Addition of Heteroatom Radicals to endo-Glycals †

Photocatalyzed Perfluoroalkylation of Endoglycals

The visible light-induced perfluoroalkyl (RF) radical reactions on peracetylglycals derived from hexoses and pentoses (galactal, glucal, arabinal, and xylal derivatives) were investigated. Various photocatalysts and perfluoroalkyl iodides (RF-I) were employed as sources of RF radicals with LEDs as the irradiation source. Particularly noteworthy was the use of an Iridium photocatalyst, Ir[dF(CF3)ppy]2(dtbpy))PF6, which yielded two distinct product types when applied to glucal. On the one hand, the 2-RF-substituted glucal was formed, a trend observed even when utilizing organic dyes as photocatalysts. On the other hand, the unexpected addition product, namely the 1-RF-2-iodo-α-manno-configured C-glycosyl derivative, was also obtained, as a result of a highly regioselective addition reaction of the RF moiety into the anomeric carbon, followed by attachment of the iodine atom on C-2 in axial disposition. This result contrasted with other radical reactions carried out on 2-unsubstituted glycals, where the incipient radical adds to C-2, generating a stabilized 1-glycosyl radical. The photocatalyzed radical perfluoroalkylations of peracetyl glycals derived from galactose, arabinose, and xylose all afforded the 2-RF-substituted glycals in good yields as a result of the expected vinylic substitution reaction. Mechanistic studies revealed that the 1-RF-2-iodo-α-manno-configured C-glycosyl derivatives arise from a radical chain reaction, whereas the 2-RF-substituted glycals proceed from inefficient chain processes.

New syntheses of thiosaccharides utilizing substitution reactions

Novel synthetic methods published since 2005 affording carbohydrates containing sulfur atom(s) are reviewed. The review is divided to subchapters based on the position of sulfur atom(s) in the sugar molecule. Only those methods that take advantage of substitution are discussed.

Behaviour of some 1-C-acceptor-substituted glycals under azidohydroxylation conditions

Azidohydroxylation of 1-carbamoyl, 1-methoxycarbonyl and 1-cyano substituted d-lyxo and d-arabino configured O-peracylated glycals was studied and the reaction conditions were optimized. Under these conditions (3 equiv. NaN₃/2 equiv. PIFA/0.3 equiv. TEMPO/50 equiv. H₂O/dry DCM/0 °C/Ar) the expected 3-azido-3-deoxy ulopyranosonic acid derivatives were isolated in good yield with α-d-galacto configuration exclusively from the reaction of the 1-carbamoyl and 1-methoxycarbonyl substituted d-lyxo configured O-peracetylated glycals, while the transformation of the 1-cyano derivative gave a 2,3-vicinal diazide in low yield. The 1-carbamoyl d-arabino configured O-perbenzoylated glycal gave a mixture of α-d-gluco and α-d-manno configured azidohydroxylated products with d-gluco preference. The analogous 1-methoxycarbonyl derivative gave an inseparable product mixture and no transformation was detected with the respective 1-cyano glycal.

Halogen addition to some 1-C-substituted pyranoid glycals

Addition of bromine and chlorine to O-peracylated 1-CN-, COOMe- and CONH2-substituted glycals was studied under ionic and radical conditions. The main or exclusive products were the corresponding 2,3-trans-diaxial (3-bromo-3-deoxy-α-d-heptopyranosylbromide)onic acid derivatives. Bromination of the O-peracetylated d-lyxo-hept-2-enopyranosononitrile and all chlorinations proved selective towards the 2-axial-3-equatorial (3-halogeno-3-deoxy-α-d-heptopyranosylhalide)onic acid derivatives. Silver triflate promoted glycosylation of methanol was successful with each 2,3-trans-diaxial (3-bromo-3-deoxy-α-d-heptopyranosylbromide)onic acid derivative, however, several attempted nucleophilic substitution and elimination reactions gave the parent glycal only.

Display of Rich Reactivities of Endo- and Exocyclic Unsaturated Sugars that Parallel the Native Sugars

Unsaturated monosaccharides expand the scope of reactivities in a sugar, directly leading to the development of newer methodologies, molecular structures and functional entities. The unsaturation as a reactive moiety can either be within the molecule, namely, endocyclic, or as a pendant moiety around the molecule, namely, exocyclic. One carbon homologations aided by reactions at the unsaturated moiety expand the molecular structures in both endo- and exocyclic sugars and lead to structures that are largely hitherto unknown. Molecular shifts and rearrangements permit interchanging the reactivities from one carbon to the other in unsaturated sugars. Activations of exocyclic unsaturated sugars also find newer possibilities to reactions central to the sugar chemistry, namely, the glycosylations. The personal reflections result from a couple of decades of explorations that traverse through the unsaturated sugars from different vantage points.