2.5Coumarone Bitter Principles
The combination of the furan and benzene rings gives rise to the formation of benzofuran nucleus, otherwise termed as ‘coumarone’ as already depicted under Section (2.3). The most important coumarone based bitter principle is rotenone which shall now be discussed in an elaborated fashion in given below.
2.5.1 Rotenone
Synonym Canex
Biological Sources The principal insecticidal constituent of the dried derris roots, Derris elliptical Roxb. and D. malaccensis Prain, belonging to family Leguminoseae; from cube roots, Lonchocarpus utilis and L. urucu, belonging to the natural order Leguminoseae; from Lonchocarpus nicou (Aubl.) D.C. (Leguminoseae); fruits and plant of Piscidia piscipula Sarg. (Jamaica Dogwood); and the roots of Tephrosia virginiana (L.) Pers (Fabaceae) (Devil’s Shoe String).
Chemical Structure
[2R-(2α, 6aα, 12aα)]-1, 2, 12, 12a-Tetrahydro-8, 9-dimethoxy-2-(1-methylethenyl)-[1] benzopyrano [3, 4-6] furo [2, 3-h] [1] benzopyran-6 (6aH)-one; (C23H22O6). It is a rotenoid.
Isolation Various steps involved in the isolation of rotenone are as follows:
1. The derris roots and rhizomes are dried, powdered, sieved and extracted with carbon tetrachloride in a Soxhlet apparatus for at least 24 hours.
2. The CCl4 extract is filtered, concentrated under vacuo and allowed to cool at an ambient temperature for 24 hours, when crystals of rotenone separate out.
3. The resulting mixture is filtered through a gouche crucible under suction, and the crystals this collected are washed with a little CCl4; and finally dried in the air.
Characteristic Features Its chemical features are:
1. It is an ‘isoflavone compound ’ wherein the 2 : 3 double bond has undergone reduction.
2. Its heterocyclic portions are:
(a) A hydrobenzopyran moiety, and
(b) A hydrocoumarone (or 2 : 3-benzofuran) function.
3. Rotenone is a derivative of tubic acid lactone and is more commonly known as 6, 7-dimethoxy-2, 3-dihydro-benzopyran tubic acid lactone.
4. Decomposition of rotenone yields derric acid and tubic acid; and the latter further gives rise to a ‘lactone’ termed as the tubic acid lactone as shown below:
The physical parameters of rotenone are:
1. It is usually obtained either as orthorhombic or as six-sided plates from trichloroethylene having mp 165–166°C; however, the dimorphic form has mp 185–186°C.
2. Its specific optical rotation [α]D20 – 228° (C=2.22 in benzene).
3. It gets decomposed upon exposure to air and light.
4. Rotenone is almost insoluble in water; and soluble in ethanol acetone, carbon tetrachloride, chloroform, ether, in addition to many other organic solvents.
Identification Tests These are as given below:
1. Dissolve 2-3 mg of rotenone in 1 ml acetone and add to it 1 ml dilute HNO3 (50% v/v), and allow it to stand for about one hour to cause the oxidation. Now, add to it a few drops of NaOH solution (10% w/v) when a distinct blue colour gets developed.
2. Its colourless solutions in organic solvents normally oxidize upon exposure and become yellow, orange and then deep red finally. It may also deposit crystals of dehydrorotenone and rotenonone that are found to be toxic to insects.
Uses
1. It is mostly used as a potent pesticide.
2. It is widely employed as an acaricide and actoparasiticide in cattles.
3. The action of rotenone closely resembles to that of pyrethrin in affecting a rapid knock-down of the flying insects (e.g., house-flies, mosquitos etc.); and is found to be comparatively harmless to the warm-blooded animals.
4. As rotenone does not leave any harmful residue, it may be employed with enormous safety for most delicate and precious garden crops and garden plants.
Note Interestingly, though the derris roots do contain a natural insecticidal principle (rotenone) they are nevertheless prone to infestation by some specific types of insects obviously unaffected by rotenone. Biosynthesis of Rotenone In general, many thousands of wide variety of isoflavonoids have been duly isolated, characterised and identified; and subsequently their structural complexity havebeen resolved logically and methodically by first carrying out the hydroxylation, and secondly by alkylation reactions. These reactions not only helped in varying the oxidation level of the heterocyclic ring, but also produced additional heterocyclic rings.
In the biosynthesis of rotenone a simple isoflavone called daidzein is the starting material which undergoes methylation in the para position of the phenyl ring attached to the pyran ring with a covalent bond thereby forming an isoflavone termed as formononetin. This undergoes further biotransformations as stated earlier to yield rotenone. It contains a C5 isoprene unit, as could be observed in most of the natural rotenoids,* which is afforded via dimethyl-allylation of demethylmunduserone.
