alkyl or cyclic ketal thioketal thioxolane which may be unsubstituted or substituted with lower alkyl aryl or heterocyclyl r a is selected from lower alkylenyl lower alkenlenyl or lower alkynlenyl preferably r a is lower alkylenyl r b is selected from halogen hydroxy methanesulfonate toluenesulfonate aryl or heterocyclyl preferably r b is halogen or hydroxy w is selected from o s u2014 nr a or u2014 chr c preferably w is o g is selected from n u2014 c u2014 cn or u2014 cr c preferably g is n z is selected from o s u2014 nr d or u2014 chr d preferably z is o or u2014 nr d r c is selected from h lower alkyl preferably r c is h r d is selected from h lower alkyl amino or alkylamino preferably r d is h r 1 r 3 and r 4 are each independently selected from h halogen halogeno lower alkyl lower alkyl lower alkoxy lower alkoxyalkoxy lower alkenyl or lower alkynyl preferably r 1 r 3 and r 4 are each independently halogen lower alkyl or lower alkoxy r 2 is selected from h halogen halogeno lower alkyl or lower alkyl preferably r 2 is h or fluorine or a pharmaceutically acceptable salt thereof the term u201c lower alkylenyl u201d as used herein unless otherwise indicated includes 1 to 6 saturated u2014 ch 2 u2014 radicals the term u201c lower alkenlenyl u201d as used herein unless otherwise indicated includes lower alkylenyl groups as defined above having at least one carbon carbon double bond such as u2014 ch 2 u2014 ch u2550 ch u2014 the term u201c lower alkynlenyl u201d as used herein unless otherwise indicated includes lower alkylenyl groups as defined above having at least one.