2.1.3 Terpenoid Alkaloids
A plethora of alkaloids solely based on mono-, sesqui-, di-, and tri-terpenoid skeletons have been isolated and characterized. However, logistic and scientific information (s) with regard to their actual formation in nature is more or less sparse. It has been observed that the monoterpene alkaloids are derived from the structurally related iridoid materials, wherein the O-atom in the heterocyclic ring is replaced by a N-containing ring as depicted below.
A few typical examples of the terpenoid alkaloids are, namely: aconine and actinitine, which shall be discussed in the sections that follow:
A. Aconine
Biological Source Aconine is the hydrolyzed product of aconitine which is obtained from the dried roots of Aconitum napellus Linn. (Ranunculaceae) and other aconites. A. napellus in also known as aconite, blue rocket and monkshood. Usually it contains upto 0.6% of the total alkaloids of aconite, of which approximately one third is the alkaloid aconitine.
Chemical Structure (1α, 3α, 6α, 14α, 15α, 16α)-20-Ethyl-1, 6-16-trimethoxy-4-(methoxymethyl) aconitane-3, 8, 13, 14, 15-pentol.
Isolation The alkaloid aconitine is subjected to hydrolysis which yields benzoyl aconine and acetic acid. The resulting benzoyl aconine is further hydrolyzed to yield aconine and benzoic acid.
Aconine being very soluble in water may be separated easily from the less water-soluble by product, i.e., benzoic acid.
Characteristic Features
(i) It is an amorphous powder with a bitter taste.
(ii) It has mp 132°C, [α]D + 23° and pKa 9.52.
(iii) It is extremely soluble in water, alcohol; moderately soluble in chloroform and slightly soluble in benzene. It is practically insoluble in ether and petroleum ether.
Identification Tests It forms two distinct derivatives as given below:
(a) Aconine Hydrochloride Dihydrate (C25H42ClNO9.2H2O): It is obtained as crystals having mp 175-176°C and [α]D–8°.
(b) Aconine Hydrobromide Sesquihydrate (C25H42BrNO9 .1½ H2O]: It is obtained as crystals from water with mp 225°C.
Uses
1. It is used in the treatment of neuralgia, sciatica, rheumatism and inflammation.
2. It is employed occasionally as analgesic and cardiac depressant.
B. Aconitine
Biological Source The bolanical source is the same as described under (A) above.
Chemical Structure
(1α, 3α, 6α, 14α, 15α, 16(β)-20-Ethyl-1, 6,-16-trimethoxy-4-(methoxymethyl) aconitane-3, 8, 13, 14, 15,-pentol 8-acetate 14-benzoate (C34H47NO11).
Characteristic Features
1. It occurs as hexagonal plates having mp 204°C.
2. Its pKa value stands at 5.88.
3. Its specific rotation [α]D + 17.3° (Chloroform).
4. It is slightly soluble in petroleum ether; but 1 g dissolves in 2 ml chloroform, 7 ml benzene, 28ml absolute ethanol, 50 ml ether, 3300 ml water.
Identification Tests Aconitine forms specific salts with HBr and HNO3 having the following physical parameters.
(a) Aconitine Hydrobromide Hemipentahydrate (C34H47O11.HBr.2½ H2O): The hexagonal tablets mp 200-207°C and the dried substance mp 115-120°C. Its crystals obtained from ethanol and ether with ½ H2O has mp 206-207°C. Its specific rotation [α]D – 30.9°.
(b) Aconitine Nitrate (C34H47NO11.HNO3): The crystals have mp about 200°C (decomposes), [α]D20 -350(c = 2 in H2O).
Uses
1. It is exclusively used in producing heart arrythmia in experimental animals.
2. It has also been used topically in neuralgia.
Biosynthesis of Aconitine-Type Alkaloids Aconite is particularly regarded as extremely toxic, due to the presence of aconitine, and closely related C19 nonditerpenoid alkaloids. It has been observed that the species of Delphinium accumulate diterpenoid alkaloids, for instance: atisine, which proved to be much less toxic when compared to aconitine. A vivid close resemblance of their structural relationship to diterpenes, such as: ent-kaurene, of course, little experimental evidence is available.
From the above course of reactions it appears quite feasible that:
(a) A pre-ent-kaurene carbocation usually undergoes Wagner-Meerwein Rearrangements,
(b) The atisine-skeleton is produced subsequently by incorporating an N–CH2–CH2–O fragment (e.g., from 2-aminoethanol) to form the resulting heterocyclic rings,
(c) The aconitine-skeleton is perhaps formed from the atisine-skeleton by further modifications as stated above,
(d) A rearrangement process converts two fused 6-membered rings into a (7 + 5)-membered bicyclic system, and
(e) One carbon from the exocyclic double bond is eliminated.
