The 4,6-O-[alpha-(2-(2- iodophenyl)ethylthiocarbonyl)benzylidene] protecting group: stereoselective glycosylation, reductive radical fragmentation, and synthesis of beta-D-rhamnopyranosides and other deoxy sugars

Chemical approaches for the stereocontrolled synthesis of 1,2-cis-β-D-rhamnosides

In carbohydrate chemistry, the stereoselective synthesis of 1,2-cis-glycosides remains a formidable challenge. This complexity is comparable to the synthesis of 1,2-cis-β-D-mannosides, primarily due to the adverse anomeric and Δ-2 effects. Over the past decades, to attain β-stereoselectivity in D-rhamnosylation, researchers have devised numerous direct and indirect methodologies, including the hydrogen-bond-mediated aglycone delivery (HAD) method, the synthesis of β-D-mannoside paired with C6 deoxygenation, and the combined approach of 1,2-trans-glycosylation and C2 epimerization. This review elaborates on the advancements in β-D-rhamnosylation and its implications for the total synthesis of tiacumicin B and other physiologically relevant glycans.

Chemical Synthesis Elucidates the Key Antigenic Epitope of the Autism-Related Bacterium Clostridium bolteae Capsular Octadecasaccharide

The gut pathogen Clostridium bolteae has been associated with the onset of autism spectrum disorder (ASD). To create vaccines against C. bolteae, it is important to identify exact protective epitopes of the immunologically active capsular polysaccharide (CPS). Here, a series of C. bolteae CPS glycans, up to an octadecasaccharide, was prepared. Key to achieving the total syntheses is a [2+2] coupling strategy based on a β-d-Rhap-(1→3)-α-d-Manp repeating unit that in turn was accessed by a stereoselective β-d-rhamnosylation. The 4,6-O-benzylidene-induced conformational locking is a powerful strategy for forming a β-d-mannose-type glycoside. An indirect strategy based on C2 epimerization of β-d-quinovoside was efficiently achieved by Swern oxidation and borohydride reduction. Sequential glycosylation, and regioselective and global deprotection produced the disaccharide and tetrasaccharide, up to the octadecasaccharide. Glycan microarray analysis of sera from rabbits immunized with inactivated C. bolteae bacteria revealed a humoral immune response to the di- and tetrasaccharide, but none of the longer sequences. The tetrasaccharide may be a key motif for designing glycoconjugate vaccines against C. bolteae.

Stereoselective synthesis of β-rhamnopyranosides via gold(i)-catalyzed glycosylation with 2-alkynyl-4-nitro-benzoate donors

An effective β-rhamnosylation protocol has been developed by using α-rhamnopyranosyl ortho-hexynyl-para-nitro-benzoates as donors and Ph₃PAuBArF4 as a catalyst.

Methodology development and physical organic chemistry: a powerful combination for the advancement of glycochemistry

This Perspective outlines work in the Crich group on the diastereoselective synthesis of the so-called difficult classes of glycosidic bond: the 2-deoxy-β-glycopyranosides, the β-mannopyranosides, the α-sialosides, the α-glucopyranosides, and the β-arabinofuranosides with an emphasis on the critical interplay between mechanism and methodology development.

Protecting groups in carbohydrate chemistry: influence on stereoselectivity of glycosylations

Saccharides are polyhydroxy compounds, and their synthesis requires complex protecting group manipulations. Protecting groups are usually used to temporarily mask a functional group which may interfere with a certain reaction, but protecting groups in carbohydrate chemistry do more than protecting groups usually do. Particularly, protecting groups can participate in reactions directly or indirectly, thus affecting the stereochemical outcomes, which is important for synthesis of oligosaccharides. Herein we present an overview of recent advances in protecting groups influencing stereoselectivity in glycosylation reactions, including participating protecting groups, and conformation-constraining protecting groups in general.

Stereocontrolled synthesis of D- and L-beta-rhamnopyranosides with 4-O-6-S-alpha-cyanobenzylidene-protected 6-thiorhamnopyranosyl thioglycosides

The synthesis of both enantiomers of a 4-O-6-S-alpha-cyanobenzylidene-protected 6-thiorhamnopyranosyl thioglycoside is described starting from D-mannnose and L-arabinose derivatives for the D- and L-series, respectively. This donor is effective in the preparation of the corresponding beta-glycosides using the 1-benzenesulfinyl piperidine/trifluoromethanesulfonic anhydride protocol. Following desulfurization and concomitant debenzylation with Raney nickel, the so-formed 6-thio-beta-mannosides are converted in high yield to the beta-rhamnopyranosides.

Electrochemical generation of glycosyl triflate pools

Glycosyl triflates, which serve as important intermediates in glycosylation reactions, were generated and accumulated by the low-temperature electrochemical oxidation of thioglycosides such as thioglucosides, thiogalactosides, and thiomannosides in the presence of tetrabutylammonium triflate (Bu(4)NOTf) as a supporting electrolyte. Thus-obtained solutions of glycosyl triflates (glycosyl triflate pools) were characterized by low-temperature NMR measurements. The thermal stability of glycosyl triflates and their reactions with glycosyl acceptors were also examined.

