Heteroatom-directed π-facial diastereoselection in diels-alder cycloadditions of plane-nonsymmetric cyclopentadienes

Article abstract of DOI:10.1021/ja00159a038

The synthesis of a series of pentamethylcyclopentadienes bearing stereogenic C-5 heteroatom substituents and their reactions in [4 + 2] cycloadditions with maleic anhydride and/or N-phenylmaleimide are described. Cyclopentadienes (13, 14, 6, 7, and 1) containing the substituents OH, OCH₃, NH₂, NHAc, and C1 reacted to form syn adducts preferentially. In contrast, with compound 8 (SH) only slight syn discrimination was observed, while the facial selectivity was reversed with other sulfur substituents (SCH₃, SPh, SCH₂Ph, SOCH₃, SO₂CH₃) (compounds 9, 3, 2, 10, and 11), and anti adducts were the major or exclusive products. This behavior is consistent with the σ donor ability of the C-X versus the C-C bond so that cycloaddition occurs preferentially anti to the best donor due to hyperconjugation of this antiperiplanar σ bond with the developing incipient bond(s).