A method of treating or preventing of a disease or condition associated with an increased level of isoforms of amyloid .beta. peptides (A.beta.) and/or with a changed ratio of levels of A.beta. isoforms and/or with the formation of plaques containing amyloid .beta. peptide (A.beta.) isoforms in a mammal comprising administering to said mammal an therapeutically effective amount of a compound selected from the formulas Ia, Ib ##STR00001## wherein V, W, Y, R.sup.2, R.sup.3, R.sup.5, R.sup.6, L1 and i are defined as in claim 1.