Abstract:conclusions to bedrawn about the topology of the environment of reactive thiol groups. The scope of the above addition can be widened considerably since lipophilic, hydrophilic, colored, or polymer-bound P-nitrostyrenes are readily accessible via suitably substituted aldehydes and could also find application in the isolation and chromatography of cysteine peptides.
N-tert-Butoxycarbonyl-S-(2-nitro-l-phenylethyl)-L-cysteinr N-Methylmorpholine (10ml) is added to a stirred solution of N-tert-butoxycarbonyl-L-cyst… Show more
“…In general, chemical Baeyer−Villiger reactions of anthraquinones are known to proceed slowly because they give the unstable seven-membered lactones as an intermediate . The only two successful examples of anthraquinone ring cleavage are the experiments using an enzyme or singlet oxygen. ,, 6 …”
Section: Resultsmentioning
confidence: 99%
“…Quinone derivative 16c was then reductively methylated to the corresponding anthracene derivative 17c by treatment with sodium hydrosulfite in the presence of tetrabutylammonium bromide as a phase-transfer catalyst followed by methylation employing dimethyl sulfate and KOH . According to the literature, an etheral solution of anthracenes 17a − c , which also served as a sensitizer, was irradiated with a halogen lamp through a Pyrex filter under oxygen bubbling. After the solvent was evaporated in vacuo, the resulting oxygen adducts were treated with a catalytic amount of sulfuric acid in acetone to give the benzophenone derivatives 18 − 23…”
The efficient total synthesis of (−)-balanol, a potent inhibitor
of the protein kinase C, is described.
(−)-Balanol consists of a chiral hexahydroazepine-containing
fragment and a benzophenone
fragment, both of which were prepared via novel synthetic routes.
The hexahydroazepine fragment
was prepared in racemic form through either Bu3SnH- or
SmI2-promoted radical cyclization of oxime
ethers 2ab intramolecularly connected with the formyl group.
SmI2-promoted radical cyclization
of 2b was found to be particularly successful in the
selective synthesis of the seven-membered
trans-amino alcohol 8b. Preparation of the
enantiomerically pure hexahydroazepine-containing
fragment was achieved through the enantioselective enzymatic
acetylation of racemic alcohol 9,
employing the immobilized lipase from Pseudomonas sp.
The benzophenone fragment was prepared
in short steps through a biomimetic oxidative anthraquinone ring
cleavage starting from
commercially available natural chrysophanic acid 15c.
This reaction proceeded via [4 + 2]-cycloaddition of singlet oxygen to anthracene derivative 17c,
followed by Baeyer−Villiger-type
rearrangement of the resulting hydroperoxide to afford the benzophenone
derivatives 22 and 23.
“…In general, chemical Baeyer−Villiger reactions of anthraquinones are known to proceed slowly because they give the unstable seven-membered lactones as an intermediate . The only two successful examples of anthraquinone ring cleavage are the experiments using an enzyme or singlet oxygen. ,, 6 …”
Section: Resultsmentioning
confidence: 99%
“…Quinone derivative 16c was then reductively methylated to the corresponding anthracene derivative 17c by treatment with sodium hydrosulfite in the presence of tetrabutylammonium bromide as a phase-transfer catalyst followed by methylation employing dimethyl sulfate and KOH . According to the literature, an etheral solution of anthracenes 17a − c , which also served as a sensitizer, was irradiated with a halogen lamp through a Pyrex filter under oxygen bubbling. After the solvent was evaporated in vacuo, the resulting oxygen adducts were treated with a catalytic amount of sulfuric acid in acetone to give the benzophenone derivatives 18 − 23…”
The efficient total synthesis of (−)-balanol, a potent inhibitor
of the protein kinase C, is described.
(−)-Balanol consists of a chiral hexahydroazepine-containing
fragment and a benzophenone
fragment, both of which were prepared via novel synthetic routes.
The hexahydroazepine fragment
was prepared in racemic form through either Bu3SnH- or
SmI2-promoted radical cyclization of oxime
ethers 2ab intramolecularly connected with the formyl group.
SmI2-promoted radical cyclization
of 2b was found to be particularly successful in the
selective synthesis of the seven-membered
trans-amino alcohol 8b. Preparation of the
enantiomerically pure hexahydroazepine-containing
fragment was achieved through the enantioselective enzymatic
acetylation of racemic alcohol 9,
employing the immobilized lipase from Pseudomonas sp.
