Monocyclic Terpenes
Classification.
Type A and Type Β
The monocyclic terpene hydrocarbons, many of which are formed readily from the acyclic terpenes by ring closure or from the bicyclic terpenes by ring fission, contain a six-carbon ring and may all be considered as derivatives of either cyclo hexane or benzene. They may also be classified, on the basis of common larger fundamental structures, into two distinct types of substituted six-carbon ring structures: Type A . Those which contain as ring substituents an iso three-carbon group and a one-carbon group, usually in para position; for a few hydrocarbons of this type, these groups are in meta or ortho position. Thus, Type A monocyclics may be considered as derivatives of menthane (isopropylmethylcyclohexane) or of cymene (isopropylmethylbenzene). Type B . Those which contain no iso three-carbon group. Most of the hydro carbons of this type, for which structures have been proved, contain two one-carbon groups in gem configuration and one or two additional isolated one-carbon or two -carbon groups, one of which is usually in meta position. These structures could be further divided into those that contain the gem-dimethyl configuration and those that do not, but their similar chemical reactivity warrants classification as a single group of compounds. Thus, Type Β monocyclics may be considered as derivatives of dimethylcyclohexane, for which no trivial name has been well established. These structural relationships are evident from the typical formulas shown in Chart 4. Although in this chart a-pyronene is numbered as 1,5,5,6-tetramethyl, the numberings 1,1,2,3- and 1,2,3,3- appear frequently in the literature.
Nomenclature Problems Nomenclature rules and practices are well established for forming the systematic cyclohexane-type names for both Type A and Type Β monocyclic hydrocarbons (13, 43). Simpler names than these systematic names will result by applying the princi -ple of largest parent compound because fewer substituents have to be named (8). However, operating against the blanket application of this principle to the naming of all monocyclic terpenes is the principle of treating like things alike (9). The latter principle, as currently interpreted by the London Chemical Society and Chemi~ cal Abstracts, does not affect formulating simpler names for the Type A structures since the ring substituents (a one-carbon and an iso three-carbon group) are unlike, and the unsaturation of the structures differs only in position and number of double bonds. Violation of this principle is encountered, however, in naming the Type Β structures on the basis of larger parents because the ring substituents in gem con15
16
A D V A N C E S IN C H E M I S T R Y
Chart 4.
Structural Relation of Monocyclic Terpene Hydrocarbons to Aromatic and Saturated C y c l i c Hydrocarbons
Numbers within the Rings Indicate the Systematic Numbering Based on Chemical Abstracts Order of Prefixes; Usual Terpene Numbering.
Numbers outside the Rings Indicate the
These Numberings Are Those in Current Usage
and Are Not Necessarily the Recommended Numberings. Saturated Cyclic
Aromatic
H
Benzene
(C
1 Q
H
1 4
)
p-Cymene
(C H )
(CeH )
Cyclohexane
gem-Dimethyl Ο cyclohexane
6
12
16
1-IsopropyM-methyl (
(C8H ) 1 6
l>3(or m)-Dimethyl( cyclohexane
benzene Type A Monocyclic Terpene CM,
*^%, < 10 16>
* 10 16* 0- Terpinene
Dipentene
1- Isopropyl-4-
4-Isopropenyl-1-
C
H
C
H
< 10 18> < 10 18> Carvoraenthene Dihydroterpinolene C
H
C
4-Isopropyl-1-
methyl-1,3-
methylO
methyl ^
cyclohexadiene
cyclohexene
cyclohexene
H
1-Isopropylide ne-4-
p-Menthane l-Isopropyl-4methyl^
methyl cyclohexane
cyclohexane
Type Β Monocyclic Terpene
From P y r o l y s i s of Allo-ocimene
C
H
Cyclodihydro C myrcene 1,2,3,5-TetraC methyl-1,3cyclohcxadieae
2-Ethyl-6,6dimethyl-1,3cyclohexadiene
1,5,5,6-TetraC methyl Ο cyclohexene
SERIES
NOMENCLATURE
FOR T E R P E N E
HYDROCARBONS
17
figuration common to most of the structures and the additional isolated groups at various ring positions are frequently identical — that is, all may be methyl groups. In addition, a trivial name, menthane, for the parent compound of Type A mono cyclics is well established but none is established for the parent compound of Type Β monocyclics. Coining a new trivial name for this latter parent compound may not be the soundest approach to systematizing the nomenclature. Therefore, recommendations for naming the Type A and Type Β monocyclic terpene hydro carbons will be discussed separately.
