Diastereoselective Reactions of the Tiglic Acid Functionality Mediated by Oxazolidine Chiral Auxiliaries: A Mechanistic Comparison of DMD and m ‐CPBA Epoxidations versus Singlet Oxygen and PTAD Ene Reactions

2,2‐Dimethyloxazolidines have been utilized as chiral auxiliaries for the diastereoselective functionalization of the optically active tiglic acid derivatives ( S )‐ 1 by means of epoxidation with DMD or m ‐CPBA and ene reactions with 1 O 2 or PTAD. In the DMD and m ‐CPBA epoxidations, high diastereoselectivities but opposite senses of diastereomer selection were observed. In contrast, the stereochemistry of the 1 O 2 and PTAD ene reactions depended on the size of the attacking enophile: whereas essentially perfect diastereoselectivity was obtained with PTAD, much lower stereoselection was observed with 1 O 2 . The stereochemical results for the DMD and m ‐CPBA epoxidations and the PTAD ene reaction are explained in terms of the energy differences for the corresponding diastereomeric transition states, dictated by steric and electronic effects. The PTAD ene reaction for these tiglic acids ( S )‐ 1 provides, after removal of the chiral auxiliaries, an attractive synthetic route for optically active β‐amino acid derivatives.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)