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 15 b, which is then transformed into 16. Reaction of 16 with diiodomethane gives the diester 14 b, and selective saponification leads to the half-ester 14 c. Degradation of 14 c to methyl 4-bromobicyclo[2.1.1]hexane-1-carboxylate (17 a), reduction of the ester to the corresponding carbinol 17 b, oxidation of 17 b to the aldehyde 17 c, and conversion of the aldehyde with triphenyl phosphite/bromine gives compound 18. Reaction of 18 with NaN(SiMe(3)) 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.