3-hydroxy-4-methylcyclobut-3-ene-1,2-dione

2-methyl-1-propan-2-yloxy-5,8-dioxaspiro[3.4]oct-1-en-3-one

3,4-dimethylcyclobut-3-ene-1,2-dione

4-hydroxy-4-phenyl-2,3-dimethylcyclobutenone

3-(1-methylethoxy)-4-methylcyclobut-3-ene-1,2-dione