Reduction of sterigmatocystin and versicolorin A hemiacetals with sodium borohydride

Chen, Paul N.; Kingston, David G. I.; Vercellotti, John R.

doi:10.1021/jo00442a034

Cited by 16 publications

(5 citation statements)

References 19 publications

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“…Dihydro- O -methylsterigmatocystin ( 32 , DHOMST) and dihydro-11-hydroxy- O -methylsterigmatocystin ( 34 , DHHOMST) were prepared from synthetic OMST and HOMST, respectively, by hydrogenation over Pd/C . After incubation with cultures of pYCapoKX transformants grown in galactose, we found that AFB 2 was produced in a yield suitable for analysis by TLC and ELISA (Figure , lanes 8 and 10).…”

Section: Resultsmentioning

confidence: 99%

Synthesis of 11-Hydroxyl O-Methylsterigmatocystin and the Role of a Cytochrome P-450 in the Final Step of Aflatoxin Biosynthesis

2002

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The major skeletal rearrangements (anthraquinone --> xanthone --> coumarin) that occur in the complex biosynthesis of aflatoxin B(1) are mediated by cytochromes P-450. Previous experiments have suggested that two successive monooxygenase reactions are required to convert the xanthone O-methylsterigmatocystin (OMST) to aflatoxin, a process we demonstrate is mediated by a single P-450, OrdA, in Aspergillus parasiticus in accord with findings in A. flavus. The first oxidative cycle is proposed to result in the formation of 11-hydroxy O-methylsterigmatocystin (HOMST), while the second entails aryl ring cleavage, demethylation, dehydration, decarboxylation, and rearrangement to give aflatoxin - a remarkable sequence of transformations. To test this hypothesis, HOMST has been synthesized by an alkylnitrilium variant of the Houben-Hoesch reaction. The troublesome xanthone carbonyl was protected as a butylene to allow further elaboration of the molecule, and then the product xanthone was restored in a uniquely facile peracid deprotection. Methods were devised to construct the sensitive dihydrobisfuran and to maintain the oxidation state of the partially methylated hydroquinone. Expression of ordA in a yeast membrane preparation enabled the intermediacy of HOMST both to be detected in the conversion of OMST to aflatoxin and to be established directly in the biosynthesis of the mycotoxin. Having secured the role of HOMST in aflatoxin formation, the mechanism of the second oxidative cycle of this P-450 is considered.

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Section: Resultsmentioning

confidence: 99%

Synthesis of 11-Hydroxyl O-Methylsterigmatocystin and the Role of a Cytochrome P-450 in the Final Step of Aflatoxin Biosynthesis

2002

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“…5-Methoxy-ST (MST) was obtained from Makor Biochemicals (Jerusalem, Israel). DHST and EST were prepared by the procedure of Chen et al (1977), and DHOMST was extracted from SRRC 2043 cultures as described earlier (Cleveland, 1989). HST, HOMST, and HMST were synthesized from ST, OMST, and MST, respectively, by trifluoroacetic acid treatment as described earlier by Cleveland et al (1987a).…”

Section: Methodsmentioning

confidence: 99%

“…The metabolites and aflatoxins were quantitated by comparison of the areas of peaks of fluorescent scans at 310 or 360 nm of spots on TLC plates against peaks of known standards run on the same plate, by using a Shimadzu dual-wavelength TLC scanner, CS-930 (Bhatnagar et al, 1987;Cleveland et al, 1987a). Quantities of metabolites were also determined spectrophotometrically (Shimadzu UV-visible dual-beam spectrophotometry, UV-160) by using established extinction coefficients (Bhatnagar et al, 1987;Cleveland et al, 1987a;Cleveland, 1989;Chen et al, 1977;Yabe et al, 1988;Cole & Cox, 1981).…”

Section: Methodsmentioning

confidence: 99%

Enzymological evidence for separate pathways for aflatoxin B1 and B2 biosynthesis

