HETEROATOM-DIRECTED PI-FACIAL DIASTEREOSELECTION IN DIELS-ALDER CYCLOADDITIONS OF PLANE-NONSYMMETRIC CYCLOPENTADIENES

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 Cl 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). © 1990, American Chemical Society. All rights reserved.