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.