The Ozonolysis of Longifolene: A Tool for the Preparation of Useful Chiral Compounds. Configuration Determination of New Stereogenic Centers by NMR Spectroscopy and X-Ray Crystallography

The ozonolysis of (+)‐longifolene (1) in different solvents (Et2O, CH2Cl2, CHCl3, acetone) at −80° provided quantitatively longifolene epoxide (3) as a single diastereoisomer in which the O‐atom is endo‐positioned (Scheme 2). Upon warming to room temperature, the epoxide remained stable only in acetone and was isolated as a low‐melting crystalline compound. In CH2Cl2, Et2O, or CHCl3 solution, epoxide 3 rapidly rearranged to the isomeric enols 4 and 5, which underwent further rearrangement to give the exo‐aldehyde 6. On standing for several weeks in CH2Cl2 solution, or in CHCl3 and Et2O as well, at room temperature, aldehyde 6 slowly rearranged into its epimer 7. The aldehydes 6 and 7 were isolated on the preparative scale for further synthetic use. The addition of methylmagnesium iodide to 6 and 7 provided the corresponding alcohols 13/14 and 15/16, respectively, which were isolated as pure diastereoisomers (Scheme 4). The configurations of the new chiral centers in 13–16 were determined by NMR methods and X‐ray crystallography.