Synthesis and Characterization of 2-Substituted 6-(Methylthio)bicyclo[2.2.1]heptanes

The synthesis and structure elucidations of several 2-endo-substituted-6-endo-(methylthio)-, 2-exo-substituted- 6-endo-(methylthio)-, 2-endo-substituted-6-exo-(methylthio)-, and 2-exo-substituted-6-exo-(methylthio)- bicyclo[2.2.1]heptanes, 2-5, respectively, are reported. The structure proofs involved a combination of spectroscopic, X-ray crystallographic, and equilibration studies. The crystal and molecular structures of 6-endo-(methylthio) bicyclo[2.2.1]heptane-2-endo-carboxylic acid (2a) and 6-exo-(methylthio)bicyclo[2.2.1]heptane-2-endo-carboxylic acid (4a) were determined by X-ray crystallographic techniques. These compounds crystallize in the space groups Pbca and P2l/n, respectively, with a = 13.040 (2) Å,b = 10.877 (3) Å,c = 13.518 (3)Å, Z = 8, and α = 12.740 (2) Å, b = 6.200 (1) Å, c = 13.146 (2) Å, β = 114.81 (1)°, 2 = 4, respectively. The nonhydrogen atoms were located in each case by direct methods. Full-matrix least-squares refinement led to conventional R factors of 0.0466 and 0.0389, respectively, after several cycles of anisotropic refinement. These molecular structure determinations revealed an unsymmetrical contra twist of the norbornyl skeleton in endo acid 2a and an unsymmetrical synchro twist in endo acid 4a.