A compound of formula (I), or stereoisomers or pharmaceutically acceptable salts thereof,
wherein, R
1a
, R
1b
, R
2a
, R
2b
, U
1
, U
2
, X
1
, X
2
and L are as defined herein.
A compound of formula (I), or stereoisomers or pharmaceutically acceptable salts thereof,
wherein, R1a, R1b, R2a, R2b, U1, U2, X1, X2 and L are as defined herein.