Bicyclic Monoterpenes

B. Bicyclic Monoterpenes

The bicyclic monoterpenes, as the name suggests essentially possess two cyclic rings which are condensd together. This class of compound is relatively more complex in nature in comparison to the monocyclic species. The second ring system usually conatin 2, 3 or 4 C-atoms in common and the rings may be having 3, 4, 5 or 6 membered rings.

The bicyclic monoterpenes may be regarded as chemical entities derived from:

(a) para-Menthane – by direct fusion of 2–C atoms and the formation of a simple bridge, and

(b) Methylated Cyclohexanes– by having a bridge with either –CH₂– or C(CH₃)₂ – moieties.

In general, the ‘bicyclic monoterpenes’ are classified into five categories, namely:

(i) Thujane; (ii) Pinane; (iii) Carane

(iv) Camphane; and (v) Fenchane.

These five distinct categories shall be discussed briefly with typical examples as given below:

I. Thujane

4-Methyl-1-(1-methyl ethyl) bicyclo[3.1.0] hexane.

Eventually, thujane is derived from p-menthane with direct union between C-2 and C-4. It comprises of a 3-memberd and a 6-membered ring. The ‘bridge’ in this particular instance does not have the isopropyl group in it.

Example: Sabinene

A Sabinene

Chemical Structure

4-Isopropyl-p-methylene bicyclo-2, 4-hexane.

Occurrence It is the major constituent (≈30%) in oil of savin obtained from young shoots of Juniperus sabina L., Cupressaceae. It is also present in oils of cardamom and majoram.

Isolation It is obtained by the fractional distillation of oil of savin under reduced pressure.

Characteristic Features It is a liquid, lighter than water. It is found to be isomeric with α-thujane.

Identification Sabinene either on boiling with dilute sulphuric acid or on shakig with cold dilute sulphuric acid yields:

(i) different forms of terpinene, and

(ii) 1, 4-terpin.

II. Pinane It is formed from p-menthane by forming a bridge between C-3 and C-6 positions, thereby resulting into the formation of a 4-membered ring system and a parent 6-membered ring system.

Example α-Pinene.

Chemical Structure 2,6,6-Trimethyl bicyclo[3,1,1] hept-2-ene;

Occurrence It is obtained from oil of turpentine which contains 58-65% α-pinene along with 30% β-pinene. It is also widely distributed in essential oils belonging to the family Coniferae. It has been reported to be present in oils of American pippermint, corriander, cumin and lemon.

Isolation

(i) It is isolated from the essential oils stated above by the help of chromatographic techniques.

(ii) Mostly isolated by the fractional distillation from essential oils, preferable under reduced pressure followed by further purification. The fraction collected between 155-165^oC is converted to crystalline form of nitrosochloride (treated with amyl nitrite and hydrochloric acid) from which the desired product is liberted by treatment with aniline.

Characteristic Features It is a colourless oil which has a tendency to resinification on exposure to air. The various physical parameters of its isomers are given below:

dl-form : bp₇₆₀ 155-156^oC; d₄²⁰ 0.8592; n_D²⁰ 1.4664

d-form : bp₇₆₀ 155-156^oC; d₄²⁰ 0.8591; n_D²⁰ 1.4661;

l-form : bp₇₆₀ 155-156^oC; d₄²⁰ 0.8590; n_D²⁰ 1.4662.

The l-form is usually found in the French Turpentine Oil, whereas the d-form is found in the American, German and Swedish Turpentines.

Identification It may be characteristized by–

(a) Preparation of its nitrosochloride derivative mp115^oC, which is devoid of optical activity,

(b) Preparation of its hydrochloride derivative mp 132^oC, and [α ]_D²⁰ – 33.24 °C (in alcohol), and

(c) Preparation of its adduct with malic anhydride (crystalline ) mp 169^oC.

Uses

1. It is abundantly used as a starting material for the large-scale preparation of synthetic camphor

as given below:

2. Turpentine oil is cooled to –10^oC first and then hydrogen chloride gas is passed through it to obtain the pinene hydrochloride. The latter undergoes isomerization to yield bornyl chloride which on treatment with alkali gives rise to borneol. This on oxidation with nitric acid yields pure synthetic camphor.

3. It also finds its application in the production of insecticides, solvents, plasticizers, perfume bases and synthetic pine oil.

