Abstract: We discuss homogeneous principal bundles endowed with invariant principal or Cartan connections and treat the problem of prolonging invariant conformal and CR structures on homogeneous spaces to Cartan geometries. For a family of CR structures on SU(l+2)/U(l) we give explicit prolongations to Cartan geometries and use the resulting curvatures of the Cartan connections to show that the only spherical CR manifold in this family is SU(2)=S^3.