- Stereoelectronic factors in the stereoselective epoxidation of glycals and 4-deoxypentenosides
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Glycals and 4-deoxypentenosides (4-DPs), unsaturated pyranosides with similar structures and reactivity profiles, can exhibit a high degree of stereoselectivity upon epoxidation with dimethyldioxirane (DMDO). In most cases, the glycals and their corresponding 4-DP isosteres share the same facioselectivity, implying that the pyran substituents are largely responsible for the stereodirecting effect. Fully substituted dihydropyrans are subject to a "majority rule", in which the epoxidation is directed toward the face opposite to two of the three groups. Removing one of the substituents has a variable effect on the epoxidation outcome, depending on its position and also on the relative stereochemistry of the remaining two groups. Overall, we observe that the greatest loss in facioselectivity for glycals and 4-DPs is caused by removal of the C3 oxygen, followed by the C5/anomeric substituent, and least of all by the C4/C2 oxygen. DFT calculations based on polarized-π frontier molecular orbital (PPFMO) theory support a stereoelectronic role for the oxygen substituents in 4-DP facioselectivity, but less clearly so in the case of glycals. We conclude that the anomeric oxygen in 4-DPs contributes toward a stereoelectronic bias in facioselectivity whereas the C5 alkoxymethyl in glycals imparts a steric bias, which at times can compete with the stereodirecting effects from the other oxygen substituents.
- Alberch, Laura,Cheng, Gang,Seo, Seung-Kee,Li, Xuehua,Boulineau, Fabien P.,Wei, Alexander
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p. 2532 - 2547
(2011/06/19)
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- Selective generation of free radicals from epoxides using a transition-metal radical. A powerful new tool for organic synthesis
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Bis(cyclopentadienyl)titanium(III) chloride reacts with epoxides by initial C-O homolysis. The regiochemistry of the opening is determined by the relative stabilities of the radicals. Depending on the reaction partners, these radicals undergo intramolecular (hex-5-enyl cyclization) or intermolecular additions to olefins. The resultant radicals are efficiently scavenged by a second equivalent of Ti(III) to afford the corresponding Ti(IV) derivative. Treatment of this intermediate with electrophiles such as H+ or halogens provides a route to functionalized cyclopentanes and other useful products. The radical initially formed from an epoxide can also be trapped by H-atom donors such as 1,4-cyclohexadiene or tert-butyl thiol, resulting in an overall reduction of the epoxide. In the absence of a H-atom donor or an olefin, this radical is trapped by Ti(III), resulting in a β-oxido-Ti organometallic species which undergoes facile elimination to give an olefin. The reaction conditions are remarkably mild and are applicable to very sensitive substrates.
- RajanBabu,Nugent, William A.
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p. 986 - 997
(2007/10/02)
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- Highly Selective Metal-Graphite-Induced Reductions of Deoxy Halo Sugars
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To explore the applicability of highly active metal-graphite reducing agents to polyfunctional compounds a variety of suitably protected chloro-, bromodeoxy-, and deoxyiodohexopyranosides and hexofuranoses were each treated with potassium-graphite laminat
- Fuerstner, Alois,Weidmann, Hans
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p. 2307 - 2311
(2007/10/02)
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- REACTION OF BUTYLLITHIUM WITH BENZYLIDENE ACETALS OF ALDOPYRANOSIDES AND 1,5-ANHYDROALDITOLS
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Under suitable conditions, butyllithium selectively cleaves 5-membered benzylidene acetals (1,3-dioxolane ring) leaving the 6-membered analogs (1,3-dioxane ring) intact in aldopyranoside and 1,5-anhydroalditol derivatives.The usual course of the reaction
- Horton, Derek,Weckerle, Wolfgang
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p. 305 - 312
(2007/10/02)
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- Absolute Configuration and Enantioselective Synthesis of Spiculisporic Acid
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The absolute configuration of spiculisporic acid (1), obtained from Penicillium spiculisporum Lehman, has been determined as (3S,4S) by a degradation into (2S,3S)-decylcitric acid (2), a metabolite of a variant of this fungus.The key step in the enantiose
- Brandaenge, Svante,Dahlman, Olof,Lindqvist, Bo,Mahlen, Anders,Moerch, Lars
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p. 837 - 844
(2007/10/02)
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- A CONVENIENT METHOD FOR THE PREPARATION OF 4,6-O-BENZYLIDENEGLYCALS FROM METHYL 2,3-ANHYDRO-4,6-O-BENZYLIDENE-α-D-HEXAPYRANOSIDES
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The reaction of methyl 2,3-anhydro-4,6-O-benzylidene-α-D-allopyranoside (1) with ethylmagnesium bromide in the presence of CuI afforded 4,6-O-benzylidene-1,2-dideoxy-D-ribo-hex-1-enopyranoside (2).Similarly, methyl 2,3-anhydro-4,6-O-benzylidene-α-D-gulopy
- Tsuda, Nobuo,Yokota, Sumio,Kudo, Takashi,Mitsunobu, Oyo
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p. 289 - 292
(2007/10/02)
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- REACTION OF 2,3-ANHYDRO-4,6-O-BENZYLIDENE-α-D-HEXOPYRANOSIDES WITH PROPENYLMAGNESIUM CHLORIDES. REGIOSELECTIVE CARBON CHAIN EXTENSION AT THE C-2 POSITION OF HEXAPYRANOSIDES
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The reaction of 2-propenylmagnesium chlorides with methyl 2,3-anhydro-4,6-O-benzylidene-α-D-allopyranoside (1) or -gulopyranoside (6) exclusively afforded the corresponding 2-deoxy-2-C-(2-propenyl)-α-D-hexopyranosides (4b, 8a, and 8b).
- Asano, Tadahiko,Yokota, Sumio,Mitsunobu, Oyo
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p. 343 - 346
(2007/10/02)
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- β-(Trimethylsilyl)ethoxymethyl chloride. A new reagent for the protection of the hydroxyl group
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Reactions of β-(trimethylsilyl)ethoxymethyl chloride with alcohols afford the corresponding ethers in high yield. Deprotection using n-Bu4NF in THF or HMPA cleanly regenerates the hydroxyl function.
- Lipshutz, Bruce H.,Pegram, Joseph J.
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p. 3343 - 3346
(2007/10/02)
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