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1. A versatile approach to the synthesis of mannosamine glycosides

   https://doi.org/10.1039/D0OB01640C  

   Alex, Catherine; Visansirikul, Satsawat; Demchenko, Alexei V. (September 2020, Organic & Biomolecular Chemistry)

   null (Ed.)

   O -Picoloyl protecting groups at remote positions can affect the stereoselectivity of glycosylation by means of the H-bond-mediated aglycone delivery (HAD) pathway. A new practical method for the stereoselective synthesis of β-glycosides of mannosamine is reported. The presence of the O -picoloyl group at the C-3 position of a mannosamine donor can provide high or complete stereocontrol. The method was also utilized for the synthesis of a biologically relevant trisaccharide related to the capsular polysaccharide of Streptococcus pneumoniae serotype 4. Also reported herein is a method to achieve complete α-manno stereoselectivity with mannosamine donors equipped with 3- O -benzoyl group. 

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2. Catalysis‐Enabled Concise and Stereoselective Total Synthesis of the Tricyclic Prostaglandin D ₂ Metabolite Methyl Ester

   https://doi.org/10.1002/anie.202115633  

   Sims, Hunter S.; de Andrade Horn, Pedro; Isshiki, Ryota; Lim, Melissa; Xu, Yan; Grubbs, Robert H.; Dai, Mingji (December 2021, Angewandte Chemie International Edition)

   Abstract A concise and stereoselective total synthesis of the clinically relevant tricyclic prostaglandin D₂metabolite (tricyclic‐PGDM) methyl ester in racemic form was accomplished in eight steps from a readily available known cyclopentene‐diol derivative. The synthesis features a nickel‐catalyzed Ueno–Stork‐type dicarbofunctionalization to generate two consecutive stereocenters, a palladium‐catalyzed carbonylative spirolactonization to build the core oxaspirolactone, and aZ‐selective cross‐metathesis to introduce the (Z)‐3‐butenoate side chain, a group challenging to introduce through traditional Wittig protocols and troublesome for the two previous total syntheses. A generalZ‐selective cross‐metathesis protocol to construct (Z)‐β,γ‐unsaturated esters was also developed that has broad functional group tolerance and high stereoselectivity. Additionally, our synthesis already accumulated 75 mg of valuable material for an¹⁸O‐tricyclic‐PGDM‐based assay used in clinical settings for inflammation. 

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3. Glycosyl Sulfonates Beyond Triflates

   https://doi.org/10.1002/tcr.202100141  

   Bennett, Clay S. (June 2021, The Chemical Record)

   Abstract While glycosyl triflates are frequently invoked as intermediates in many chemical glycosylation reactions, the chemistry of other glycosyl sulfonates remains comparatively underexplored. Given the reactivity of sulfonates can span several orders of magnitude, this represents an untapped resource for the development of stereoselective glycosylation reactions. This personal account describes our laboratories efforts to take advantage of this reactivity to develop β‐specific glycosylation reactions. Initial investigations led to the development of 2‐deoxy‐sugar tosylates as highly selective donors for β‐glycoside synthesis, an approach which has been used to great success by our group and others for the construction of deoxy‐sugar oligosaccharides and natural products. Subsequent studies demonstrate that “matching” the reactivity of the sulfonate to that of the sugar donor leads to highly selective S_N2‐glycosylations with a range of substrates. 

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4. Synthesis of AD-Dihydrodipyrrins Equipped with Latent Substituents of Native Chlorophylls and Bacteriochlorophylls

   https://doi.org/10.1021/acs.joc.1c01239  

   Wang, Pengzhi; Lindsey, Jonathan S. (July 2021, The Journal of Organic Chemistry)

   null (Ed.)

