful by-products of pulping are derived from these extractives, the most important of which are turpentine and tall oil. Turpentine is a mixture of bicyclic hydrocarbons with the empirical formula C10H16, the dominant components of which are a- and /J-pinene (Figure 2.8). They are produced as volatile by-products at a yield of around 4-5 litres per tonne of wood (for pine) and are used as a solvent and as a chemical feedstock. 

Although this route to a,cu-diallenes from terminal bisolefmic precursors has often been used in allene hydrocarbon chemistry, the reaction is actually complex and yields product mixtures containing cyclic and bicyclic hydrocarbons when the spacer between the double bond and the allene group is short (between one and three methylene groups), see (a) Th. Miebach, H. Wiister,... 

Products conjugated bicyclic hydrocarbons with the same number of carbon atoms as the parent hydrocarbon. 

The bicyclic hydrocarbon full-valence isomer (28) of the 2-isopropylidene-cyclopentane-l,3-diyl (15b) can be prepared and kept at temperatures below... 

K. It too gives the characteristic adducts obtained from the singlet, but the addition does not occur directly between the bicyclic hydrocarbon (28) and the alkene. Instead, the reaction occurs in two steps, first the reversible unimolecular ring opening of 28 to singlet biradical 14b, followed by a bimolecular capture of the latter (Scheme 5.5). Another hydrocarbon isomer 29 can be prepared as a transient intermediate. Its thermal conversion to the biradical 14b apparently occurs at even lower temperature. 

Kinetic observations suggest that of the four species, the biradicals 14b and 14b, and the bicyclic hydrocarbons 28 and 29, the triplet biradical 14b is the global energy minimum. Thus, the two related full-valency molecules 28 and 29 both have a negative bond dissociation energy (BDE). 

In a related type of reaction, the sulfur ylide of mintsulfide on treatment with methyllithium gave, via a 1,4-diradical, an epimer of the naturally occurring ( —)-j8-bourbonene (8, 29%) in addition to the bicyclic hydrocarbon.143... 

It has been observed that the formation of the olefin and carbon monoxide, 45, is ten times more important than the formation of the bicyclic hydrocarbon and carbon monoxide, 46, at 80° and 80 mm. pressure even at 3130 A. The formation of the strained bicyclic hydrocarbon is evidently not a favorable reaction although this may not be the only consideration. In the case of camphor it should be interesting to find out if an optically active isomer of the ketone on photolysis will give rise to an optically active trimethyl bicyclo [2.1.1] hexane (XXVI). A concerted reaction, analogous to the formation of cyclobutane from cyclopentanone, may lead to only an optically active product. 

The application of this reaction to other ring systems has shown that the yield of the bicyclic hydrocarbon decreases as the strain involved in the product increases. For example, the mercury sensitized decomposition of bicyclo [3.2.0] heptanone-3 (XXIII) leads to very little of bicyclo [2.2.0] hexane and gives more of bicyclo [2.1.1] hexane than of bicyclo [2.2.0] hexane. The reaction has been successfully applied to the synthesis of a substituted bicyclo [2.1.0] pentane (XXVIII) from a sub-... 

The crucial structural feature which underlies the aromatic character of benzenoid compounds is of course the cyclic delocalised system of six n-electrons. Other carbocyclic systems similarly possessing this aromatic sextet of electrons include, for example, the ion C5Hf formed from cyclopentadiene under basic conditions. The cyclopentadienide anion is centrosymmetrical and strongly resonance stabilised, and is usually represented as in (7). The analogous cycloheptatrienylium (tropylium) cation (8), with an aromatic sextet delocalised over a symmetrical seven-membered ring, is also demonstrably aromatic in character. The stable, condensed, bicyclic hydrocarbon azulene (Ci0H8) possesses marked aromatic character it is usually represented by the covalent structure (9). The fact that the molecule has a finite dipole moment, however, suggests that the ionic form (10) [a combination of (7) and (8)] must contribute to the overall hybrid structure. 