The benzophenone fragment was prepared
in short steps through a biomimetic oxidative anthraquinone ring
cleavage starting from
commercially available natural chrysophanic acid 15c.
This reaction proceeded via [4 + 2]-cycloaddition of singlet oxygen to anthracene derivative 17c,
followed by Baeyer−Villiger-type
rearrangement of the resulting hydroperoxide to afford the benzophenone
derivatives 22 and 23.
“…Otherwise, the Baeyer−Villiger oxidation of an unactivated‘ anthraquinone may not be possible. The difficulty of this rearrangement is clear from determined efforts by Franck and co-workers some time ago to model this reaction chemically. , Synthetically and biosynthetically, the addition of an activated oxygen species to the carbonyl of a rigid, planar anthraquinone would generate a tetrahedral intermediate/transition state with such poor orbital alignment that aryl migration to oxygen is difficult, if not prohibitive. Alternatively, an initial A-ring epoxidation of the anthraquinone disrupts the planar nature of the substrate and introduces strain into the ring system that may be critical for Baeyer−Villiger rearrangement to occur.…”
The biosynthesis of the potent environmental carcinogen aflatoxin B1 involves ca. 15 steps beyond the first polyketide intermediate. Central among these is the rearrangement of the anthraqinone versicolorin A to the xanthone demethylsterigmatocystin. Genetic evidence strongly suggests that two enzymes are required for this process, a cytochrome P450, AflN, and a probable NADPH-dependent oxidoreductase, AflM. Given the overall redox change evident in this skeletal rearrangement, two rounds of oxidation and a reduction necessarily occur. Earlier experiments indicated that reductive deoxygenation of versicolorin A is not the first step. In the present report we consider a mechanistic alternative that AflM-mediated reduction is instead the last of these three reactions prior to formation of the xanthone intermediate. To this end, 9-hydroxydihydrodemethylsterigmatocystin was prepared by total synthesis as was its 9-deoxy analogue, an established aflatoxin precursor. During the final isolation of the "angular" synthetic xanthone targets it was found that acid catalysis promoted their isomerization to thermodynamically favored "linear" xanthones. Whole-cell and ground-cell incubations of the 9-hydroxy- and 9-deoxyxanthones were conducted with a mutant strain of Aspergillus parasiticus blocked at the first step of the pathway and examined for their ability to support aflatoxin production. The 9-deoxyxanthone gave dramatically enhanced levels of the mycotoxin. The 9-hydroxyxanthone, on the other hand, afforded no detectable increase in aflatoxins above controls, indicating that reductive deoxygenation at C-9 of a xanthone precursor does not take place in aflatoxin biosynthesis. Constraints imposed by earlier studies and the experiments in this paper serve to eliminate simple and intuitive conversions of versicolorin A to demethylsterigmatocystin and lead inescapably to a more subtle reaction sequence of oxidation-reduction-oxidation. Previous puzzling observations of extensive A-ring hydrogen exchange in the course of the rearrangement of versicolorin A to demethylsterigmatocystin have now been explained by a new mechanism that is consistent with all extant data. We propose that P450-mediated aryl epoxidation (AflN) initially disrupts the aromatic A-ring of versicolorin A. Oxirane opening enables A-ring proton exchange, as does the subsequent AflM-mediated reductive step. A second cycle of P450 oxidation (AflN), this time a Baeyer-Villiger cleavage, enables decarboxylation and the formation of demethylsterigmatocystin. Mechanistic and stereoelectronic principles that underlie this proposal are described and may prove general as illustrated in biogenetic hypotheses for four other fungal anthraquinone --> xanthone transformations.
“…[9] Aus dem stabileren Ozonid (1) wird durch Photolyse (A>290 nm) in CCI, in guter Ausbeute gebildet. nicht aber bei der Reaktion von (5) mit TCNE gebildet wird, ist zu erklaren.…”
Das erste Primärozonid, das aufgrund seiner außergewöhnlichen Stabilität isoliert werden konnte, wurde aus 1,2‐Dichloracenaphthylen erzeugt und als (1) charakterisiert. Neben‐ und zugleich Umlagerungsprodukt von (1) ist das Normalozonid (2).
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
