six membered non aromatic heterocycle having one oxygen heteroatom within the genus the invention encompasses a first sub genus of compounds of formula ia within the first subgenus the invention encompasses a first class of compounds of formula ia wherein within the first class the invention encompasses a sub class of compounds of formula ia wherein within the sub class the invention encompasses a group of compounds of formula ia wherein r 2 is h also within the group the invention encompasses a second sub group of compounds of formula ia wherein r 1 is u2014 ch 2 n u2014 c o u2014 or 4 within the genus the invention encompasses a second sub genus of compounds of formula ib within the second sub genus the invention encompasses a second class of compounds of formula ib wherein the invention also encompasses a pharmaceutical composition comprising a compound according to formula i or a pharmaceutically acceptable salt thereof and a pharmaceutically acceptable carrier the invention also encompasses a method of treatment of a subject suffering or prone to a condition associated with the deposition of u03b2 amyloid which comprises administering to that subject an effective amount of a compound according to formula i or a pharmaceutically acceptable salt thereof the invention also encompasses the use of a compound according to formula i or a pharmaceutically acceptable salt thereof for the manufacture of a medicament for treating or preventing alzheimer 39 s disease where a variable occurs more than once in formula i or in a substituent thereof the individual occurrences of that variable are independent of each other unless otherwise specified for.