specified number of carbon atoms and having one point of attachment examples include n propyl a c 3 alkyl isopropyl also a c 3 alkyl and t butyl a c 4 alkyl u201c alkoxyalkyl u201d refers to an alkylene group substituted with an alkoxy group for example methyloxyethyl ch 3 och 2 ch 3 u2014 and ethoxymethyl ch 3 ch 2 och 2 u2014 are both c 3 alkoxyalkyl groups u201c alkanoyloxy u201d refers to an ester substituent wherein the ether oxygen is the point of attachment to the molecule examples include propanoyloxy ch 3 ch 2 c o u2014 o u2014 a c 3 alkanoyloxy and ethanoyloxy ch 3 c o u2014 o u2014 a c 2 alkanoyloxy u201c alkoxy u201d refers to an o atom substituted by an alkyl group for example methoxy u2014 och 3 a c 1 alkoxy u201c alkoxycarbonyl u201d refers to an ester substituent wherein the carbonyl group is the point of attachment to the molecule examples include ethoxycarbonyl ch 3 ch 2 oc o u2014 a c 3 alkoxycarbonyl and methoxycarbonyl ch 3 oc o u2014 a c 2 alkoxycarbonyl u201c aryl u201d refers to aromatic groups which have at least one ring having a conjugated pi electron system and includes carbocyclic aryl heterocyclic aryl also known as heteroaryl groups and biaryl groups all of which may be optionally substituted u201c thioalkyl u201d refers to a sulfur atom substituted by an alkyl group for example thiomethyl ch 3 s u2014 a c 1 thioalkyl certain compounds of the present invention or for use in the pharmaceutical compositions or methods of the present invention are represented by formulas i xxiii r 1 r 2 r 7 r 8 are independently selected from hydrogen and c 1 c 3 alkyl r 3 r 4 r.