Jak inhibitor

A compound of formula (I) or a pharmaceutically acceptable salt thereof,

• wherein,
• ring A is selected from optionally substituted:
• 5˜12 membered heterocycloalkyl or 5˜6 membered heteroaryl;
• R1 is selected from H or optionally substituted C1-6 alkyl, C1-6 heteroalkyl, C3-6 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;
• optionally, R1 and the N in L forms a optionally substituted 3˜6 membered ring;
• R2 is selected from H, or optionally substituted C1-3 alkyl and 3˜6 membered cycloalkyl;
• optionally, the structural unit
• can be replaced by
• and “hetero” indicates O, S or NO, and the number is selected from 1, 2 or 3.