Quinoline alkaloids from higher plants

The distribution of quinoline alkaloids among higher plants is limited, and the number of quinoline alkaloids is also limited. Thus, O. japonica is definitely one of the treasure houses of quinoline alkaloids, and more than 23 quinoline alkaloids have been isolated from this plant material to date [9]. 

With a year production of 300-500 tons (26), the Cinchona alkaloids (quinine 1 and quinidine 2) probably form one of the largest markets of fine chemicals derived from higher plants. They are extracted from stem and rootbark of Cinchona trees, containing 5-18Z of alkaloid, with an average of about 8X (27). Because of the high demand a number of attempts have been made to develop a commercial synthesis (28 and references cited therein) of the quinoline alkaloids. Although successful syntheses have been reported they could not be commercialized. 

QUINOLINE ALKALOIDS ISOLATED FROM HIGHER PLANTS OTHER THAN THE RUTACEAE FAMILY... 

One of the simplest quinoline alkaloids obtained from higher plants is echinopsine, which was isolated from the seeds of Echinops ritro (Astraceae) [2]. The chemical structure of this alkaloid was determined in 1922 [3] and it was synthesized by several groups [4-8]. No studies on the biosynthesis of this alkaloid have been conducted, but it is possible that anthranilic acid is the biosynthetic precursor. 

Many of the modifications of the amino acid units that are found in fungal metabolites differ from those in higher plants. In particular, the decarboxylation of amino acids is far less common in fungi. For example, the decarboxylation of phenylalanine or tyrosine to form an arylethylamine is far less common in fungi and hence alkaloids derived from (3-arylethylamine units such as the benzyliso-quinoline alkaloids, which are common in plants, are unusual in fungi. 

Quinoline Alkaloids withont 3-Substitnents.—4-Quinolones with a long-chain alkyl group in the 2-position were obtained originally from fungi, but in recent years have been shown to be present in some higher plants. A new alkaloid, 2-(n-undecyl)-4-quinolone (5 n = 10), has now been isolated from Ptelea trifoliata and its structure established by spectral data. The roots of Ruta graveolens contain an unresolved mixture of alkylquinolones (5 n = 10—13). ... 

None of the precursors in the biosynthesis of quinoline alkaloids is available at a price lower than the alkaloids thus, bioconversion is not of interest, except for one step. This step would be the stereospecific conversion of quinidinone (obtained by oxidation of quinine or quinidine) to quinidine (8/ ,95) or quinine (85,97 ). This reaction offers the possibility of converting an excess of one of the naturally occurring stereoisomers to the other. It is now performed chemically as the demand for quinidine is higher than production from plant extracts. 

Quinoline-4-carbaldehyde derivatives also occur in higher plants (Table Vlll). For example, 8-hydroxyquinoline-4-carbaldehyde oxime (J.3.1 Scheme 9) was isolated from Broussonetia zeylanica (Moraceae) (151-153). (+)-Tuberosine B (J.1.2), an unprecedented tetrahydroquinoline alkaloid, has been reported in the inaccessible Chinese literature as a new metabolite of Allium tuberosum (Alliaceae) (157). Eichhornia crassipes (Pontederiaceae) is an invasive plant and often jams rivers and lakes with uncounted thousands of tons of floating plant matter. It has yielded 1,4-dimethylquinolinium iodide (J.4) (142). 

These alkaloids constitute two small groups, and only a few derivatives occur sporadically in microorganisms and higher plants. For example, 8-hydroxyquino-line-4-carbaldehyde oxime (J.3.1) and broussonetine (J.2.1), a dimeric quinoline, were isolated from B. zeylanica (Moraceae) (151), (+)-tuberosine B (J.1.2) from... 

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