Synthesis and Crystal and Molecular Structure of the Conformationally Restricted Methionine Analogue (±)-2-exo-Amino-6-endo-(methylthio)bicyclo[2.2.1]heptane-2-endo-carboxylic Acid and Neighboring Group Participation in its Anodic Oxidation

(±)-2-exo-Amino-6-endo-(methylthio)bicyclo[2.2.1]heptane-2-endo-carboxylic acid (lc) was synthesized by amination of the lithium enolate of methyl 6-endo-(methylthio)bicyclo[2.2.1]heptane-2-ercdo-carboxylate with 0-(mesitylenesulfonyl)hydroxylamine followed by hydrolysis. Its crystal and molecular structure was determined by single-crystal X-ray analysis. It crystallizes in the monoclinic space group P2_1/c with a = 9.681 (6) Å, b = 10.276 (5) Å, c = 9.773 (4) Å, β = 91.23 (4)°, and Z = 4. The structure was solved by direct methods. Full-matrix least-squares refinement led to a conventional R factor of 0.048 after several cycles of anisotropic refinement. The structure is compared both with 6-exo-(methylthio)bicyclo[2.2.1]heptane-2-endo-carboxylic acid (3) and the HBr salt of 2-exo-aminobicyclo[2.2.1]heptane-2-endo-carboxylic acid (2). Electrochemical oxidation of lc in acetonitrile, using the technique of cyclic voltammetry, revealed two oxidation waves with peak potentials of 0.90 and 1.35 V. Controlled potential electrolysis of lc provided the corresponding sulfoxides as a mixture of diastereomers (in 60 and 25-30% yield, respectively), which were also prepared by chemical oxidation, derivatized, separated, and characterized. The remarkable cathodic shift of 450 mV for lc is ascribed to neighboring carboxylate participation in oxidation of the thioether moiety.