Reversal of Stereoselectivity in Cycloadditions of Five‐Membered Cyclic Nitrones Derived from Hexoses

Abstract

Two five‐membered cyclic nitrones derived from hexoses, a newly synthesized d‐allo‐configured nitrone and an already known d‐talo‐configured nitrone, have been examined in 1,3‐dipolar cycloaddition reactions with vinyl acetate, allyl benzoate, methyl acrylate, acrylonitrile, and vinylene carbonate. The reactions of the d‐allo‐configured nitrone proceeded preferentially through the expected exo‐anti transition state, but all the cycloadditions of the d‐talo‐configured nitrone surprisingly showed high exo‐syn selectivity. This reversal of stereoselectivity has been explained by 3D analysis of the preferred conformations of the nitrones, obtained from X‐ray data. The structures were further inspected using CONFLEX, PM5, and DFT calculations. All these methods revealed that the dioxolane substituents attached to C‐5 of both nitrones had opposite spatial locations.

A Convenient Synthetic Route towards 3,5‐Bis(hydroxymethyl)pyrrolizidines: Stereoselective Synthesis of Unnatural (–)‐Hyacinthacine B2

“…Acetoxyisoxazolidine 15 was prepared, as previously described, from known D‐mannose‐derived nitrone 5 by the syn ‐stereoselective 1,3‐dipolar cycloaddition with vinyl acetate (neat, 75 °C, 24 h) in 72% yield . Its hydrolysis according to Moffat's method with potassium carbonate in aqueous methanol gave isoxazolidinol 10 (Scheme ) in almost quantitative 96% isolated yield as a mixture two undefined anomers in a 65:35 ratio.…”