Simple Tetrahydro Isoquinoline Alkaloids

2.7.2 Simple Tetrahydro Isoquinoline Alkaloids

The typical representatives of the simple tetrahydroisoquinoline derivatives are the closely-related alkaloids occurring along with mescaline are, namely: anhalamine, anhalonine and anhalonidine.

These three alkaloids shall be discussed in the pages that follow:

A. Anhalamine

Biological Sources It is obtained from the plant Lophophora williamsii (Lemaire) Coult. (Coctaceae) (Peyote), and Anhalonium lewinii. Henn. (Cactaceae).

Chemical structures

1, 2, 3, 4-Tetrahydro-6, 7-dimethoxy-8-isoquinolinol, (C₁₁H₁₅NO₃).

Characteristic Features

1. The crystals have mp 189-191°C.

2. Its uv_max (ethanol) is 274 nm (log € 2.90).

3. Solubility Profile: It is found to be almost insoluble in cold water, cold ethanol, ether and freely soluble in hot water, ethanol, acetone and dilute acids.

Identification Test Anhalamine Hydrochloride Dihydrat (C₁₁H₁₅NO₃.HCl.2H₂O): It is obtained as crystals from water having mp 258°C.

Uses It may play a minor role in causing hallucinatious.

B. Anhalonine

Synonym Anhalanine

Biological Sources It is obtained from the mescal buttons [Lophophora williamsii (Lemaire) Coult. (Anhalonium lewinii Henn). Cactaceae]; and also in Ariocarpus, in Gymnocalycium gibbosum.

Chemical Structure

6, 7, 8, 9-Tetrahydro-4-methoxy-9-methyl-1, 3-dioxolo [4, 5-h] isoquinoline, (C₁₂H₁₅NO₃).

Characteristic Features

1. It is obtained as rhombic needles from petroleum ether having mp 86°C and bp_0.02 140°C.

2. Its specific optical rotation [α]_D²⁵-63.8⁰ (methanol); and [α]_D²⁵  -56.3⁰ (chloroform).

3. It is found to be freely soluble in ethanol, ether, chloroform, benzene and petroleum ether.

Identification Test

Anhalonine Hydrochloride (C₁₂H₁₅NO₃.HCl): It is obtained as orthorhombic prisms decomposing at 255°C. Its aqueous solution is almost neutral. It is found to be freely soluble in hot water.

Uses It may be employed as a mild hallucinating agent.

C. Anhalonidine

Biological Source It is invariably obtained from the mescal buttons, the buds of

Lophophora williamsii (Lemaire) Coult. (Anhalonium lewinii Henn.) belonging to the natural order Coctaceae.

Chemical Structure

1, 2, 3, 4-Tetrahydro-6, 7-dimethoxy-1-methyl-8-isoquinolinol; (C₁₂H₁₇NO₃).

Characteristic Features

1. It is mostly obtained as small octahedral crystals from benzene having mp 160-161°C.

2. Its uv_max (ethanol) is 270 nm (log € 2.81).

3. Its aqueous solution acts as a strong base.

4. It is freely soluble in water, ethanol, chloroform and hot benzene; sparingly soluble in ether; and practically insoluble in petroleum ether.

5. It has been observed that the solutions of anhalonidine acquire a reddish colouration on standing.

Uses It may be used as a mild hallucinogen.

Biosynthesis of Anhalamine, Anhalonine and Anhalonidine Interestingly, the two additional C-atoms present in anhalonidine and anhalonine are provided by pyruvate; whereas, the C-atom for anhalamine is supplied by glyoxylate, as shown below. However, in each instance, a carboxyl group is lost from this aforesaid additional precursor. The pyruvate i.e., the keto-acid eventually reacts with an appropriate phenylethylamine, in this particular instance the dimethoxy-hydroxy derivative, thereby yielding a Schiff Base. Further, a Mannich-like mechanism helps in the cyclization to produce the heterocyclic isoquinoline nucleus, whereby the mesomeric effect of an oxygen substituent caters for the nucleophilic site on the aromatic ring. Evidently, restoration of aromaticity via proton loss yields the tetrahydroquinoline nucleus, thus representing overall a biosynthetic equivalent of the Pictet-Spengler Isoquinoline Synthesis.* Subsequently, the carboxyl group is eliminated, not by means of a simple decarboxylation process, but via an unusual oxidative decarboxylation process that essentially involves the following steps, namely:

(i) First, producing the intermediate imine,

(ii) Secondly, subjecting to reduction yielding anhalonidine,

(iii) Thirdly, subjecting to methylation giving rise to anhalonine,

(iv) Fourthly, subjecting phenylethylamine precursor employing the glyoxylic acid instead of pyruvic acid generating anhalamine.

Source:Pharmacognosy And Pharmacobiotechnology By Ashutosh Kar