Disclosed are the compounds of formula I ##STR1## wherein X represents oxygen or sulfur; ring A is unsubstituted or substituted by one to three identical or different substituents selected from hydroxy, hydroxy-lower alkyl, etherified hydroxy, etherified hydroxy-lower alkyl, acyloxy, acyloxy-lower alkyl, halogen, lower alkyl, trifluoromethyl, amino, mono- and di-lower alkylamino and acylamino; or ring A is substituted by one lower alkylenedioxy; R represents hydrogen, lower alkyl or aryl-lower alkyl; R.sub.1 represents hydrogen, lower alkyl, lower alkylthio-lower alkyl, amino, acylamino, (amino, mono- or di-lower alkylamino)-lower alkyl, carboxy, lower alkoxycarbonyl, carbamoyl or mono- or di-lower alkylcarbamoyl; R.sub.2 to R.sub.5 represent hydrogen or lower alkyl; the dehydro derivatives thereof with a double bond at the 1,2-position, or at the 1,10b-position in which case R.sub.5 is absent; or a pharmaceutically acceptable salt thereof; methods for their synthesis; pharmaceutical compositions thereof; and use thereof as psychactive agents for the treatment of central nervous system disorders.