A compound of the formula: or a pharmaceutically acceptable salt thereof; wherein, R 1 is hydrogen, halogen, -OH, -OR 5, -N(R 5) 2, alkyl, carbocyclyl, heterocyclyl, aryl, heteroaryl, carbocyclylalkyl, heterocyclylalkyl, aralkyl, or heteroarylalkyl; wherein the aryl and aralkyl groups are optionally substituted by halo; R 2 is hydrogen, halogen, -OH, -OR 5, -N(R 5) 2, alkyl optionally substituted by OH or N(R 5) 2, carbocyclyl, heterocyclyl, aryl, heteroaryl, carbocyclylalkyl, heterocyclylalkyl, aralkyl, or heteroarylalkyl; R 3 is OH or is -O-X-Y; R 4 is hydrogen or alkyl; each R 5 is independently hydrogen, alkyl optionally substituted by OH, carbocyclyl, heterocyclyl, aryl, heteroaryl, carbocyclylalkyl, heterocyclylalkyl, aralkyl, or heteroarylalkyl; X is C 1 -C 8 alkylene or where n is 0 to 4; wherein the C 1 -C 8 alkylene is optionally substituted by carbocyclylaklyl; and Y is hydrogen, carbocyclyl, aryl, or heteroaryl, wherein the carbocyclyl, aryl and heteroaryl groups are independently optionally substituted by one or more substituents selected from halo; fluoroalkyl; alkyl; OR a wherein R a is H, alkyl, carbocyclylalkyl, fluoroalkyl or aralkyl optionally substituted by halo or alkyl; carbocyclyl; cyano; or aralkyl optionally substituted by halo; wherein alkyl is defined as a C 1 -C 15 alkyl; carbocyclyl is defined as a C 3 -C 15 carbocyclyl; heterocyclyl is defined as a C 3 -C 18 heterocyclyl; aryl is defined as a C 5 -C 18 aryl and wherein at least one of the rings in the aryl group is fully unsaturated; heteroaryl is defined as a C 3 -C 18 heteroaryl and wherein at least one of the rings in the heteroaryl group is fully unsaturated; wherein carbocyclylalkyl has the formula R c - C 3 -C 15 carbocyclyl; aralkyl has the formula R c C 5 -C 18 aryl; heterocyclylalky has the formula R c C 3 -C 18 heterocyclyl; heteroarylalkyl has the formula R c C 3 -C 18 heteroaryl; and R c is C 2 -C 12 alkyl R 1 is hydrogen, halogen, -OH, -OR 5, -N(R 5) 2, alkyl, carbocyclyl, heterocyclyl, aryl, heteroaryl, carbocyclylalkyl, heterocyclylalkyl, aralkyl, or heteroarylalkyl; wherein the aryl and aralkyl groups are optionally substituted by halo; R 2 is hydrogen, halogen, -OH, -OR 5, -N(R 5) 2, alkyl optionally substituted by OH or N(R 5) 2, carbocyclyl, heterocyclyl, aryl, heteroaryl, carbocyclylalkyl, heterocyclylalkyl, aralkyl, or heteroarylalkyl; R 3 is OH or is -O-X-Y; R 4 is hydrogen or alkyl; each R 5 is independently hydrogen, alkyl optionally substituted by OH, carbocyclyl, heterocyclyl, aryl, heteroaryl, carbocyclylalkyl, heterocyclylalkyl, aralkyl, or heteroarylalkyl; X is C 1 -C 8 alkylene or where n is 0 to 4; wherein the C 1 -C 8 alkylene is optionally substituted by carbocyclylaklyl; and Y is hydrogen, carbocyclyl, aryl, or heteroaryl, wherein the carbocyclyl, aryl and heteroaryl groups are independently optionally substituted by one or more substituents selected from halo; fluoroalkyl; alkyl; OR a wherein R a is H, alkyl, carbocyclylalkyl, fluoroalkyl or aralkyl optionally substituted by halo or alkyl; carbocyclyl; cyano; or aralkyl optionally substituted by halo; wherein alkyl is defined as a C 1 -C 15 alkyl; carbocyclyl is defined as a C 3 -C 15 carbocyclyl; heterocyclyl is defined as a C 3 -C 18 heterocyclyl; aryl is defined as a C 5 -C 18 aryl and wherein at least one of the rings in the aryl group is fully unsaturated; heteroaryl is defined as a C 3 -C 18 heteroaryl and wherein at least one of the rings in the heteroaryl group is fully unsaturated; wherein carbocyclylalkyl has the formula R c - C 3 -C 15 carbocyclyl; aralkyl has the formula R c C 5 -C 18 aryl; heterocyclylalky has the formula R c C 3 -C 18 heterocyclyl; heteroarylalkyl has the formula R c C 3 -C 18 heteroaryl; and R c is C 2 -C 12 alkyl