The 1-norbornene skeleton by carbene rearrangement

4-Bromo-1-(dibromomethyl)bicyclo[2.1.1]hexane (18) was synthesized by formation of the dienolate of dimethyl cyclopentanedicarboxylate 15b, which is then transformed into 16. Reaction of 16 with diiodomethane gives the diester 14b, and selective saponification leads to the half-ester 14c. Degradation of 14c to methyl 4-bromobicyclo[2.1.1]hexane-1-carboxylate (17a), reduction of the ester to the corresponding carbinol 17b, oxidation of 17b to the aldehyde 17c, and conversion of the aldehyde with triphenyl phosphite/bromine gives compound 18. Reaction of 18 with NaN(SiMe3) in diethyl ether in the presence of diphenylisobenzofuran afforded a 3.3:1 mixture of the Diels-Alder adducts 22 and 23, indicating the presence of 2,4-dibromobicyclo[2.1.1]hept-1-ene as a reactive intermediate generated by rearrangement of carbene 19.