Versiconal acetate biosynthesis

Dichlorvos possesses a strong inhibitory effect on aflatoxin biosynthesis in A.flavus and A. parasiticus, presumably by specific enzyme inhibition. Dichlorvos inhibited 90% of the aflatoxin production by A. parasiticus ATCC 15517 (Hsieh, 1973). An early step in the secondary metabolic pathway was inhibited, as only additions of dichlorvos prior to the initiation of toxin production, which occurs 40 hr after incubation effectively stopped the toxin synthesis. The reduction in aflatoxin synthesis was accompanied by the excretion of versiconal acetate (19). Yao and Hsieh (1974) reported the efficient conversion of versiconal acetate (13.7%) into aflatoxin Bj by a nitrogen-free resting cell medium oiA. parasiticus. Averufin is converted into versiconal acetate, and not into aflatoxin Bj, in the presence of dichlorvos (Yao and Hsieh, 1974). 

Several specific enzyme activities associated with precursor conversions in the aflatoxin pathway " " have been partially purified (Fig.l 1.1) whereas others such as methyltransferases " have been purified to homogeneity. Several other enzymes involved in aflatoxin biosynthesis, such as a reductase and a cyclase, " have also been purified from A. parasiticus. A desaturase that converts VERA to VERB has been found in cell-free fungal extracts. Matsushima et al. have purified and characterized two versiconal hemiacetal acetate reductases involved in toxin synthesis, whereas Kusumoto and Hsieh purified to homogeneity an esterase that converts VHA to versiconal. ... 

YABE, K., ANDO, Y., HAMASAKI, T., A metabolic grid among versiconal hemiacetal acetate, versiconol acetate, versiconol and versiconal during aflatoxin biosynthesis, y. Gen. Microbiol., 1991,137, 2469-2475. 

YABE, K., HAMASAKI, T., Stereochemistry during aflatoxin biosynthesis Cyclase reaction in the conversion of versiconal to versicolorin B and racemi2ation of versiconal hemiacetal acetate, Appl. Environ. Microbiol., 1993, 59, 2493-2500. 

BiosyntheticaUy, the aflatoxins are all formed from the same precursor, versiconal hemiacetal acetate (12) (25). Compound 12 is formed from acetate, the units of which are converted into a polyketide. The polyketide is then metabolized to the xanthone 12 (see Scheme 2.1) (26). Intermediate 12 can then be transformed either into versicolorin A (13) or versicolorin B (14) in several steps. Versicolorin A (13) may be converted to sterigmatocystin (15), while 14 can lead to dihydro-sterigmatocystin (16). Sterigmatocystin (15) can be metabolized to aflatoxins Gi (3) or Bi (1) and the latter may then be transformed to aflatoxin Mi (5). Aflatoxins B2 (2) and G2 (4) are formed from dihydrosterigmatocystin (16) and aflatoxin M2 (6) is formed by conversion from B2 (2). Pathways also exist to convert aflatoxin Bi (1) to B2 (2), Ml (5) to M2 (6), and Gj (3) to G2 (4), and vice versa. Important biosynthesis steps are shown in Scheme 2.1. 

In 1985, O Malley et al. published the total syntheses of rac-averufin (103) and rac-nidurufin (104) (65). These are both early precursors of the aflatoxins in their biosynthesis. Nidurufin (104) is the direct successor of averufin (103) and the direct precursor of versiconal hemiacetal acetate (12, see Scheme 2.1). Nidurufin (104) and averufin (103) are accessible by the same synthesis route only the two last steps differ firom each other (see Scheme 2.17). The first reaction was a double Diels-Alder reaction with dichloro-p-benzoquinone (97) and two equivalents of diene 98. Then, three of the four alcohol functions were selectively MOM-protected (—> 99). The remaining alcohol was converted into the allyl ether and then subjected to a reductive Claisen rearrangement, followed by MOM-protection of the redundant alcohol ( 100). By addition/elimination of PhSeCl, 101 was formed. Deprotonation of t-butyl 3-oxobutanoate, followed by reaction with 101 yielded the pivotal intermediate 102. This could be converted into rac-averufin (103) by deprotection of the alcohols and decarboxylation at the side chain. The last step was a p-TsOH-catalyzed cyclization to give 103. By treating 102 with /m-CPBA, the double bond is epoxidized. rac-Nidurufin (104) was then formed by cyclization of this epoxide under acidic conditions. 

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