Rational approach to selective and direct 2-O-alkylation of 5,6-O-isopropylidine-L-ascorbic acid

Novel Multifunctional Ascorbic Triazole Derivatives for Amyloidogenic Pathway Inhibition, Anti-Inflammation, and Neuroprotection

Alzheimer's disease (AD) is a common neurodegenerative disorder. The number of patients with AD is projected to reach 152 million by 2050. Donepezil, rivastigmine, galantamine, and memantine are the only four drugs currently approved by the United States Food and Drug Administration for AD treatment. However, these drugs can only alleviate AD symptoms. Thus, this research focuses on the discovery of novel lead compounds that possess multitarget regulation of AD etiopathology relating to amyloid cascade. The ascorbic acid structure has been designated as a core functional domain due to several characteristics, including antioxidant activities, amyloid aggregation inhibition, and the ability to be transported to the brain and neurons. Multifunctional ascorbic derivatives were synthesized by copper (I)-catalyzed azide-alkyne cycloaddition reaction (click chemistry). The in vitro and cell-based assays showed that compounds 2c and 5c exhibited prominent multifunctional activities as beta-secretase 1 inhibitors, amyloid aggregation inhibitors, and antioxidant, neuroprotectant, and anti-inflammatory agents. Significant changes in activities promoting neuroprotection and anti-inflammation were observed at a considerably low concentration at a nanomolar level. Moreover, an in silico study showed that compounds 2c and 5c were capable of being permeated across the blood-brain barrier by sodium-dependent vitamin C transporter-2.

Surprisingly long-lived ascorbyl radicals in acetonitrile: concerted proton-electron transfer reactions and thermochemistry

Proton-coupled electron transfer (PCET) reactions and thermochemistry of 5,6-isopropylidene ascorbate (iAscH-) have been examined in acetonitrile solvent. iAscH- is oxidized by 2,4,6-tBu3C6H2O. and by excess TEMPO. to give the corresponding 5,6-isopropylidene ascorbyl radical anion (iAsc.-), which persists for hours at 298 K in dry MeCN solution. The stability of iAsc.- is surprising in light of the transience of the ascorbyl radical in aqueous solutions and is due to the lack of the protons needed for radical disproportionation. A concerted proton-electron transfer (CPET) mechanism is indicated for the reactions of iAscH-. Redox potential, pKa and equilibrium measurements define the thermochemical landscape for 5,6-isopropylidene ascorbic acid and its derivatives in MeCN. These measurements give an O-H bond dissociation free energy (BDFE) for iAscH- of 65.4 +/- 1.5 kcal mol-1 in MeCN. Similar studies on underivatized ascorbate indicate a BDFE of 67.8 +/- 1.2 kcal mol-1. These values are much lower than the aqueous BDFE for ascorbate of 74.0 +/- 1.5 kcal mol-1 derived from reported data.

Ascorbate analogs for use in medical imaging: synthesis and radical scavenging activity of 5-O-(4'-iodobenzyl)-L-ascorbic acid

As part of our program to develop potential imaging agents for ascorbate bioactivity in the brain, 5-O-(4'-iodobenzyl)-L-ascorbic acid was prepared through a seven-step sequence which involved C5-O-alkylation with p-iodobenzyl bromide in the presence of Ag2O and CaSO4 as the key step, starting from L-ascorbic acid. The scavenging activity of the p-iodobenzylated analog against 2,2-diphenyl-1-picrylhyrazyl (DPPH) radical was almost the same as that of L-ascorbic acid itself.

A Convenient Entry to C2- and C3-Substituted Gulono-γ-lactone Derivatives from l-Ascorbic Acid

A convenient method to obtain unknown chiral C2- and C3-functionalized aldono-1,4-lactone derivatives starting from l-ascorbic acid, which would be valuable in the synthesis of derivatives of various pharmacologically active agents for structure-activity studies, is described. The practicality of this approach is demonstrated by the synthesis of a series of 5,6-O-isopropylidene-2-allyl-3-keto-l-galactono-gamma-lactone and 5,6-O-isopropylidene-3-allyl-2-keto-l-galactono-gamma-lactone derivatives using the thermal Claisen rearrangement of the corresponding 3-O- and 2-O-allyl derivatives of 5,6-O-isopropylidene-l-ascorbic acid, respectively, followed by stereospecific reduction to the corresponding alcohols. The synthetic steps are shown to be efficient, and enantiospecific, and they proceed with high yields.