Dementia is divided into cerebrovascular dementia and neurodegenerative dementia, and various agents such as cerebral blood flow improvers and nootropics are used for treating these dementias. 
Senile plaques characteristic of Alzheimer's disease, which is most typical as neurodegenerative dementia, are mainly composed of amyloid β protein (Aβ) derived from β amyloid precursor protein.  Aβ is considered as a substance that is deposited on the neurons or blood vessels of brain to cause a disease such as dementia.  In addition, it has been reported that Aβ itself injures neurons.  Inhibitors of neurotoxicity induced by Aβ are investigated as therapeutic agents for Alzheimer's disease. 
As compounds having inhibitory activity against neurotoxicity induced by Aβ, there are known, for example, the 1,2-ethanediol derivatives disclosed in JP-A-3-232830 and JP-A-4-95070, and the N-alkoxyalkyl-N,N-dialkylamine derivatives disclosed in International Publication WO 00/76957. 
The 1,2-ethanediol derivatives disclosed in JP-A-3-232830 and JP-A-4-95070, in particular, (R)-1-(benzo[b]thiophen-5-yl)-2-[2-(N,N-diethylamino)-ethoxy]ethanol hydrochloride has protective activity against the neuronal death caused by Aβ (SOCIETY FOR NEUROSCIENCE, Abstracts, Vol.  24, Part 1, p. 228, 1998) and activity to enhance the activity of nerve growth factor (NGF) (WO 96/12717) and hence is useful as a therapeutic agent for diseases in central and peripheral nerves.  However, there is desired the development of a compound possessing properties such as a higher activity to protect neurons and a higher activity to accelerate nerve regeneration, which are required as a therapeutic agent for diseases in central and peripheral nerves.