The present invention relates to compounds of formula I wherein Ar.sub.1, Ar.sub.2, R, R.sup.1, R.sup.2, R.sup.3 and n are as defined herein or to pharmaceutically suitable acid addition salts, optically pure enantiomers, racemates or diastereomeric mixtures thereof. Compounds of formula I are orexin receptor antagonists and may be useful in the treatment of disorders, in which orexin pathways are involved like sleep disorders.