The scope of this problem can be indicated by a brief consideration of the stereochemical nomenclature of hydroxy derivatives of the bicyclic terpenes. For the parent bicyclic hydrocarbon compound known in the literature at present as cam-phane, the name "bomane is recommended in this report. (Reasons for choosing "bornane are discussed later under "The Bornane Hydrocarbons .) A mono-hydroxy derivative of bornane, on the basis of IUPAC rules (41), would be named as a bornanol. Two such derivatives have the common terpene names, bomeol and isobomeol these names, on the basis of recommendations in this report would both become 2-bornanol. Obviously, this is inadequate and additional designations are necessary to distinguish between die two stereoisomers. 

Reducing the chemical structures of the remaining simple monocyclic and bicyclic hydrocarbons to five fundamental types. 

The recommended systematic names of the thujane, carane, and pinane bicyclic hydrocarbons are formed from the common terpene names of the saturated fundamental structures larger than the unsubstituted ring systems. Where these do not apply, the nor- forms of these terpene names are recommended for the carane and pinane types of structures, and the bicyclo names for the thujane type. For purposes of uniformity and convenience, skeletal formulas resembling the familiar terpene formulas are used they are oriented so that the isolated methyl group is at the top and the numbering runs clockwise. The general rules for forming the names are ... 

Furthermore, the use of special subgroup names such as fenchenes, which applies to a series of synthetic compounds to indicate their derivation from fenchyl and iso-fenchyl derivatives and reportedly (33) to distinguish them from closely related naturally occurring terpenes, obviously complicates the problem of systematizing nomenclature. Names such as cyclofenchene and /8-difenchene are two additional memory exercises. Such irregularities in nomenclature have, of course, been carried over into the nomenclature of derivatives of these bicyclic hydrocarbons. 

Gradual replacement of the currently used common terpene names for this group of bicyclic hydrocarbons by the recommended new norbornane-bornane names should not be difficult, and may well be accomplished much more speedily than now antici-... 

Form the names of simple acyclic, monocyclic, and bicyclic hydrocarbon terpene radicals systematically from the recommended names of the terpene hydrocarbons. For cyclic compounds, radicals may be formed with points of attachment in side chains which have a fixed numbering as well as in rings. Preference in numbering is given to the point of attachment over double bonds when there is a choice. 

The rules for naming bicyclic hydrocarbon terpene radicals are ... 

Rule 7 Form the above types of terpene-like names for all hydrocarbon terpene radicals derived from the four fundamental types of bicyclic hydrocarbons regardless of the position of the point or points of attachment that is, the H or H s may be removed from carbons in the ring or from carbons in the numbered side chains. Use position numbers to designate the position of the point or points of attachment in the radicals. 

The cyclopropane synthesis is also suitable for the preparation of highly strained bicyclic hydrocarbons such as [2.1.0]bicyclopentanes (14) and spiropentanes (16) [14a,b]. The formation of the spiropentane 16 is particularly remarkable as it is the result of a homolytic hydrogen abstraction from a cyclopropane ring. Those processes are very rarely observed due to the relatively high C-H-bond energies of cyclopropanes (Sch. 8). 

Beginning with diethynylbenzene derivatives, it is obvious that the para-isomer 45 could lead to rod-like hydrocarbons, whereas the meta- (46) and the ortho-isomers (47), respectively, could provide angular and circular structures. Of course, an increase in the number of ethynyl groups results in an increased number of coupling possibilities. 1,3,5-Triethynylbenzene (48), for example, is of interest since it can be used for the construction of bicyclic hydrocarbons - it constitutes a blown-up version of a bridgehead. Clearly, hexa-ethynylbenzene (49) [32] can function as a tile for the preparation of novel forms of... 

A cell-free extract obtained from T. roseum has been shown to convert trans,trans-farnesyl pyrophosphate into trichodiene (87), the bicyclic hydrocarbon precursor of the trichothecane sesquiterpenoids.42 Related studies in this area have also been reported.43,44 Chemical and spectroscopic evidence has been reported which is consistent with the structure assigned to vetisporin (88), a new antibiotic isolated from Verticimonosporium diffractum.45... 

---