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 (Et 2 O, CH 2 Cl 2 , CHCl 3 , 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 CH 2 Cl 2 , Et 2 O, or CHCl 3 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 CH 2 Cl 2 solution, or in CHCl 3 and Et 2 O 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.