An aryl glycoside compound of formula I or formula I', or a pharmaceutically acceptable salt, or an optical isomer thereof: wherein, X is O, S, SO, SO 2 CO, or a single bond; L is C 1 -C 6 alkylene, (C 1 -C 6 alkylene)-(C 3 -C 10 cycloalkylene) or (C 1 -C 6 alkylene)-(C 3 -C 10 cycloalkylene)-(C 1 -C 6 alkylene), and each methylene group in said cycloalkylene is optionally replaced by O, N, or S; M is 4-membered cycloheteroalkyl; with the proviso that when M is azetidinyl and L is linked with the nitrogen atom of M, then X-L is not O(CH 2) m CH(OR 6f)CH 2 where m is 1 to 3 and R 6f is hydrogen, alkyl or alkylcarbonyl; R 1, R 2 and R 3 are independently selected from hydrogen, OH, -OR 7, alkyl, -SR 5i or halogen, or two of R 1, R 2 and R 3 together with the carbons to which they are attached can form an annelated five, six or seven membered carbocycle or heterocycle which may contain 1 to 4 heteroatoms in the ring which are N, O, S, SO and/or SO 2; R 4 and R 5 are independently selected from hydrogen, OH, -OR 5a, -O-Aryl, -OCH 2 -Aryl, alkyl, cycloalkyl, halogen, -CN, -CO 2 R 5b, -CO 2 H, COR 6b, -CH R 6c, -CH(OR 5h)R 6d, -CONR 6a R 6e, -NHCOR 5c, -NHSO 2 R 5d, -NHSO 2 -Aryl, Aryl, -SR 5e, -SOR 5f, -SO 2 R 5g, -SO 2 -Aryl, or a five, six or seven membered heterocycle which may contain 1 to 4 heteroatoms in the ring which are N, O, S, SO and/or SO 2; or R 4 and R 5 together with the carbons to which they are attached form an annelated five, six or seven membered carbocycle or heterocycle which may contain 1 to 4 heteroatoms in the ring which are N, O, S, SO and/or SO 2; R 7, R 5a, R 5b, R 5c, R 5d, R 5e, R 5f, R 5g, R 5h and R 5i are independently selected from alkyl; R 6a, R 6b, R 6c, R 6d and R 6e are independently selected from hydrogen, alkyl, aryl, alkylaryl or cycloalkyl, or R 6a and R 6e together with the nitrogen to which they are attached form an annelated five, six or seven membered heterocycle which may contain 1 to 4 heteroatoms in the ring which are N, O, S, SO and/or SO 2; A is O, S, 1,1-cyclopropyfidene, CHF, CF 2 or (CH 2) n where n is 1 to 3; wherein said compound I' is selected from any of the following compounds: X is O, S, SO, SO 2 CO, or a single bond; L is C 1 -C 6 alkylene, (C 1 -C 6 alkylene)-(C 3 -C 10 cycloalkylene) or (C 1 -C 6 alkylene)-(C 3 -C 10 cycloalkylene)-(C 1 -C 6 alkylene), and each methylene group in said cycloalkylene is optionally replaced by O, N, or S; M is 4-membered cycloheteroalkyl; with the proviso that when M is azetidinyl and L is linked with the nitrogen atom of M, then X-L is not O(CH 2) m CH(OR 6f)CH 2 where m is 1 to 3 and R 6f is hydrogen, alkyl or alkylcarbonyl; R 1, R 2 and R 3 are independently selected from hydrogen, OH, -OR 7, alkyl, -SR 5i or halogen, or two of R 1, R 2 and R 3 together with the carbons to which they are attached can form an annelated five, six or seven membered carbocycle or heterocycle which may contain 1 to 4 heteroatoms in the ring which are N, O, S, SO and/or SO 2; R 4 and R 5 are independently selected from hydrogen, OH, -OR 5a, -O-Aryl, -OCH 2 -Aryl, alkyl, cycloalkyl, halogen, -CN, -CO 2 R 5b, -CO 2 H, COR 6b, -CH R 6c, -CH(OR 5h)R 6d, -CONR 6a R 6e, -NHCOR 5c, -NHSO 2 R 5d, -NHSO 2 -Aryl, Aryl, -SR 5e, -SOR 5f, -SO 2 R 5g, -SO 2 -Aryl, or a five, six or seven membered heterocycle which may contain 1 to 4 heteroatoms in the ring which are N, O, S, SO and/or SO 2; or R 4 and R 5 together with the carbons to which they are attached form an annelated five, six or seven membered carbocycle or heterocycle which may contain 1 to 4 heteroatoms in the ring which are N, O, S, SO and/or SO 2; R 7, R 5a, R 5b, R 5c, R 5d, R 5e, R 5f, R 5g, R 5h and R 5i are independently selected from alkyl; R 6a, R 6b, R 6c, R 6d and R 6e are independently selected from hydrogen, alkyl, aryl, alkylaryl or cycloalkyl, or R 6a and R 6e together with the nitrogen to which they are attached form an annelated five, six or seven membered heterocycle which may contain 1 to 4 heteroatoms in the ring which are N, O, S, SO and/or SO 2; A is O, S, 1,1-cyclopropyfidene, CHF, CF 2 or (CH 2) n where n is 1 to 3; wherein said compound I' is selected from any of the following compounds: