The present invention relates to compounds of formula I, or pharmaceutically acceptable salts thereof, wherein R1 and R5 are each independently H, C(ORj′) or a hydrocarbyl group optionally substituted by one or more R6 groups; R2, R3, and R4 are each independently H, alkyl or alkenyl, each of which may be optionally substituted with one or more R7 groups; R6 and R7 are each independently halogen, NO2, CN, (CH2)mORa, O(CH2)nORb, (CH2)pNRcRd, CF3, COORe, CONRfRg, CORh, SO3H, SO2Ri, SO2NRjRk, (CH2)qNRa′CORg′, Rf′, (CH2)rNRb′SO2Rh′, SO2NRd′Ri′, SO2NRe′(CH2)sORc′, heterocycloalkyl or heteroaryl, wherein said heterocycloalkyl and heteroaryl may be optionally substituted by one or more substituents selected from aralkyl, sulfonyl, Rm and CORn; Rg′, Rh′, Ri′ and Rj′ are each independently selected from alkyl, aryl, aralkyl and heteroaryl, each of which may be optionally substituted with one or more substituents selected from halogen, OH, NO2, NH2 CF3 and COOH; m, p, q and r are each independently 0, 1, 2 or 3; n and s are each independently 1, 2, or 3; and Ra-n and Ra′-f′ are each independently H or alkyl. Further aspects of the invention relate to pharmaceutical compositions comprising such compounds, and their use in the preparation of a medicament for treating one or more of the following: a proliferative disorder, a viral disorder, a CNS disorder, a stroke, alopecia and diabetes.