5,6-trimethylenepyrimidin-4-one compounds

A method of treating Alzheimer's disease wherein the method comprises administering to a patient in need thereof an effective amount of a compound formula (I), neat, or admixed with a pharmaceutically acceptable carrier, wherein formula (I) is:

• in which:
• Ra and Rb together with the pyrimidine ring carbon atoms to which they are attached form a fused 5-membered carbocyclic ring;
• R2CH2X is 4-fluorobenzylthio;
• R3 is C(1-3)alkyl substituted by NR8R9;
• or
• R3 is Het-C(0-2)alkyl in which Het is a 5- to 7-membered heterocyclyl ring comprising N and in which N is unsubstituted or substituted by C(1-6)alkyl;
• R4 and R5 form a 4-(4-trifluoromethylphenyl)phenyl moiety;
• R8 and R9, which may be the same or different, are selected from the group consisting of hydrogen, or C(1-6)alkyl);
• and
• X is S;
• or a pharmaceutically acceptable salt thereof.