Product 11 forms only one stereoisomer because in this step, the bond that breaks is between oxygen and C2 which does not affect the stereochemistry in molecule 11, enabling C3 and C1 to retain its stereochemistry.
C4 forms a carbocation in this step, creating a stereocenter when H attacks. However, it favors the cis product because the electrons of the methoxy group has a favorable interaction with the hydrogen on the -OH group, stabilizing the configuration and making the cis product more favorable.
Stereochemistry of Product 11
