A compound selected from 5-((3-((4aS,5S,7aS)-2-amino-5-(trifluoromethyl)-4a,5,7,7a-tetrahydro-4H-furo[3,4-d][1,3]thiazin-7a-yl)-4-fluorophenyl)carbamoyl)pyrazine-2-carboxylic acid (A), N-(3-((4aS,5S,7aS)-2-amino-5-(trifluoromethyl)-4a,5,7,7a-tetrahydro-4H-furo[3,4-d][1,3]thiazin-7a-yl)-4-fluorophenyl)-5-(hydroxymethyl)pyrazine-2-carboxamide (B) and N-(3-((4aS,5S,7aS)-2-amino-5-(trifluoromethyl)-4a,5,7,7a-tetrahydro-4H-furo[3,4-d][1,3]thiazin-7a-yl)-4-fluorophenyl)acetamide (C) or pharmaceutically acceptable salts thereof. The compounds have an amyloid β (Aβ) production inhibitory effect or a beta-secretase (BACE1) inhibitory effect and which may be useful as a prophylactic or therapeutic agent for a neurodegenerative disease caused by Aβ such as Alzheimer-typedementia (AD), Down's syndrome, cerebrovascular amyloid angiopathy (CAA), mild cognitive impairment (MCI), memory loss, presenile dementia, senile dementia, hereditary cerebral hemorrhage with amyloidosis, and other degenerative dementias such as dementias of mixed vascular and degenerative origin, dementia associated with supranuclear palsy, dementia associated with cortical basal degeneration, dementia associated with Parkinson's Disease (PD), and dementia associated with diffuse Lewy Body type of AD.
