Ochratoxin B

These compounds were found to be important mycotoxin constituents of Aspergillus species shortly after the discovery of the aflatoxins (see Chap. 2.). The ochratoxins are isocoumarin derivatives coupled with p-phenylalanine. The major representatives of this class of pentaketides are shown in Fig. 6.2. Ochratoxin B (324) is the dechloro analog of ochratoxin A (323), which is the most important member of this group with respect to its toxicity. The corresponding methyl and ethyl esters of ochratoxin A (323) and B (324) were found also in Aspergillus species and the ethyl ester of ochratoxin A (323) has been named ochratoxin C (325). Ochratoxin a (326) is a free carboxylic acid that represents the dihydroi-socoumarin nucleus of ochratoxin A (323). 

Ochratoxin a (326) or the dechlorinated derivative 331 can be converted easily to ochratoxin A (323) and C (325) or ochratoxin B (324), respectively. Steyn et al. formulated this approach in 1967. They converted the free acid to the acid chloride, converted this into the acid azide and coupled the product with L-phenylalanine... 

Sibi MP, Chattopadhyay S, Dankwardt JW, Snieckus V (1985) Combinational 0-Aryl Carbamate and Benzamide Directed Ortho Metalation Reactions. Synthesis of Ochratoxin A and Ochratoxin B. J Am Chem Soc 107 6312... 

From a toxicological point of view, the most important representative of this group of mycotoxins is ochratoxin A, 3R)-N-[(5-chloro-3,4-dihydro-8-hydroxy-3-methyl-1-0X0-lH-2-benzo-pyran-7-yl)carbonyl]-L-phenylalanine (12-92), the molecule of which phenylalanine JV-substituted with a derivative of 3R)-3,4-dihydro-3-methylisocoumarine that contains at C-5 a chlorine atom to which are attributed the toxic effects of ochratoxin A. The incorporation of the chlorine atom into the skeleton of ochratoxin A is done by the action of chloroperoxidase the chlorine donor is inorganic chloride. Ochratoxin B (12-92) differs from ochratoxin A only in the absence of the chlorine atom, while ochratoxin C (12-92) is an ethyl ester of ochratoxin A. Ochratoxins a, p and y resulting from from parent compounds by the loss of phenylalanine caused by peptide bond hydrolysis, are virtually non-toxic, in addition to ochratoxin B, and are not routinely monitored. 

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