JAK inhibitor

1. A compound of formula (I),

or a pharmaceutically acceptable salt thereof,

wherein:

ring A is selected from optionally substituted: 5-12 membered heterocycloalkyl having from 1 to 4 ring heteroatoms selected from the group consisting of N, O and S;

R 1 is selected from H or optionally substituted C 1-6 alkyl, C 1-6 heteroalkyl, C 3-6 cycloalkyl, 3˜6 membered heterocycloalkyl, 5˜6 membered aryl, or 5˜6 membered heteroaryl;

L is selected from single bond, —C(═O)O—, acyl or optionally substituted: amino, aminoacyl, acyl amino methylene and aminoacyl methylene; and when L contains a nitrogen atom, R 1 and optionally form an optionally substituted 3˜6 membered ring with the nitrogen atom of L; and

R 2 is selected from H, or optionally substituted C 1-3 alkyl and 3˜6 membered cycloalkyl;

optionally, the structural unit

can be replaced by

2. The compound of formula (I) or the pharmaceutically acceptable salt thereof according to claim 1 , wherein substituents of the said 5-12 membered heterocycloalkyl, 5˜6 membered heteroaryl, C 1-6 alkyl, C 1-6 heteroalkyl, C 3-6 cycloalkyl, 3˜6 membered heterocycloalkyl, 5˜6 membered aryl, 5˜6 membered heteroaryl, amino, aminoacyl, acylamino methyl, aminoacyl methylene, C 1-3 alkyl, and 3˜6 membered ring are selected from halogen, cyano, hydroxy, amino or selected from optional halogenated, hydroxylated and/or ammoniated C 1-3 alkyl and C 1-3 heteroalkyl;

the number of the substituents is selected from 0, 1, 2, 3, 4 or 5; and

the substituent is selected from F, Cl, Br, I, OH, NH 2 , CN, Me, ethyl, n-propyl, isopropyl, cyclopropyl and trifluoromethyl.

3. The compound of formula (I) or the pharmaceutically acceptable salt thereof according to claim 1 , wherein ring A is selected from optionally substituted piperidyl, piperazinyl, morpholinyl, pyrrolidinyl, oxazolinyl and 5-12 membered heterocyclyl group with 1˜2 heteroatoms.

4. The compound of formula (I) or the pharmaceutically acceptable salt thereof according to claim 3 , wherein ring A is selected from

5. The compound of formula (I) or the pharmaceutically acceptable salt thereof according to claim 1 , wherein R 1 is selected from H, or optionally substituted: C 1-4 alkyl, C 1-2 alkyl-O—C 1-2 alkyl-, C 1-2 alkyl-S—C 1-2 alkyl, C 4-5 cycloalkyl, 6 membered aryl, or 5 membered heteroaryl;

or optionally substituted: Me,

C 1-2 alkyl-O—C 1-2 alkyl-, C 1-2 alkyl-S—C 1-2 alkyl, imidazolyl and phenyl; or

R1 is selected from

6. The compound of formula (I) or the pharmaceutically acceptable salt thereof according to claim 1 , wherein R 1 and L together form an optionally substituted 4-5 membered ring;

form an optionally substituted

or form an optionally substituted

7. The compound of formula (I) or the pharmaceutically acceptable salt thereof according to claim 1 , wherein R 1 -L- is selected from optionally substituted: Me,

or R 1 -L- is selected from

8. The compound of formula (I) or the pharmaceutically acceptable salt thereof according to claim 1 , wherein R 2 is selected from H, methyl, ethyl or cyclopropyl.

9. The compound according to claim 1 , wherein the compound is selected from:

10. A method of inhibiting JAK in a subject comprising administering to a subject who is in need thereof a therapeutically effective amount of compound of formula (I) according to claim 1 or the pharmaceutically acceptable salt thereof.

