The present invention relates to compounds of formula I ##STR00001## or pharmaceutically acceptable salts thereof, wherein one of R.sup.1 and R.sup.2 is methyl, ethyl or isopropyl, and the other is H; R.sup.3 and R.sup.4 are each independently H, branched or un branched C.sub.1-C.sub.6 alkyl, or aryl, and wherein at least one of R.sup.3 and R.sup.4 is other than H; R.sup.5 is a branched or unbranched C.sub.1-C.sub.5 alkyl group or a C.sub.1-C.sub.6 cycloalkyl group, each of which may be optionally substituted with one or more OH groups; R.sup.6, R.sup.7, R.sup.8 and R.sup.9 are each independently H, halogen, NO.sub.2, OH, OMe, CN, NH.sub.2, COOH, CONH.sub.2, or SO.sub.2NH.sub.2.A further aspect of the invention relates to pharmaceutical compositions comprising compounds of formula 1, and the use of said compounds in treating proliferative disorders, viral disorders, CNS disorders, diabetes, stroke, alopecia or neurodegenerative disorders.