There are many compounds that taste sweet. Many of the artificial ones taste much sweeter than natural sugars (which is why they contribute little to energy intake when use as sweeteners).
A range of sweeteners is shown in the figure below:
Structurally there are common features for some. We can see why sucralose (a non-metabolisable sweetener based on sucrose) might taste sweet. Rebaudioside a (a major ingredient of the sugar substitute Steria) shares some of those features. But the very sweet artificial sweeteners seem to have less in common.
Do we know what makes molecules generate the sensation of sweetness? Which features matter and why are some sweeter than others?