   Native chlorophylls and bacteriochlorophylls share a common trans-substituted pyrroline ring D (17-propionic acid, 18-methyl), whereas diversity occurs in ring A particularly at the 3-position. Two dihydrodipyrrins equipped with native-like D-ring substituents and tailorable A-ring substituents have been synthesized. The synthesis relies on a Schreiber-modified Nicholas reaction to construct the stereochemically defined precursor to ring D, a dialkyl-substituted pent-4-ynoic acid. The carboxylic acid group of the intact propionic acid proved unworkable, whereupon protected propionate (−CO2tBu) and several latent propyl ethers were examined. The tert-butyldiphenylsilyl-protected propanol substituent proved satisfactory for reaction of the chiral N-acylated oxazolidinone, affording (2S,3S)-2-(3-((tert-butyldiphenylsilyl)-oxy)propyl)-3-methylpent-4-ynoic acid in ∼30% yield over 8 steps. Two variants for ring A, 2-tert-butoxycarbonyl-3-Br/H-5-iodo-4- methylpyrrole, were prepared via the Barton−Zard route. Dihydrodipyrrin formation from the pyrrole and pentynoic acid entailed Jacobi Pd-mediated lactone formation, Petasis methenylation, and Paal−Knorr-type pyrroline formation. The two AD- dihydrodipyrrins bear the D-ring methyl and protected propanol groups with a stereochemical configuration identical to that of native (bacterio)chlorophylls, and a bromine or no substitution in ring A corresponding to the 3-position of (bacterio)chlorophylls. The analogous β-position of a lactone−pyrrole intermediate on the path to the dihydrodipyrrin also was successfully brominated, opening opportunities for late-stage diversification in the synthesis of (bacterio)chlorophylls. 

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5. Conformational disorder in the crystal structure of methyl 2-acetamido-2-deoxy-β- D -glucopyranosyl-(1→4)–2-acetamido-2-deoxy-β- D -glucopyranoside (methyl β-chitobioside) methanol monosolvate

   https://doi.org/10.1107/S2053229624005199  

   Shit, Pradip; Tetrault, Timothy; Zhang, Wenhui; Yoon, Mi-Kyung; Oliver, Allen G; Serianni, Anthony S (July 2024, Acta Crystallographica Section C Structural Chemistry)

   Methyl 2-acetamido-2-deoxy-β-D-glucopyranosyl-(1→4)-2-acetamido-2-deoxy-β-D-glucopyranoside (methyl β-chitobioside), (IV), crystallizes from aqueous methanol at room temperature to give a structure (C₁₇H₃₀N₂O₂₂·CH₃OH) containing conformational disorder in the exocyclic hydroxymethyl group of one of its βGlcNAc residues. As observed in other X-ray structures of disaccharides containing β-(1→4)O-glycosidic linkages, inter-residue hydrogen bonding between O3H of the βGlcNAc bearing the OCH₃aglycone and O5 of the adjacent βGlcNAc is observed based on the 2.79 Å internuclear distance between the O atoms. The structure of (IV) was compared to that determined previously for 2-acetamido-2-deoxy-β-D-glucopyranosyl-(1→4)-2-acetamido-2-deoxy-β-D-glucopyranose (β-chitobiose), (III). TheO-glycosidic linkage torsion angles,phi(ϕ) andpsi(ψ), in (III) and (IV) differ by 6–8°. TheN-acetyl side chain conformation in (III) and (IV) shows some context dependence, with the C1—C2—N—C_cartorsion angle 10–15° smaller for the βGlcNAc residue involved in the internalO-glycosidic linkage. In (IV), conformational disorder is observed in the exocyclic hydroxymethyl (–CH₂OH) group in the βGlcNAc residue bearing the OCH₃aglycone, and a fitting of the electron density indicates an approximate 50:50 distribution of thegauche–gauche(gg) andgauche–trans(gt) conformers in the lattice. Similar behavior is not observed in (III), presumably due to the different packing structure in the vicinity of the –CH₂OH substituent that affects its ability to hydrogen bond to proximal donors/acceptors. Unlike (IV), a re-examination of the previously reported electron density of (III) revealed conformational disorder in theN-acetyl side chain attached to the reducing-end βGlcNAc residue caused by rotation about the C2—N bond. 

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