Caranes and Thujanes

Additional chemical diversity of monoterpenes is apparent from the natural occurrence of their bicyclic analogs that bear cyclopropane (carane and thujane types), cyclobutane (pinane type), and cyclopentane (camphene/bornane, isoeamphane and fenchone types) rings (Figs. 6 and 7). The carane type of bicyclic monoterpenoids in plants is represented by (+)-3-carene... 

Bicyclic cyclopropanes carane and thujane, bicyclic cyclobutane pinane, and bi-cyclo[2.2.1]heptanes such as camphane, isocamphane and fenchane are the most important skeletons of naturally occurring bicyclic monoterpenes... 

Bicyclic Compounds with Carane and Thujane Skeletons... 

SCHEME 6 Biosynthetic pathways toward carane- and thujane-type monoterpenes. 

Preliminary testing of the two schemes for configurational names indicates strong preference for the a,p method. Although this method was applied above only to bornane-type bicyclics, it can be applied equally as well to configurational names for the other fundamental bicyclic structures — that is, the thujane, carane, and pinane types. The reference point of these structures could be the same as in the case of bomane — that is, the. most substituted bridge carbon atom. For example ... 

Recommended systematization of the nomenclature of the bicyclic terpene hydrocarbons will now be discussed for each of the four fundamental types. The thujane, carane, and pinane groups present very few and similar nomenclature problems. Some drastic steps are indicated, however, to resolve some of the difficulties encountered in naming the bomane-group compounds. For this reason, one set of rules is formulated below for the first three groups, and the rules for naming the bornane-group bicyclics will be discussed separately. 

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 ... 

Rule 2. Retain the names thujane, carane, and pinane for the fundamental saturated structures larger than the unsubptituted ring systems, and form series of names based on these common terpene names. 

For a double bond outside the ring, two prefix numbers are necessary — that of the ring carbon to which the double bond is attached, and following this in parentheses that of the substituent carbon to which the double bond is attached — e.g., 3(10)-carene(Formula 58,Chart 11) 2(10),3-pinadiene (Formula 70, Chart 12). If the number of the substituent carbon in such a double bond were the next higher-numbered carbon to the ring carbon, which is impossible on the basis of the fixed numberings of the thujane, carane, and pinane structures (see Rule 2), then only one prefix number would be necessary. 

The bicyclic terpene hydrocarbons classified in this report as the bomane group (bicycloQ2.2.1 heptane type), but currently called the camphane group in terpene literature, are more numerous and have many more important derivatives (especially the camphors) than the other three fundamental bicyclic types — the thujane, carane, and pinane groups — already discussed. As would be expected, a greater variety in the arrangement of substituents and double bonds and in the number of substituents is found. 

The historically developed nomenclature of the compounds classified as the bornane group in this report contributes much more confusion to the terpene literature than the common tetpene names of the compounds belonging to the thujane, carane, and pinane groups. Campbane (Formula 75, Chart 13) is a logical name for the saturated parent hydrocarbon of camphor, which differs from this parent only in that it has an oxo group instead of two hydrogens attached to ring carbon No. 2. 

FI) that occurs in Pinus longifolia (Pinaceae) and the related carboxylic acid, (+)-chaminic acid (F2), in Chamaecy-paris nootkatensis (Cupressaceae). Compared with caranes, the thujane type of monoterpenoids is more abundant in plants. The hydrocarbon analog (—)-3-thujene (F3) has been found... 

Rule 2. One bridge-end carbon is No. 1, and the numbering proceeds clockwise around the entire ring skeleton, first by the longer route to the other bridge-end carbon and then by the shorter route back to carbon No. 1. Hence, in thujane and pinane, the bridge ends are 1-5 instead of 1-3 in carane, 1-6 instead of 1-3 in bornane, 1-4. 

Rule 3. When a valence bridge — i.e., contains no carbon — is present, the ring carbon skeleton is completely numbered by Rule 2. This is the case for thujane and carane. 

To number the positions if double bonds are present, follow the bicyclo plan of assigning to double bonds as low positions as are consistent with the numbering imposed by the bridges. For structures containing one double bond, this means that only 2- -ene structures can exist in the thujane and pinane groups but both the 2- and 3 -ene structures can exist in the carane group. Thus ... 

Different methods of computer-matching the mass spectra of 122 monoterpenoids give the best results when six to eight of the most intense peaks are matched.21 The mass spectral fragmentation patterns of various bicyclic ketones of the thujane and carane series, and of their deuteriated [e.g. (6 X = D)] analogues, did not produce any generalizations on the fragmentation patterns or on the deuterium content.22... 

Most common bicyclic monoterpenes are members of the pinane, bor-nane, or thujane families, while members of the fenchane and carane series are less widely distributed. 

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