The present invention provides compounds of the general formula
wherein
R
4
is one of the following groups
and R
1
, R
2
, R
3
, R
7
, R
8
, and R
9
are as defined in the specification and pharmaceutically acceptable salts, optically pure enantiomers, racemates or diastereomeric mixtures thereof. The compounds are useful for the treatment of Alzheimer's disease.