Regioselectivity in the ring opening of 2-phenyl-1,3-dioxan-2-yl radicals derived from cyclic benzylidene acetals and comparison with deoxygenation of a carbohydrate diol via its cyclic thionocarbonate

Regioselectivity in the ring-opening β-scission of 2-phenyl-1,3-dioxan-2-yl radicals derived from bicyclic benzylidene acetals

“…We reported earlier 2 that, according to molecular mechanics calculations, the most stable conformation of the trans-fused radical 41 (derived from the glucosidic acetal 6) is more stable than the lowest-energy conformer of the cis-fused analogue 42 (from the galactosidic acetal 9) by 12.8 kJ mol Ϫ1 and we made use of this relative stability as part of our rationalisation for the different regiochemistry observed in the redox rearrangements of 6 and 9 (experimental regioselectivities indicated on structures 41 and 42). 2 || However, a more extensive conformational || It should be noted that the two β-C-O bonds are aligned similarly with respect to the π SOMO in all the dioxanyl radicals considered here and thus stereoelectronic effects that depend on this alignment are unlikely to influence the regioselectivity of their β-scission, particularly in view of the 'product-like' nature of the transition states. search, involving exhaustive rotation of the three methoxy groups, has revealed the existence of another rotamer of 42 that is 2.6 kJ mol Ϫ1 more stable than the lowest-energy conformation of 41.…”

Deoxy sugars. General methods for carbohydrate deoxygenation and glycosidation

“…This reaction was first described by Roberts’ group. 161–163 In the presence of a thiol, such as ( t -BuO) 3 SiSH (TBST) or (i-Pr) 3 SiSH (TIPS), and di- tert -butyl peroxide or 2,2-bis( tert -butylperoxy)butane as radical initiators, these acetals rearrange, resulting in the deoxygenation of one of the hydroxyls involved in the acetal and the formation of a benzoate group in the other one (Fig. 6A).…”

Efficient routes toward the synthesis of the D-rhamno-trisaccharide related to the A-band polysaccharide of Pseudomonas aeruginosa

“…Keeping these limitations in mind we selected the protocol devised by Pedersen et al [ 28 ] and subsequently studied by Dang et al [ 29 – 32 ] which seemed to address all the inadequacies described above. However, the compatibility of the method with thioglycosides had to be verified.…”