Cycloadditions of Allyl Cations, 21. Synthesis and 1H NMR Spectra of Bicyclo[3.2.1]octa‐2,6‐Dienes and 3‐Methylenebicyclo[3.2.1]oct‐6‐enes from Bicyclo[3.2.1]oct‐6‐en‐3‐ones Methylated at C‐2 and C‐4

Bicyclo[3.2.1]oct‐6‐en‐3‐ones 5–8, methylated at C‐2 and C‐4, have been transformed into methylated bicyclo[3.2.1]octa‐2,6‐dienes 19–24 and 3‐methylenebicyclo[3.2.1]oct‐6‐enes 16–18 by three routes: 1) Conversion into tertiary alcohols 9–11 with methylmagnesium halide and dehydration. 2) Reduction to epimeric secondary alcohols 12–15 with LiAlH4 and also i‐Bu2AlH, followed by dehydration. 3) Methylenation with CH2Br2, Zn/TiCl4 in THF/CH2Cl2. — The reduction of 2,4‐methylated bicyclo[3.2.1]oct‐6‐en‐3‐ones 5–8 with LiAlH4 and i‐Bu2AlH gives axial and equatorial alcohols; the sterically more demanding i‐Bu2AlH attacks preferentially from the exo side, yielding predominantly axial alcohols. Physical, spectroscopic, and chemical properties of the resulting bicyclo[3.2.1]oct‐6‐en‐3‐ols 12–15 have been correlated by conformational analysis. Axial alcohols form hydrogen bonds intramolecularly, giving inter al. lower melting points and shorter retention times than equatorial alcohols, a sharp OH band in the IR, and a downfield 1H NMR shift of the signals of the olefinic protons 6‐H, 7‐H (in CCl4 solvent). They also show marked coupling (3J = 10–12 Hz) of the OH proton in solvent CCl4. Exceptions to these trends are ascribed to flattening of the six‐membered ring. The dehydration of the sterically hindered tertiary 3‐methylbicyclo[3.2.1]oct‐6‐en‐3‐ols 9–11 with phosphoryl chloride in pyridine produces bicyclo[3.2.1]octa‐2,6‐dienes 19–21 in high yields, but requires forcing conditions (≈︁100°C) compared to secondary alcohols (0–25°C). Bicyclo[3.2.1]octa‐2,6‐dienes, unlike the isomeric 3‐methylenebicyclo[3.2.1]oct‐6‐enes, form crystalline AgNO3 complexes. 2,3,4,4‐Tetramethylbicyclo[3.2.1]octa‐2,6‐diene (21) reacts with atmospheric oxygen at ambient temperature and −20°C to give 2,4,5,5‐tetramethyl‐3‐oxatricyclo[4.2.1.02,4]non‐7‐ene (26).