Influence of Various Silyl Protecting Groups on Stereoselective 2-Deoxyrhamnosylation.

The influence of various silyl protecting groups on 2-deoxyrhamnosylation using 2-deoxyrhamnosyl acetates, thioglycosides, and (p-methoxyphenyl)vinylbenzoate (PMPVB) donors has been presented. C-Glycosylation reactions reveal that tert-butyldimethylsilyl (TBDMS), triisopropylsilyl (TIPS), and tert-butyldiphenylsilyl (TBDPS) silyl protected rhamnosyl oxocarbenium ions have no facial selectivity except for the conformationally (4H3) locked tetraisopropyldisiloxane (TIPDS) protected rhamnose donor, which provides complete α-selectivity. However, TBDPS protected rhamnosyl donors are found to be superior protecting groups for α-stereoselective O-glycosylation reactions with various acceptors. The observed results are found consistent across donors and donor activation conditions. Most importantly, the study was conducted at room temperature unlike the other energy-intensive low-temperature studies and was bound to have more practical utility. The outcomes have been explained using kinetic and thermodynamic analyses.

Sterically Strained Brønsted Pair Catalysis by Bulky Pyridinium Salts: Direct Stereoselective Synthesis of 2-Deoxy and 2,6-Dideoxy-ß-thioglycosides from Glycals.

A sterically strained ionic Brønsted pair complex obtained from a sterically bulky base 2,4,6-tri-tert-butylpyridine and hydrochloric acid imbues unusual reactivity to the anionic chloride. The complete shielding of the cationic [N-H]+ by the bulky ortho-tert-butyl groups weakens the possible hydrogen-bonding interactions with the chloride anion, and the [N-H]+···Cl- distance is unusually longer (3.10 Å). This results in strained/frustrated electrostatic interactions between the ion-pair, thus infusing an increased reactivity in both of the ions, which results in the activation of a third molecule like thiol via hydrogen-bonding. This intriguing weak interaction-based reactivity has been utilized to develop an organocatalytic synthesis of 2-deoxy-ß-thioglycosides from glycals. While the 1H NMR studies showcase the diamagnetic activation of thiols in the presence of the catalyst, the electron paramagnetic resonance (EPR) studies reveal the generation of a radical species that suggests a possible frustrated radical pair catalysis. Besides, IR spectroscopic studies explain the intriguing influence of size/charge density of the anion on the solvation-insusceptible, cationic [TTBPyH]+ and on the observed reactivity.

Influence of Anion-Binding Schreiner's Thiourea on DMAP Salts: Synergistic Catalysis toward the Stereoselective Dehydrative Glycosylation from 2-Deoxyhemiacetals.

Amines are used as additives to facilitate or increase the host-guest chemistry between the thiourea and the anions of Bronsted acids. However, we here demonstrate, for the first time, the synergistic effect of the combination of DMAP/HCl/Schreiner's thiourea in catalyzing dehydrative glycosylation. The variations in the electronic effects of the cationic Bronsted acid part (the protonated DMAP) in the presence of chloride binding Schreiner's thiourea have been discussed using NMR and X-ray crystallographic techniques.

C-H···Anion Interactions Assisted Addition of Water to Glycals by Sterically Hindered 2,4,6-Tri-tert-butylpyridinium Hydrochloride.

The conjugate acid of the bulky base 2,4,6-tri-tert-butylpyridine, under mild conditions, catalyzes the synthesis of silyl-protected 2-deoxy-hemiacetals and their dimerized products from glycals at varying concentrations of water. The criticality of the concentration of water in the reaction outcome is indicative of a unique mechanistic pathway for the bulky pyridine salt and not via the general Brønsted acid mechanism. The various silyl-protected hemiacetals thus synthesized were successfully utilized in the stereoselective synthesis of both α and ß glycosides.

Sterically Hindered 2,4,6-Tri- tert-butylpyridinium Salts as Single Hydrogen Bond Donors for Highly Stereoselective Glycosylation Reactions of Glycals.

We demonstrate here that the strained and bulky protonated 2,4,6-tri- tert-butylpyridine salts serve as efficient catalysts for highly stereoselective glycosylations of various glycals. Moreover, the mechanism of action involves an interesting single hydrogen bond mediated protonation of glycals and not via the generally conceived Brønsted acid pathway. The counteranions also play a role in the outcome of the reaction.

Secondary amine salt catalyzed controlled activation of 2-deoxy sugar lactols towards alpha-selective dehydrative glycosylation.

A new organocatalytic glycosylation method exploiting the lactol functionality has been disclosed. The catalytic generation of glycosyl oxacarbenium ions from lactols under forcible conditions via weakly Brønsted-acidic, readily available secondary amine salts affects the diastereoselective glycosylation of 2-deoxypyranoses and furanoses. This operationally simple iminium catalyzed activation of 2-deoxy hemi-acetals is a potential alternative to the existing cumbersome methods that need specialized handling. The mechanisms for this unique transformation and kinetic/thermodynamic effects have been discussed based on both experimental evidence and theoretical studies.

Hypokalaemic paralysis in an adult case of Plasmodium vivax malaria.

Hypokalaemia and its subsequent complications are more often seen in children rather than in adults and are more common with falciparum malaria. This is a case of a 26-year-old male with Plasmodium vivax malaria who had developed paraparesis secondary to hypokalaemia. His treatment involved correction of the potassium level as well as the treatment of malaria. Such an atypical manifestation of malaria in an adult has not been previously documented.