p-Methylbenzyl Group: Oxidative Removal and Orthogonal Alcohol Deprotection

Oxidative Deprotection of Benzyl Protecting Groups for Alcohols by an Electronically Tuned Nitroxyl-Radical Catalyst

The oxidative deprotection of benzyl (Bn) groups using nitroxyl-radical catalyst 1 and co-oxidant phenyl iodonium bis(trifluoroacetate) (PIFA) is reported. This catalyst is highly active for the oxidation of benzylic ethers because of the electronic tuning on account of the electron-withdrawing ester groups next to the catalytically active center. This catalytic system promotes deprotections at ambient temperature and has a broad substrate scope, including substrates possessing hydrogenation-sensitive functional groups, while the deprotection hardly proceeds when using well-known nitroxyl-radical catalysts such as 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO). The 1/PIFA system also promotes the deprotection of several benzylic protecting groups, including 2-naphthylmethyl (NAP) and 4-methylbenzyl (MBn) groups. Catalyst 1 was also effective for the direct synthesis of ketones and aldehydes from Bn ethers via deprotected alcohols using an excess of the co-oxidant PIFA.

Lewis Acid Promoted Deprotection of Benzylidene Acetals and p-Methoxybenzyl Ethers with Mercaptoacid Acid as Scavenger

We demonstrate a new and high-yielding method for removal of benzylidene acetals and para-methoxybenzyl ethers under catalytic conditions (BF₃:OEt₂ orFeCl₃, 10 mol%) with mercaptoacetic acid as a scavenger. The reaction coproducts are converted to water-soluble molecules, which can be removed by aqueous extraction, thereby bypassing the need for chromatographic purification. The reaction was demonstrated on both multimilligram and multigram scale.

Synthesis of a Bifunctionalized Glycosylphosphatidylinositol (GPI) Anchor Useful for the Study of GPI Biology

A new, bifunctional glycosylphosphatidylinositol (GPI) derivative containing the highly conserved core structure of all natural GPI anchors with a photoactivable diazirine in the lipid chain and clickable alkynes in the glycan was synthesized by a convergent [3+2] glycosylation strategy with late stage protecting group manipulation and regioselective phosphorylation. The challenges of this synthesis were due to the presence of several distinctive functional groups in the synthetic target, which complicated the protection tactics, in addition to the inherent difficulties associated with GPI synthesis. This bifunctional GPI derivative can cross-react with molecules in proximity upon photoactivation and be subsequently labeled with other molecular tags via click reaction. Therefore, it should be a valuable probe for biological studies of GPIs, such as analysis of GPI-interacting membrane proteins, and gaining insights into their functional mechanisms.

Total Synthesis and Antibacterial Investigations of 1-Hydroxyboivinianin A

Synthetic investigations of natural products has been instrumental in the development of novel antibacterial small molecules. 1-hydroxyboivinianin A, a lactone containing phenolic bisabolane isolated from marine sediment, has reported antibacterial activity against the aquatic pathogen Vibrio harveyi. The total synthesis of 1-hydroxyboivinianin A and its enantiomer was completed in a six-step sequence in 42 % overall yield. The synthesis leveraged a key diastereoselective nucleophilic addition with chiral imidazolidinone to establish the benzylic tertiary alcohol and intramolecular Horner-Wadsworth Emmons to furnish the lactone. Both enantiomers were found to have negligible antibacterial activity against a panel of gram-positive and negative bacteria and minimal antifungal activity against phytopathogens. Investigations of a possible in vitro lactone hydrolysis to produce an inactive linear acid led to the discovery of a spontaneous cyclization, suggesting the lactone is resistant to hydrolysis and the lactone is not degrading to produce an inactive species.

Design and Synthesis of a Doubly Functionalized Core Structure of a Glycosylphosphatidylinositol Anchor Containing Photoreactive and Clickable Functional Groups

A bifunctional derivative of the core structure of glycosylphosphatidylinositol (GPI) anchors having a clickable alkynyl group and a photoreactive diazirine group attached to the GPI glucosamine and lipid moieties, respectively, was synthesized from myo-inositol, d-glucosamine, and (R)-1,2-O-acetonized glycerol. The target molecule should be useful for the investigation of GPI-interacting components in the cell membrane that play a key role in the signal transduction and other biological functions of GPI-anchored proteins.

