“…Since the re -face of the aldehyde is attacked, a 1,4-chelation involving the primary alcoholate anion appears most likely ( I ). In fact, a similar re -selectivity has also been observed in the additions of other organomagnesium compounds to the l -erythrosefuranose 2 . , …”
Section: Discussionsupporting
confidence: 66%
“…(b) There is a single report of syn -diastereoselectivity in the addition of methyl magnesium chloride to the erythrofuranose 2 (see ref ). However, the authors of ref do not state how the configurational assignment was done.…”
Ribonucleotide reductases (RNRs) catalyze the 2'-reduction of ribonucleotides, thus providing 2'-deoxyribonucleotides, the monomers for DNA biosynthesis. The current mechanistic hypothesis for the catalysis effected by this class of enzymes involves a sequence of radical reactions. A reversible 3'-hydrogen abstraction, effected by a radical at the enzyme's active site, is believed to initiate the catalytic cycle. For the study of this substrate-enzyme interaction, a series of 4'-benzophenone-substituted model compounds was designed and synthesized. In these models, the benzophenone carbonyl group is oriented such that irradiation is expected to result in an enzyme-like, reversible 3'-hydrogen abstraction. The key step of our synthetic approach is the highly diastereoselective (dr > 95:5) Grignard-addition of carbonyl-protected o-benzophenone magnesium bromide to 2,3-O-isopropylidene-beta-L-erythrofuranose. The configuration of the newly established chiral center was unambiguously proven by X-ray crystallography. The erythritol derivative thus obtained was dehydrated to a base-free, 4'-benzophenone-substituted nucleoside analog. This first model system was further modified by transforming the free 2',3'-hydroxyl groups into the mono- and bis-methyl ethers, into the cyclic carbonate, and into the mono- and bis-mesylates. Alternatively, the primary hydroxyl group of the erythritol intermediate was selectively oxidized to the aldehyde. In the furanose thus obtained, the stage is set for the additional introduction of a nucleobase at the 1'-position.
“…Since the re -face of the aldehyde is attacked, a 1,4-chelation involving the primary alcoholate anion appears most likely ( I ). In fact, a similar re -selectivity has also been observed in the additions of other organomagnesium compounds to the l -erythrosefuranose 2 . , …”
Section: Discussionsupporting
confidence: 66%
“…(b) There is a single report of syn -diastereoselectivity in the addition of methyl magnesium chloride to the erythrofuranose 2 (see ref ). However, the authors of ref do not state how the configurational assignment was done.…”
Ribonucleotide reductases (RNRs) catalyze the 2'-reduction of ribonucleotides, thus providing 2'-deoxyribonucleotides, the monomers for DNA biosynthesis. The current mechanistic hypothesis for the catalysis effected by this class of enzymes involves a sequence of radical reactions. A reversible 3'-hydrogen abstraction, effected by a radical at the enzyme's active site, is believed to initiate the catalytic cycle. For the study of this substrate-enzyme interaction, a series of 4'-benzophenone-substituted model compounds was designed and synthesized. In these models, the benzophenone carbonyl group is oriented such that irradiation is expected to result in an enzyme-like, reversible 3'-hydrogen abstraction. The key step of our synthetic approach is the highly diastereoselective (dr > 95:5) Grignard-addition of carbonyl-protected o-benzophenone magnesium bromide to 2,3-O-isopropylidene-beta-L-erythrofuranose. The configuration of the newly established chiral center was unambiguously proven by X-ray crystallography. The erythritol derivative thus obtained was dehydrated to a base-free, 4'-benzophenone-substituted nucleoside analog. This first model system was further modified by transforming the free 2',3'-hydroxyl groups into the mono- and bis-methyl ethers, into the cyclic carbonate, and into the mono- and bis-mesylates. Alternatively, the primary hydroxyl group of the erythritol intermediate was selectively oxidized to the aldehyde. In the furanose thus obtained, the stage is set for the additional introduction of a nucleobase at the 1'-position.
“…Methyllithium effected an analogous rearrangement, the C-methyl analogue of 40 being isolated in 45 % yield together with 2 % of the epimer at the tertiary center. In a consistent manner, the isomeric 2,3-O-isopropylidene-D-manno-anhydride 41 gave the carbocycles 42 and 43 (52 and 12%, respectively),28 and the altro isomer 44 afforded 45 (71 %) 29 showing that the reaction has some general applicability.…”
Section: Biosynthesis Of Hydroxyiated Cyclohexanesmentioning
“…Chiral γ-alkenyl butenolides and their derivatives are widespread in bioactive natural products and are also valuable synthetic intermediates (Scheme a) . When synthesizing such moieties, stoichiometric quantities of chiral reagents were usually applied .…”
The asymmetric synthesis of γ-alkenyl butenolides was accomplished by conjugated addition of butenolides to alkynones. Both terminal alkynones and nonterminal alkynones were applicable to the N,N′-dioxide−scandium(III) catalytic system. The corresponding products were obtained in good to excellent yields (up to 99%) with high E/Z ratios and high enantioselectivities (up to 98% ee). The novel methods of building both γ-alkenyl butenolides and continuing epoxidation products facilitated constructing core structure of biologically active natural products and synthetic intermediates. Additionally, one-pot Michael addition/epoxidation performed well with our catalytic system.
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