III. Carane para-Menthane with a bridge between C-3 and C-8 results into the formation of carane, which comprises of a 3-membered ring imbeded into the 6-membered parent ring as given below:

Example

A 3-Carene

Chemical Structure 3,7,7 Trimethylbicyclo [4,1,0] hept-3-ene (a); 4,7,7–Trimethyl-3 norcarene (b).

Occurrence It is a constituent of turpentine. The turpentine obtained from Pinus sylvestris L., contains upto 42%; turpentine from Pinus longifolia Roxb; Pinaceae about 30%.

Isolation It is isolated from the turpentine oil by the usage of chromatographic techniques.

Characteristic Features It is a sweet and pungent odour essential oil having a more agreeable odour than that of turpentine. It is practically insoluble in water but miscible with most fat solvents

and oils. The d-form possess physical characteristics, e.g.; d₁₅¹⁵ 0.8668; d₃₀³⁰ 0.8586; bp₇₀₅ 168-169^oC; [α]_D²⁰ + 17.69; n_D³⁰ 1.468.

Identification The d-form gives rise to the nitrosoate derivative (C₁₀H₁₆ N₂O₄), which may be prepared by treating d-Carene with amyl nitrile, acetic acid and nitric acid. Its prism decomposes at 147.5°C.

Uses It is used as an antiseptic, carminative, stimulant, stomachic and diuretic.

IV. Camphane It is formed with a direct bondage between C-1 and C-8 in the structure of p-menthane. It essentially comprise of two five-membered rings besides a six-membered ring.

Example

A Camphene

Chemical Structure 2,2, Dimethyl-3-methylenebicyclo-[2,2,1] heptane;

Occurrence It mostly occurs in a large variety of essential oils, for instance:

(i) Turpentine oil (levo and dextro forms),

(ii) Cypress oil (dextro form),

(iii) Camphor oil (dextro form in species of Lauraceae),

(iv) Bergamot oil, and

(v) Oils of Citronella, Neroli, Ginger, and Valerian).

Camphene occurs in a number of species, namely: Achillea, Milefolium, Acorus calamus, Anethum graveolens, Artemisia, Cinnamonum, Foeniculum vulgare, Juniperus, Kaempferia galanga, Myristica fragans, Peumus boldus, Pinus ellottii, Piper nigrum, Pistacia lentiscus, Rosamarins officinalis, Satureja, Schinus molle, Thymus, a and Valeriana officinalis.

Isolation Camphene is isolated by the chromatographic techniques from rectified turpentine oil.

Characteristic Features Camphene obtained from alcohol found in cubic crystals (dl-form) having an insipid odour.

dl-form: mp 51 to 52^oC; bp₇₆₀- 158.5 to 159.5^oC; d⁵⁴₄ 0.8422; n⁵⁴_D 1.45514.

Solubility Soluble in ether, dioxane, cyclohexane, cyclohexene and chloroform. Practically insoluble in water and moderately soluble in alcohol.

d-form : mp 52°C; [α]¹⁷_D + 103.5°; (C=9.67 in ether); d⁵⁰₄ 0.8486; n⁵⁰_D 1.4605;

l-form : mp 52°C; [α]²¹_D – 119.11°; (C=2.33 in benzene); d⁵⁴₄ 0.8422; n⁴⁰_D 1.4620.

Identification It forms large dodecahedra on being subjected to slow sublimation

Uses

1. As an important constituent of eucalyptus oil which is used as a counter-irritant, antiseptic and expectorant.

V. Fenchane It is a trimethyl cyclohexane with a methylene (—CH₂—) bridge. It consists of two five-membered and a six-membered ring.

Example d-Fenchone

Chemical Structure

(1S) – 1,3,3,-Trimethylbicyclo [2,2,1]-heptan-2-one.

Occurrence It occurs in fennel oil and in the essential oil of Lavondula stochas L., Libitatae.

Isolation It is isolated from the fennel oil by column chromatography which mostly contains this ketone to the extent of 20%.

Characteristic Features It is a colourless oily liquid having a camphor like odour. It attributes the bitter taste to the drug. It is very soluble in absolute alcohol and ether; but practically insoluble in water.

D¹⁸₄ 0.948; mp 6.1°C; bp₇₆₀ 193.5°C; [α]²⁰_D + 66.9°; n¹⁸_D 1.4636.

Identification The pH of its saturated solution is 6.82.

Uses

1. It is employed extensively in foods and in perfumes.

2. It also finds its application as counterirritant.