Complex sentence: Van Garderen would attack his leading companions at the foot of the first ascent of Alpe d'Huez, with only Moser and Riblon able to keep pace and caught up with him away from the top of the climb.
Split into simpler sentences: Van Garderen would attack his leading companions at the foot of the first ascent of Alpe d'Huez. Only Moser and Riblon were able to keep pace with Van Garderen and caught up with him away from the top of the climb.