Monocyclics. Type A.
Derivatives of Menthane
Structural Features Emphasis will be placed on the para-substituted Type A monocyclic terpene hydrocarbons in this discussion, but by analogy the material may be applied equal ly to the few known meta and ortho derivatives. The fundamental largest carbon skeleton common to the para-substituted monocyclics, as has been pointed out previously, is identical to that of the saturated hydrocarbon ^-menthane. (For pmenthane, the systematic name based on Chemical Abstracts order of prefixes is l-isopropyl-4-methylcyclohexane; Chart 4). These Type A monocyclic compounds may contain saturated or unsaturated rings. They may contain one or two double bonds; for the latter type, both bonds may be inside or outside the ring, or one bond may be in each position. The one-carbon group substituent may be methyl or methylene; the iso three-carbon group may be isopropyl, isopropenyl, or isopropylidene (structural formulas of known Type A monocyclics are given in Chart 5). E a r l y Attempts at Systematizing Nomenclature Attempts at systematizing the nomenclature of monocyclic terpenes (classed in this report as Type A) were made by Baeyer and by Wagner over 50 years ago. Baeyer (2), by applying the Geneva system, suggested that the dihydro-£-cymenes ( C ^ Q H ^ ) be called terpadienes; the tetrahydro-p-cymenes ( C J Q H J Q ) , the terpenes; and hexahydro-p-cymene (^1^26^* P - There is no evidence in the literature, however, that such names were used by other investigators. Wagner (54), in addition to pro posing the name p-menthane for the completely saturated p-cymene ( C ^ Q ^ O ) , pro posed p-menthenes for the tetrahydrocymene derivatives (CjQH^g), and p-menthadienes for the dihydrocymene derivatives (CjpHjg). p-Cymene on this scheme could be named 1,3,5-menthatriene. The fixed numbering of the p-menthane carbon skeleton — that is, the methyl group attached to the No. 1 ring carbon and the isopropyl group to No. 4; Chart 5 — was suggested by Wallach (55). These menthane-type names and Wallach's fixed numbering of the carbon skeleton of menthane were received favorably and began to appear occasionally in the literature before the end of the nineteenth century. t e r
a n e
Current Nomenclature The menthane-type names have been generally recognized for some time as good names for Type A monocyclics (15). The use of the prefixes p-, 0-, and m- in names of this type to designate the positions of substituents conforms to recognized usage of these prefixes with trivial names. Many of the common monocyclic terpene names appear in current literature with no mention of the corresponding menthane-type names. However, the latter have been used almost invariably for recently identified monocyclic terpene derivatives and for those having no familiar common terpene names.
18
A D V A N C E S IN C H E M I S T R Y
SERIES
The historical fixed numbering of the p-menthane carbon skeleton has been used consistently for over 50 years and hence is well established in the literature. The systematic cyclohexane names have also been used, but only infrequently, for some of these monocyclics. Both types of names and the corresponding formulas are given in Chart 5. Recommendation For the Type A monocyclic terpene hydrocarbons (para, ortho, and meta deriva tives of menthane) use the menthane, ment bene, and menthadiene names and the well-established fixed numbering of menthane. See Chart 5 for the para structures and the recommended names. The familiar terpene skeletal formulas and a consistent orientation of these formulas were chosen for Chart 5 merely for convenience. The recommendation of the American committees concerns only the names and fixed numbering of the menthane skeleton — it carries no stated preference for orientation or for the form in which formulas are drawn. Rules for Forming the Menthane-Type Names Rule 2.
Use the name menthane with the prefix p~, m~, or o- for the three funda
mental saturated Type A monocyclic hydrocarbons. Rule 2. Retain the well-established fixed numbering for the menthane carbon skeletons. (The familiar orientation of the menthane structures is used in these rules merely as an aid to memory.)
7 λ 3
/o o-Menthane
m-Menthane
p-Menthane
Draw unsaturated formulas so that the ring double bonds have as low numbers as are consistent with this fixed numbering — for example, in the para series:
Draw This
And Not This
é
