A compound of formula I: wherein: X is -(C 1 -C 8)alkyl-, aryl or -aryl(C 1 -C 8)alkyl-; Y is -(C 1 -C 8)alkyl- or absent; W is heteroaryl, (C 3 -C 7)carbocycle or aryl wherein any heteroaryl, (C 3 -C 7)carbocycle or aryl of W is optionally substituted with one or more Z 1 groups; R 1 is (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl or aryl, wherein aryl is optionally substituted with one or more groups selected from (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl, (C 3 -C 7)carbocycle, halo(C 1 -C 3)alkyl, -CN, NO 2, halogen, -OR a, -SR a, -S 2 NR b R c, -NR b R c, -NR a COR d, -C R a, -C OR a and -C NR b R c; R 2 is (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl or aryl, wherein aryl is optionally substituted with one or more groups selected from (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl, (C 3 -C 7)carbocycle, halo(C 1 -C 3)alkyl, -CN, NO 2, halogen, -OR e, -SR e, -S 2 NR f R g, -NR f R g, -NR e COR h, -C R e, -C OR e and -C NR f R g; each R a is independently selected from H, (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl, (C 3 -C 7)carbocycle and aryl; R b and R c are each independently selected from H, (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl, (C 3 -C 7)carbocycle and aryl, or R b and R c together with the nitrogen to which they are attached form a 5, 6 or 7-membered heterocycle; R d is independently selected from (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl, (C 3 -C 7)carbocycle and aryl; each R e is independently selected from H, (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl, (C 3 -C 7)carbocycle and aryl; R f and Rg are each independently selected from H, (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl, (C 3 -C 7)carbocycle and aryl, or R f and R g together with the nitrogen to which they are attached form a 5, 6 or 7-membered heterocycle; R h is selected from (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl, (C 3 -C 7)carbocycle and aryl; each Z 1 is independently selected from (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl, aryl, heteroaryl, -OR i, -NR j R k and -NR i COR m, wherein any aryl or heteroaryl of Z 1 is optionally substituted with one or more (C 1 -C 8)alkyl; each R i is selected from H and (C 1 -C 8)alkyl; R j and R k are each independently selected from H and (C 1 -C 8)alkyl; R m is (C 1 -C 8)alkyl optionally substituted with one or more -N(R n) 2 or -N(R n) 3 + Q - wherein Q- is halogen; and each R n is independently H or (C 1 -C 8)alkyl; or a salt thereof. X is -(C 1 -C 8)alkyl-, aryl or -aryl(C 1 -C 8)alkyl-; Y is -(C 1 -C 8)alkyl- or absent; W is heteroaryl, (C 3 -C 7)carbocycle or aryl wherein any heteroaryl, (C 3 -C 7)carbocycle or aryl of W is optionally substituted with one or more Z 1 groups; R 1 is (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl or aryl, wherein aryl is optionally substituted with one or more groups selected from (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl, (C 3 -C 7)carbocycle, halo(C 1 -C 3)alkyl, -CN, NO 2, halogen, -OR a, -SR a, -S 2 NR b R c, -NR b R c, -NR a COR d, -C R a, -C OR a and -C NR b R c; R 2 is (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl or aryl, wherein aryl is optionally substituted with one or more groups selected from (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl, (C 3 -C 7)carbocycle, halo(C 1 -C 3)alkyl, -CN, NO 2, halogen, -OR e, -SR e, -S 2 NR f R g, -NR f R g, -NR e COR h, -C R e, -C OR e and -C NR f R g; each R a is independently selected from H, (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl, (C 3 -C 7)carbocycle and aryl; R b and R c are each independently selected from H, (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl, (C 3 -C 7)carbocycle and aryl, or R b and R c together with the nitrogen to which they are attached form a 5, 6 or 7-membered heterocycle; R d is independently selected from (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl, (C 3 -C 7)carbocycle and aryl; each R e is independently selected from H, (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl, (C 3 -C 7)carbocycle and aryl; R f and Rg are each independently selected from H, (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl, (C 3 -C 7)carbocycle and aryl, or R f and R g together with the nitrogen to which they are attached form a 5, 6 or 7-membered heterocycle; R h is selected from (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl, (C 3 -C 7)carbocycle and aryl; each Z 1 is independently selected from (C 1 -C 8)alkyl, (C 2 -C 8)alkenyl, (C 2 -C 8)alkynyl, aryl, heteroaryl, -OR i, -NR j R k and -NR i COR m, wherein any aryl or heteroaryl of Z 1 is optionally substituted with one or more (C 1 -C 8)alkyl; each R i is selected from H and (C 1 -C 8)alkyl; R j and R k are each independently selected from H and (C 1 -C 8)alkyl; R m is (C 1 -C 8)alkyl optionally substituted with one or more -N(R n) 2 or -N(R n) 3 + Q - wherein Q- is halogen; and each R n is independently H or (C 1 -C 8)alkyl; or a salt thereof.