There’s a sweetener on the market (forgot the name) that say’s it’s made 100% from regular sugar (sucrose), but that it’s been modified into this sucralose that provides no calories yet all of the taste of sugar.
I’ve read a little on the web about this stuff and it does seem to be pretty straightforward. Here’s a link about the chemistry/biology and other bits and pieces about the compound.
It seems that the the replacement of three of the eight hydroxyl groups (as in free sucrose) with chlorine atoms reduces the body’s ability to metabolise this molecule to a great degree, but allows the maintainance of the interaction (spatially) which gives rise to the sweet taste.
I don’t know enough about carbohydrate metabolism (yet, I’ll read up) to understand why metabolism is inhibited, but I don’t think it’s a coincidence that (chemical) chlorination (naturally) occurs at the more reactive primary hydroxyl sites (and one of the secondary sites), perhaps blocking a normal initial degradation pathway.
[This could be nonsense, but maybe the first step in the pathway for sucrose metabolism involves phosphorylation to give sucrose-6-phosphate, with phosphorylation having occurred in the fructose ring; this pathway would be blocked]
Aside: Chlorination, or any kind of halogenation of a molecule does not necessarily make it any more toxic than it’s predecessor.