11. The compound of formula (I) or the pharmaceutically acceptable salt thereof according to claim 2 , wherein ring A is selected from optionally substituted piperidyl, piperazinyl, morpholinyl, pyrrolidinyl, oxazolinyl and 5-12 membered heterocyclyl group with 1˜2 heteroatoms.

12. The compound of formula (I) or the pharmaceutically acceptable salt thereof according to claim 11 , wherein ring A is selected from

13. The compound of formula (I) or the pharmaceutically acceptable salt thereof according to claim 2 , wherein R 1 is selected from H, or optionally substituted: C 1-4 alkyl, C 1-2 alkyl-O—C 1-2 alkyl-, C 1-2 alkyl-S—C 1-2 alkyl, C 4-5 cycloalkyl, 6 membered aryl, or 5 membered heteroaryl; or

R 1 is selected from optionally substituted: Me,

C 1-2 alkyl-O—C 1-2 alkyl-, C 1-2 alkyl-S—C 1-2 alkyl, imidazolyl and phenyl;

or R1 is selected from

14. The compound of formula (I) or the pharmaceutically acceptable salt thereof according to claim 2 , wherein R 2 is selected from H, methyl, ethyl or cyclopropyl.

15. A method of inhibiting JAK in a subject comprising administering to a subject who is in need thereof a therapeutically effective amount of compound of formula (I) according to claim 2 or the pharmaceutically acceptable salt thereof.

16. A method of inhibiting JAK in a subject comprising administering to a subject who is in need thereof a therapeutically effective amount of compound of formula (I) according to claim 3 or the pharmaceutically acceptable salt thereof.

17. A method of inhibiting JAK in a subject comprising administering to a subject who is in need thereof a therapeutically effective amount of compound of formula (I) according to claim 4 or the pharmaceutically acceptable salt thereof.

18. A method of inhibiting JAK in a subject comprising administering to a subject who is in need thereof a therapeutically effective amount of compound of formula (I) according to claim 5 or the pharmaceutically acceptable salt thereof.

19. A method of inhibiting JAK in a subject comprising administering to a subject who is in need thereof a therapeutically effective amount of compound of formula (I) according to claim 6 or the pharmaceutically acceptable salt thereof.

20. A method of inhibiting JAK in a subject comprising administering to a subject who is in need thereof a therapeutically effective amount of compound of formula (I) according to claim 7 or the pharmaceutically acceptable salt thereof.

21. A method of inhibiting JAK in a subject comprising administering to a subject who is in need thereof a therapeutically effective amount of compound of formula (I) according to claim 8 or the pharmaceutically acceptable salt thereof.

22. A method of inhibiting JAK in a subject comprising administering to a subject who is in need thereof a therapeutically effective amount of compound of formula (I) according to claim 9 or the pharmaceutically acceptable salt thereof.

23. A method of inhibiting JAK in a subject comprising administering to a subject who is in need thereof a therapeutically effective amount of compound of formula (I) according to claim 11 or the pharmaceutically acceptable salt thereof.

24. A method of inhibiting JAK in a subject comprising administering to a subject who is in need thereof a therapeutically effective amount of compound of formula (I) according to claim 12 or the pharmaceutically acceptable salt thereof.

25. A method of inhibiting JAK in a subject comprising administering to a subject who is in need thereof a therapeutically effective amount of compound of formula (I) according to claim 13 or the pharmaceutically acceptable salt thereof.

26. A method of inhibiting JAK in a subject comprising administering to a subject who is in need thereof a therapeutically effective amount of compound of formula (I) according to claim 14 or the pharmaceutically acceptable salt thereof.

27. A method of treating one of rheumatoid arthritis, organ transplant rejection, psoriasis, Crohn's disease, systemic lupus erythematosus, ulcerative colitis and atopic dermatitis in a subject comprising administering to a subject who is in need thereof a therapeutically effective amount of compound of formula (I) according to claim 1 or the pharmaceutically acceptable salt thereof.