Bhatnagar¹,

Cleveland²,

Kingston³

1991

Biochemistry

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Aflatoxins B1 (AFB1) and B2 (AFB2) are biologically active secondary metabolites of Aspergillus flavus and Aspergillus parasiticus. These toxins are synthesized by the fungi from pathway precursors: sterigmatocystin (ST)----O-methylsterigmatocystin (OMST)----AFB1; dihydrosterigmatocystin (DHST)----dihydro-O-methylsterigmatocystin (DHOMST)----AFB2. The late stages of AFB1 synthesis are carried out by two enzyme activities, a methyltransferase (MT) (ST----OMST), and an oxidoreductase (OR) (OMST----AFB1). Properties of the purified MT have been identified in a previous investigation [Bhatnagar et al. (1988) Prep. Biochem. 18, 321]. In the current study, the OR was partially purified (150-fold of specific activity) from fungal cell-free extracts and characterized with extended investigation of the late stages of AFB1 and AFB2 synthesis. Whole cells of an isolate of A. flavus (SRRC 141), which produce only AFB2, were able to produce AFB1 in ST and OMST feeding studies; the results suggested that the enzymes involved in AFB2 biosynthesis also carry out AFB1 synthesis. Substrate competition experiments carried out with the OR showed that an increasing concentration of either OMST or DHOMST in the presence of a fixed, nonsaturating concentration of either DHOMST or OMST, respectively, resulted in a decline in production of one aflatoxin (B1 or B2) with a corresponding increase in the synthesis of the other toxin (B2 or B1). OMST was a preferred substrate (Km, 1.2 microM) for the oxidoreductase as compared to DHOMST (Km, 13.4 microM). Similar, substrate competition experiments showed that ST (Km, 2.0 microM) was a preferred substrate over DHST (Km, 22.5 microM) for a homogeneous MT.(ABSTRACT TRUNCATED AT 250 WORDS)

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“…Any partially deprotected anthraquinones were resubjected to the acidic conditions. Recrystallization from acetone produced versicolorin A hemiacetal 54 as a fine orange crystalline powder: 48.9 mg (96.6%); mp 264−265 °C (dec) (lit . mp 269−270 °C from acetone); TLC R f 0.2 in 30% EtOAc/hexanes + 1% AcOH.…”

Section: Methodsmentioning

confidence: 99%

Silyl Triflate-Mediated Ring-Closure and Rearrangement in the Synthesis of Potential Bisfuran-Containing Intermediates of Aflatoxin Biosynthesis

et al. 1999

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The biosynthetic pathway to the potent mycotoxin aflatoxin B1 is unusually long and complex, proceeding from anthraquinone to xanthone to coumarin nuclear types bearing fused tetrahydro- and bisdihydrofuran rings. A synthetic strategy is described involving two silyl triflate-mediated cyclization and rearrangement processes that have enabled both furofuran oxidation states to be readily achieved and undesired but thermodynamically favorable side reactions to be avoided in the preparation of these ring systems. In the first an o-methoxymethyl phenylacetaldehyde is cyclized directly to the five-membered, differentially protected hemiacetal, while in the second this group, appropriately substituted, can be rearranged to a 4-trialkylsilyloxy-2,5-methano-1,3-benzodioxepane. The latter masked dialdehyde is sufficiently stable to strong base, mild acid, and oxidants to allow all needed aryl ring systems to be constructed. Using these methods, total syntheses of (±)-versicolorin B, (±)-versicolorin A, its hemiacetal, and its 6-deoxy derivative, (±)-6-deoxyversicolorin A, have been achieved, and these are reported herein, as well as preparation of the methyl ester of a putative o-carboxybenzophenone biosynthetic intermediate. In work described elsewhere, incorporation experiments with 13C-labeled forms of these compounds have made possible the complete elucidation of bisfuran biosynthesis characteristic of the first major phase of aflatoxin formation in vivo.

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Reduction of sterigmatocystin and versicolorin A hemiacetals with sodium borohydride

Cited by 16 publications

References 19 publications

Synthesis of 11-Hydroxyl O-Methylsterigmatocystin and the Role of a Cytochrome P-450 in the Final Step of Aflatoxin Biosynthesis

Synthesis of 11-Hydroxyl O-Methylsterigmatocystin and the Role of a Cytochrome P-450 in the Final Step of Aflatoxin Biosynthesis

Enzymological evidence for separate pathways for aflatoxin B1 and B2 biosynthesis

Silyl Triflate-Mediated Ring-Closure and Rearrangement in the Synthesis of Potential Bisfuran-Containing Intermediates of Aflatoxin Biosynthesis

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