1,3-Dipolar cycloaddition of nitrile oxides with C1-substituted 7-oxabenzonorbornadienes

1,3-Dipolar cycloadditions of unsymmetrical C1-substituted 7-oxabenzonorbornadienes with acetonitrile oxide and benzonitrile oxide were investigated to probe the regio- and stereoselectivities of these reactions. All novel oxabicycle-fused isoxazoline adducts were obtained in moderate to excellent yields ( 53%-98%) and with complete exo-stereoselectivity. The major regioisomer in almost all instances showed positioning of the nitrile oxide's methyl or phenyl group anti to the C1-substituent. The relative ratios of the regioisomers suggested nonnegligible electronic and steric contributions of the C1-substituent in these reactions. Several useful applications of these 2-isoxazolines toward natural product synthesis are proposed via reductive cleavage of their N-O bonds.