1,3‐Dipolar Cycloaddition of Benzonitrile Oxide to Substituted 4,5‐Dihydro‐3‐methylene‐2(3H)‐furanones
Benzonitrile oxide cycloadds to the CC bond of substituted 4,5-dihydro-3-methylene-2(3H)-furanones 1a–1g. Thus, (E)-1a and (E)-1b are converted into the corresponding isoxazolinespirodihydrofuranones (4RS,5RS)-2a and (4RS,5RS)-2b, and (E)-1c is converted into a mixture of isomers (4RS,5RS)-2c and (4RS,5RS)-3c. The reactions of (E)-1d–1g yield mixtures of the corresponding diastereomeric isoxazolinespirodihydrofuranones (4RS,5RS,8RS)-2d and (4RS,5RS,8SR)-2d, (4RS,5RS)-3e and (4RS,5SR)-3e, (4RS,5RS)-3f and (4RS,5SR)-3f, (4RS,5RS)-3g and (4RS,5SR)-3g. The addition of benzonitrile oxide to (Z)-1a gives (4RS,5SR)-2a and 4a. In the cycloaddition with (E)-1a the p-tolylsulfonyloxy group in the intermediate (4RS,5RS)-2a is partially replaced by chlorine to give (4RS,5RS)-2h. – The structures of (4RS,5RS)-2h, (4RS,5RS)-3e, (4RS,5SR)-3e, and 4a are determined by X-ray analysis. The 1H- and 13C-NMR spectra of the products are discussed and the reaction course is rationalized on the basis of HOMO-LUMO energy gaps.