Oxosumanenes were synthesized through benzylic oxidation. The electronic and redox properties were revealed to exhibit the expanded 蟺-conjugation compared to sumanene. Single-crystal X-ray analysis of monooxosumanene showed columnar 蟺-stacking in a concave鈥揷onvex fashion. Stereoselective trimethylation of the trioxo derivative was performed via 1,2-addition to the carbonyl groups.