Stereochemistry of cycloaddition of (S)-N-(1-phenylethyl)-C-phosphorylated nitrone with cyclopentene and 2,3-dihydrofuran

Cycloaddition of (S)-N-(1-phenylethyl)-C-(diethoxyphosphoryl)nitrone to cyclopentene gave a mixture of diethyl (3S,3aR,6aS)- and (3R,3aS,6aR)-hexahydro-2-[(S)-1-phenylethyl]-2H-cyclopenta[d]isoxazol-3-yl-3-phosphonates with moderate (68:32) diastereoselectivity, while the reaction with 2,3-dihydrofuran led regiospecifically to the formation of an easily separable mixture of diethyl (3S,3aR,6aR)- and (3R,3aS,6aS)-hexahydro-2-[(S)-1-phenylethyl]furo[3,2-d]isoxazol-3-yl-3-phosphonates in a 65:35 ratio. Absolute configurations of cycloadducts were established based on conformational analyses employing 1H and 13C NMR data and confirmed by 2D NOE experiments. Diastereoisomeric diethyl (3S,3aR,6aS)- and (3R,3aS,6aR)-hexahydro-2-[(S)-1-phenylethyl]-2H-cyclopenta[d]isoxazol-3-yl-3-phosphonates were transformed into both enantiomers of tert-butyl (2S,3S,3aR,6aS)- and (2R,3R,3aS,6aR)-2-ethoxy-2-oxo-cyclopenta[d](1,2-oxaphospholan-3-yl)carbamates in good yields.