Sonic CD is probably one of the stranger entries in the whole Sonic series -- certainly as far as the classic Sonic games. There's the oddities of the American soundtrack, the whole Past/Present/Future mechanic, and a lot of the game relying on a sort of controlled chaos where levels almost seem to be based on throwing everything at a wall and seeing what sticks, when in actuality they're somewhat more complex. Sonic CD was also pivotal to the resurrection of what made Sonic great -- a fan remake by Christian Whitehead led to an official Sega port on modern systems, which led to a mobile version of Sonic 1 and arguably to Sonic Mania. And now we have more info on another piece of this puzzle: A port of Sonic CD to the Nintendo DS that never happened.
A prototype/proof of concept of the Nintendo DS version of Sonic CD, featuring Palmtree Panic Acts 1-3, is downloadable from Headcannon's official site -- a name that you'll probably recognise if you've played Sonic Mania. This port was developed by Simon "Stealth" Thomley in 2009 -- mainly out fo his own interest, but also as something that could potentially be floated to Sega. The proof of concept has actually been available from Headcannon for a couple of years, although it seems as if interest has picked up again thanks to the success of Sonic Mania. Simon wished to see if a game such as Sonic CD could be ported to the DS, and was particularly interested in getting the game's Special Stage to work on there. As the Nintendo DS was too weak to support software such as Christian Whitehead's Retro Engine, this remake would have to be done primarily from scratch.
While this proof of concept was taken to completion, it was scuppered once Sega gave Headcannon the official go ahead to remake Sonic CD for modern platforms thanks to Christian Whitehead's work -- there would have been too many legal issues for it to overcome. Still, what's here is quite intriguing -- the Palmtree Panic Zone is largely recreated here in full, and it is also possible to play as Knuckles in this prototype, something which you could not do in the later officially released remake of Sonic CD. As mentioned the prototype is downloadable from Headcannon, and is also playable on either a Nintendo DS emulator or through using a flashcart -- if you're intrigued in this little scrap of Sonic history, then it's certainly worth dusting off your old DS for.