You're going to have to use some trigonometry to calculate the coordinates of the pentagon vertices.

I'd do it in a loop before you actually draw it. Something like this -

Code :

[FONT=Courier New]int v;
float pent[5][2];
float ang, da = 6.2832 / 5.0; // central angle between vertices in radians
// Compute vertex coordinates.
for (v = 0; v < 5; v++) {
ang = v * da;
pent[v][0] = cos (ang);
pent[v][1] = sin (ang);
}
// Draw loop.
glBegin (GL_LINE_LOOP);
for (v = 0; v < 5; v++) glVertex2fv(pent[v]);
glEnd();
[/FONT]

This should draw a pentagon with vertices on the unit circle.

Haven't tested this code. But it should be close to what you need.

Did you know that glVertex can only be called between glBegin and glEnd?