This invention provides a compound of the formula (I):wherein the ring A is an optionally substituted carbocyclic group or an optionally substituted heterocyclic group R1 is optionally substituted lower alkyl, optionally substituted lower alkenyl, optionally substituted lower alkynyl R2a and R2b are each independently hydrogen, optionally substituted lower alkyl or optionally substituted acyl R3a, R3b, R3c and R3d are each independently hydrogen, halogen, hydroxy or optionally substituted lower alkyl etc. a pharmaceutically acceptable salt or solvate thereof, which is useful for treating diseases induced by production, secretion and/or deposition of amyloid β protein.
