Electron spin resonance spectroscopic investigation of carbohydrate radicals. Part 3. Conformation in deoxypyranosan-2-, -3-, and -4-yl radicals

Abstract

E.s.r. spectra of C-2, -3, and -4 carbohydrate free radicals, generated regiospecifically by halogen abstraction from acylated deoxyhexopyranoses are recorded in benzene solution. Analysis of the hyperfine splittings reveals the retention of the 4C, chair conformation of the parent compounds in the radicals.In Part 2 ' the structures of various pyranosyl radicals with the radical centre at the anomeric (C-1) carbon atom have been examined by e m . spectroscopy. In non-aqueous media the pyranos-1-yl radicals exist as planar n-type radicals and their equilibrium conformations strongly depend on the type of carbohydrate. Thus, diastereoselectivities in radical C-C coupling reactions of carbohydrates have to be discussed in terms of carbohydrate conformations2 rather than in terms of 0configuration at the radical centre as had been suggested earlier. 3 Recently, it has been shown that diastereoselective C-C coupling reactions could also be performed at the C-2 position of the carbohydrate backbone, giving 2-deoxy sugars in reasonably good yield^.^ Similarly, radical-induced C-C bond formation is also possible at C-3 and -4 of the pyranosyl rings5The e.s.r. spectroscopic investigation of C-2, -3, and -4 deoxypyranosanyl radicals, which is presented here, should provide better insight into the dependence of diastereoselectivity on radical conformation. Structurally related radicals have been investigated in the reaction of unprotected sugar derivatives with O H radicals in aqueous s ~l u t i o n . ~. ~ Results and DiscussionThe deox ypyranosanyl radicals (Rl)-(R8) were regiospecifically generated in benzene solution from the corresponding bromoor iodo-compounds (1)-( 8) by reaction with trimethylstannyl radicals, generated photolytically from hexamethylditin (see Table ). The carbohydrates (lb), (2b), (3), and (6)-( 8) are new compounds that were synthesized by standard procedures. The e.s.r. spectra, recorded in the temperature range from -8 to

Chemistry of β-(Acyloxy)alkyl and β-(Phosphatoxy)alkyl Radicals and Related Species: Radical and Radical Ionic Migrations and Fragmentations of Carbon−Oxygen Bonds

“…By the same token, the tetraacetyl-1-mannopyranosyl radical retains the 4 C 1 chair conformation ( 484 ) . Radicals located at other positions on the pyranose framework, for example, the 3,4,6-triacetyl-1β-methoxy-2-glucopyranosyl radical ( 485 ), retain the more perpendicular geometry of simple β-acyloxyalkyl radicals and a 4 C 1 chairlike conformation . The extended anomeric interaction also seems to operate in much less complicated cyclic systems, with the butyrate and even the bulky pivalate esters, ( 486 ) adopting an axial orientation, coplanar with the SOMO on the adjacent carbon. , By working in acetone solution, rather than the more polar water where fragmentation was too rapid, the Schulte-Frohlinde group were also able to observe the ESR spectrum of an α-methoxy-β-(phosphatoxy)ethyl radical and conclude that it, too, adopts the eclipsed conformation ( 487 ) .…”

Excited-State Palladium-Catalyzed 1,2-Spin-Center Shift Enables Selective C-2 Reduction, Deuteration, and Iodination of Carbohydrates

“…Such an extended anomeric interaction weakens the C-2−OAc bond and promotes the 1,2-SCS through a concerted [2,3]-acyloxy rearrangement with a cyclic five-membered ring transition state IIIb, 11b,17a forming the deoxypyranosan-2-yl radical IVa that prefers the 4 C 1 chair conformation. 18 Although the anomeric radical is more stable than the secondary alkyl radical, the molecular stability gained from the formation of an anomeric C−O bond in IVa drives the desired 1,2-SCS. 19 Under visiblelight irradiation, the intermediate IVa is in equilibrium with alkyl-Pd(II)X complex IVb, which allows access to both openand closed-shell reactivities.…”

“…Such an interaction is more pronounced in glycosyl radicals because the lone pair electron of the endocyclic- O (η O , anomeric interaction) raises the SOMO energy level. Such an extended anomeric interaction weakens the C-2–OAc bond and promotes the 1,2-SCS through a concerted [2,3]-acyloxy rearrangement with a cyclic five-membered ring transition state IIIb , , forming the deoxypyranosan-2-yl radical IVa that prefers the 4 C 1 chair conformation . Although the anomeric radical is more stable than the secondary alkyl radical, the molecular stability gained from the formation of an anomeric C–O bond in IVa drives the desired 1,2-SCS .…”

Nickel-Catalyzed Radical Migratory Coupling Enables C-2 Arylation of Carbohydrates

“…The 1-glucosyl radical prefers the B 2,5 boat conformation IV by 0.6 kcal/mol, which stems from the extended anomeric interaction between the lone-pair electrons of the endocyclic O, the singly occupied molecular orbital (SOMO), and the σ C–O * orbital of the C-2 OAc group . This interaction weakens the C-2 OAc bond and promotes the 1,2-SCS through a concerted 1,2-acyloxy rearrangement via a cyclic five-membered-ring transition state ( TS5 ), , affording deoxypyranosan-2-yl radical V . Although a typical secondary alkyl radical would be less stable than an anomeric radical, in this case the molecular stability gained from the formation of an anomeric C–O bond in V drives the desired 1,2-SCS and makes this step ( IV → V ) exergonic by 2.0 kcal/mol.…”