Cyanomethyl (CNMe) ether: an orthogonal protecting group for saccharides

Logical manipulation of protecting groups is one of the vital strategies involved in the synthesis of complex oligosachharides. As opposed to the robust permanent protecting groups, the chemoselective protection-deprotection processes on orthogonal protecting groups have facilitated the synthesis of the target molecules with higher effeciency. While the derivatives of benzyl ethers are the most popular orthogonal ether based protecting groups for hydroxyls, the exploration of methyl ethers for similar synthetic application is much limited. We herein report cyanomethyl (CNMe) ether as a readily synthesized orthogonal protecting group for saccharides. The ether moiety was rapidly removed under Na-naphthalenide conditions in good to excellent yields and was found to be compatible with other well-known benzyl/methyl/silyl ether and acetal protecting groups. Additionally, the CNMe group was observed to be tolerant to standard reagents used for the deprotection of ether, ester and acetal protecting groups. The protection and deprotection steps remained unaffected by the position of hydroxyl, the configuration of monosaccharides or the presence of olefins in the skeleton.

Advancement of the Cleavage Methods of Carbohydrate-derived Isopropylidene and Cyclohexylidene Ketals

Abstract:

Carbohydrates, amino acids, and nucleosides, the fundamental building blocks of complex biomolecules in nature, are essential starting materials for the fabrication of natural and unnatural structural entities, which necessitate the masking and demasking of various functional groups with the utmost chemoselectivity, mildness, and efficiency to avoid unintended bond breaking and formation, as well as associated reactions. Ketals, benzylidene, methoxymethyl, p-methoxybenzyl, silyl ethers, trityl, tert-butyl carbamate, and other functional groups are widely used in modern organic synthesis. In carbohydrate chemistry, the commonly used protecting functionality of isopropylidene and cyclohexylidene ketals necessitates effective methods for selective cleavage. This review summarises different methods for deblocking isopropylidene and cyclohexylidene ketals using inorganic acids, Lewis acid, silica-supported inorganic acids, Amberlite-120 (H+) resin, phosphotungstic acid, Nafion-H, NaBArF4.2H2O, montmorillonite clay, Dowex 50W-X8, camphorsulphonic acid (CSA), ceric ammonium nitrate, molecular iodine, ionic liquids, zeolites and so on.

Stereoselective β-mannosylations and β-rhamnosylations from glycosyl hemiacetals mediated by lithium iodide

Stereoselective β-mannosylation is one of the most challenging problems in the synthesis of oligosaccharides. Herein, a highly selective synthesis of β-mannosides and β-rhamnosides from glycosyl hemi-acetals is reported, following a one-pot chlorination, iodination, glycosylation sequence employing cheap oxalyl chloride, phosphine oxide and LiI. The present protocol works excellently with a wide range of glycosyl acceptors and with armed glycosyl donors. The method doesn't require conformationally restricted donors or directing groups; it is proposed that the high β-selectivities observed are achieved via an S_N2-type reaction of α-glycosyl iodide promoted by lithium iodide.

Nickel Hydride Catalyzed Cleavage of Allyl Ethers Induced by Isomerization

This report discloses the deallylation of O- and N-allyl functional groups by using a combination of a Ni-H precatalyst and excess Brønsted acid. Key steps are the isomerization of the O- or N-allyl group through Ni-catalyzed double-bond migration followed by Brønsted acid induced O/N–C bond hydrolysis. A variety of functional groups are tolerated in this protocol, highlighting its synthetic value.

Total Synthesis and Bioactivity Studies of Fungal Metabolite (-)-TAN-2483B

The first total synthesis of (-)-TAN-2483B, a fungal metabolite possessing a densely functionalized furo[3,4-b]pyran-5-one framework, is achieved in 14 steps from d-mannose. Generation of the 2,6-trans-pyran is by cyclopropane ring expansion followed by α-selective alkynylation. Julia-Kocienski olefination introduces the E-propenyl side chain. Alkyne functionalization and carbonylation stereoselectively establish the bicyclic core of (-)-TAN-2483B. Inhibition of kinases Btk and Bmx, bacterial priority pathogens, and cytokine production in splenocytes indicates promising therapeutic potential.