Wipeout or Image Clip using a Circle
After you create the multi-sided polygon, just use the WIPEOUT or IMAGECLIP command. If you use IMAGECLIP, make sure the Invert option is set correctly.
See below for examples. The top image is a inverted image clip created from a 200-sided polygon, which approximates a circle very well. The bottom image contains another 200-sided polygon with a wipeout applied, and a closed spline converted to a polyline, to which an inverted image clip has been applied.