2,2-dimethyl-5-(7'-oxabicyclo<2.2.1>hept-5'-en-2'-ylidene)-1,3-dioxan-4,6-dione

5-chloro-7-oxabicyclo<2.2.1>hept-5-en-2-one

2-endo-acetoxy-6-(2-nitrobenzenesulfonyl)-7-oxabicyclo<2.2.1>hept-5-ene-2-exo-carbonitrile