The present application relates to compounds of formula (I′) or formula (I), such as formula (Ia), and their pharmaceutical compositions/preparations. This application further relates to methods of treating or preventing neurodegenerative disorders, such as Alzheimer's disease (AD), Parkinson's disease (PD), Huntington disease (HD) and other CAG-triplet repeat (or polyglutamine) diseases, amyotrophic lateral sclerosis (ALS; Lou Gehrig's disease), diffuse Lewy body disease, chorea-acanthocytosis, primary lateral sclerosis, multiple sclerosis (MS), frontotemporal dementia, Friedreich's ataxia, acute head injury, and epilepsy (repression of microglia activation).