Stereochemical control in radical cyclization routes to N-glycosides: role of protecting groups and of the configuration (E versus Z) of the acceptors

On the Use of 3,5-O-Benzylidene and 3,5-O-(Di-tert-butylsilylene)-2-O-benzylarabinothiofuranosides and Their Sulfoxides as Glycosyl Donors for the Synthesis of β-Arabinofuranosides: Importance of the Activation Method

“…Thus, a pair of 3,5- O -benzylidene protected arabinofuranose derivatives, unstable in chloroform, was obtained in a DAST-mediated contraction of a 4,6- O -benzylidene protected pyranoside . 3,5- O -Benzylidene protected tetrahydrofurans were also accessed from the solvolysis of a 1,3;4,6-di- O -benzylidene protected mannitol derivative, by radical ring closure reactions on existing benzylidene acetal frameworks, and by fusion of a furanose ring onto an existing benzylidene acetal through a nucleophilic ring closure …”

N,N-Dialkylhydrazones in Organic Synthesis. From Simple N,N-Dimethylhydrazones to Supported Chiral Auxiliaries

“…On the other hand, use of tributyltin hydride in this reaction resulted in no cyclization but deoxygenation, giving product 348 in 41% yield. When 352 was used as a substrate in the cyclization, a 10:3 mixture of anomers of the cyclized N -furanosides 353 was formed with preference for the α-anomer. , …”

1,1′-Thiocarbonyldiimidazole

“…39 While thionocarbonates have not seen much use as protecting groups for diols, they are similar to carbonates in their general insensitivity to acids and lability in bases. 49 In these reactions, the radical intermediate formed from stannyl radical attack on sulfur undergoes rapid cyclization prior to the expected "normal" Barton Bimolecular radical chain processes have been observed as well. Fragmentations of radicals derived from imidazoyl thiocarbonates have become increasingly common.…”

“…38 Thionocarbonates, the intermediate usually associated with the Corey-Winter olefination, undergo radical reductions to form methylene derivatives, as shown in eq 11. 49 Analogous cyclization chemistry has been observed with acrylonitriles, oxime ethers, and dimethyl hydrazones as radical acceptors, and thiocarbonyl ditriazole has also been used to generate radical precursors. 39 This procedure converts an acid-stable protecting group into an acetal, which can be removed under acidic conditions.…”