Conformation of Glycosyl Radicals: Radical Stabilization by β‐CO Bonds

Abstract

iron(o) 6['.'] reveals an unusual trigonal bipyramidal type structure with the q '-bonded phosphaalkene ligand occupying an axial position. In view of its spatial requirements, this ligand should occupy an equatorial (bond angle 120") rather than an axial position (bond angle 90°) as found for the recently described carbonyliron complex with a phosphaalkene ligandI6l and for a tetracarbonyliron compound with the very bulky diphosphene ligand"].

Synthesis of Non‐Classical Arylated C‐Saccharides through Nickel/Photoredox Dual Catalysis

“…Not surprisingly,t he more flexible radical leading to 2i afforded lower diastereoselectivity. [6,21,22] Finally,t he choice of TBS as ap rotecting group allowed the formation of unprotected derivative 2e upon TBAF addition to the crude reaction mixture of 2d.…”

Stereoselective Thioconjugation by Photoinduced Thiol‐ene Coupling Reactions of Hexo‐ and Pentopyranosyl d‐ and l‐Glycals at Low‐Temperature—Reactivity and Stereoselectivity Study

“…While the C‐2‐centered hexopyranosyl radicals preferentially adopt chair conformation independently of the configuration, the conformation of glycosyl radicals is determined by the configuration of the C‐2 substituent . It has been demonstrated by Giese and co‐workers on the basis of electron paramagnetic resonance (ESR) spectroscopy study that the most preferred conformation of glycopyranosyl radicals is the one in which the C2‐alkoxy/acyloxy substituent adopts a quasi axial position, because this conformer is stabilized by the so‐called quasi‐homo‐anomeric effect . Accordingly, while C‐1 mannosyl radicals exist in 4 C 1 chair conformation, C‐1 glucosyl radicals preferably flip to the most stable twisted B 2,5 conformation—which is practically identical with the 1 S 5 skew‐boat conformer—and galactosyl radicals adopt the 4 H 5 half chair conformation .…”

“…[40,41] It has been demonstrated by Giese and co-workers on the basis of electron paramagnetic resonance (ESR) spectroscopy study that the most preferred conformation of glycopyranosyl radicals is the one in which the C2-alkoxy/acyloxy substituent adopts aquasi axial position, because this conformer is stabilized by the so-called quasi-homo-anomeric effect. [42][43][44] Accordingly, while C-1 mannosyl radicals exist in 4 C 1 chair conformation, C-1 glucosyl radicals preferably flip to the mosts table twisted B 2,5 conformation-which is practically identical with the 1 S 5 skew-boat conformer [38] -and galactosyl radicals adopt the 4 H 5 half chair conformation. [40][41][42][43][44][45] Consequently,p roducts 55, 57 and 59 with an axially linked SR group can be formed throught he 4 Upon attacks of thiyl radicals at either side to the other halfchair conformation of the startingg lycals the pyranosyl ring would only flip to high-energys kew boat and chair conformations bearinga xial C6-group which decompose rapidly.…”

Mechanism of Anaerobic Ether Cleavage

“…17) to eliminate adjacent leaving groups as a result of their electron-rich character (38,39). The cleavage of the ␤-C,O-bond can also be facilitated by stereoelectronic effects in the appropriate conformation of the radical anion 17 (40). It is well known that ␣-hydroxy radicals are up to 10 5 times more acidic than the corresponding alcohols (CH 2 OH-⅐ CHOH has pK a values of ϳ10 -12) (41).…”