2,2,3,3-tetraethoxycyclobutan-1-one

3,4,4-triethoxy-2-cyclobutenone

7,8-Carbonyldioxy-7,8-dichlor-2,5-dioxa-bicyclo[4.2.0]octan

1,4-dichloro-3,3,4-trifluoroyclobutene

2,2,4,4-Tetrabrom-cyclobutanon-(1)