It was made at the same time as sonic 2,just it was made in japan,as they were originaly supposed to be the same game,but development teams were split up,but sonic 2 was relesed before sonic cd,but still made at the same time.
Sonic 4 EP 2 should not have anything to do with CD. If Sonic 4 EP2 is canon, then Sonic did not save Little Planet, due to Stardust Speedway being in it's Bad Future when Metal Sonic is rejuvenated there by Eggman. Sonic CD, at least the Christian Whitehead build is the same game made in 1993, which was made from the ground up with a completely different physics engine, to counter the Collision Detection issues the original CD had.
The re-release has no change on the Plot, it's the same game, but with added features that make it more than just an average port.
Sonic: Stand back Tails, this guy is no robo to play around with. Yo Metal Sonic! Tryin' to help my friend here, so if yer' hoping to battle then later, okay? Metal Sonic: T_T" Sonic: Tails, meet MEtal Sonic, my counterpart rival. Metal Sonic, this is Tails. He has 2 tails. Metal Sonic: . . . . I'M NOT AN ALLY TO YOU, YOU KNOW o_o" Sonic: . . . . Tails: . . . . Metal Sonic: . . . . Tails: IF YOUR NOT HIS ALLY THAN GET OUT! Metal Sonic: 0.0"