Synthesis of a beta-(1-->3)-D-rhamnotetraose by a one-pot, multiple radical fragmentation

A naturally occurring beta-(1-->3)-D-rhamnotetraose has been constructed under conditions of sequential beta-selective mannosylation controlled by the 4,6-O-[1-cyano-2-(2-iodophenyl)-ethylidene] protecting group. The route is concise, proceeding through a late-stage radical deoxygenation that successfully uncovers all four deoxy subunits at once.

Generation of Phosphorus-Centered Radicals via Homolytic Substitution at Sulfur

A novel radical domino process relying on the homolytic cleavage of P-S bonds allows the preparation of phosphorus-containing molecules through addition of P-centered radicals onto olefins. The key step of this reaction is a homolytic substitution on a sulfur atom. The scope of the reaction is broad. Diaminophosphonyl radicals whose reactivity was unknown react smoothly with olefins. Use of tin hydride can be avoided. A radical thiophosphinoylation of triple bonds has been uncovered. [reaction: see text]

Stereocontrolled synthesis of the D- and L-glycero-beta-D-manno-heptopyranosides and their 6-deoxy analogues. Synthesis of methyl alpha-l-rhamno-pyranosyl-(1-->3)-D-glycero-beta-D-manno-heptopyranosyl- (1-->3)-6-deoxy-glycero-beta-D-manno-heptopyranosyl-(1-->4)-alpha-L- rhamno-pyranoside, a tetrasaccharide subunit of the lipopolysaccharide from Plesimonas shigelloides

The synthesis of d- and l-glycero-alpha-manno-thioheptopyranosides, protected with 4,6-O-alkylidene-type acetals is described. In glycosylations carried out with preactivation with the 1-benzenesulfinylpiperidine/trifluoromethanesulfonic anhydride couple, both the D- and L-glycero series exhibit excellent beta-selectivity with a range of glycosyl acceptors. In contrast, a 4,7-O-alkylidene acetal was found not to afford beta-selectivity. With a 4,6-O-[1-cyano-2-(2-iodophenyl)ethylidene] acetal protected thioglycoside, excellent beta-selectivity was obtained in glycosylation reactions, and subsequent treatment with tributyltin hydride and azoisobutyronitrile brought about clean fragmentation to the 6-deoxy-glycero-beta-D-manno-heptopyranosides. This chemistry was applied to the stereocontrolled synthesis of methyl alpha-L-rhamno-pyranosyl-(1-->3)-D-glycero-beta-D-manno-heptopyranosyl-(1-->3)-6-deoxy-glycero-beta-D-manno-heptopyranosyl-(1-->4)-alpha-L-rhamno-pyranoside, a component of the lipopolysaccharide from Plesimonas shigelloides.

4,6-O-[1-cyano-2-(2-iodophenyl)ethylidene] acetals. improved second-generation acetals for the stereoselective formation of beta-D-mannopyranosides and regioselective reductive radical fragmentation to beta-D-rhamnopyranosides. scope and limitations

The [1-cyano-2-(2-iodophenyl)]ethylidene group is introduced as an acetal-protecting group for carbohydrate thioglycoside donors. The group is easily introduced under mild conditions, over short reaction times, and in the presence of a wide variety of other protecting groups by the reaction of the 4,6-diol with triethyl (2-iodophenyl)orthoacetate and camphorsulfonic acid, followed by trimethylsilyl cyanide and boron trifluoride etherate. The new protecting group conveys strong beta-selectivity with thiomannoside donors and undergoes a tin-mediated radical fragmentation to provide high yields of the synthetically challenging beta-rhamnopyranosides. The method is also applicable to the glucopyranosides when high alpha-selectivity is observed in the coupling reaction and alpha-quinovosides are formed selectively in the radical fragmentation step. In the galactopyranoside series, beta-glycosides are formed selectively on coupling to donors protected by the new system, but the radical fragmentation is unselective and gives mixtures of the 4- and 6-deoxy products. Variable-temperature NMR studies for the glycosylation step, which helped define an optimal protocol, are described.

2-O-propargyl ethers: readily cleavable, minimally intrusive protecting groups for beta-mannosyl donors

[reaction: see text]. The use of 2-O-propargyl ethers as protecting groups in 4,6-O-benzylidene-protected mannopyranosyl donors bearing either bulky silyl groups or glycosidic linkages on O3 overcomes the poor stereoselectivity achieved with the corresponding 2-O-benzyl ethers, due to a reduction of steric buttressing. Deprotection is conducted by treatment with potassium tert-butoxide followed by catalytic osmium tetroxide and N-methylmorpholine N-oxide.

Is There a Homolytic Substitution Chemistry (SH2) of Sulfones?

[reaction: see text] A series of 2-alkylsulfonyl-2'-biphenyl radicals, in which the alkyl group is primary, secondary, or tertiary, were generated and the products of their reactions investigated. Dibenzothiophene S,S-dioxide was not identified among the products, which arose mainly from intramolecular hydrogen abstraction from the alkyl group or addition to the solvent, benzene. On this basis, it is concluded that homolytic substitution at sulfonyl sulfur, if possible at all, is too slow to take precedence over a number of competing decomposition pathways. Previous literature results suggesting the possibility of intramolecular homolytic substitution at sulfonyl sulfur may be explained by alternative processes.