The Reaction of Diazo Ketones in the Presence of Metal Chelates. VIII. The Stereochemistry of the 1,3-Dipolar Cycloaddition of 1-Methoxy-2-benzopyrylium-4-olates to Ethylenic Dipolarophiles

Article abstract of DOI:10.1246/bcsj.54.240

1,3-Dipolar cycloadducts were obtained in the reactions of 1-methoxy-2-benzopyrylium-4-olate (3a) with ethylenic dipolarophiles, such as dimethyl fumarate, dimethyl maleate, maleic anhydride, trans-1,2-dibenzoylethylene, N-substituted maleimides, and acenaphthylene.The configuration of the adducts was determined on the basis of the NMR coupling pattern of the methine protons of the adducts in connection with a deuterium experiment.The previously reported structure of the adduct of 3a with dimethyl fumarate was corrected according to the experimental results obtained using 3-deuterated 3a.The stereospecifity of the cycloaddition was confirmed in the reactions of dimethyl fumarate, dimethyl maleate, and trans-1,2-dibenzoylethylene; this stereospecifity was explained by the concerted (_?2_s + _?4_s) mechanism.The exo/endo ratios of the adducts of N-substituted maleimides may be explained by the combination of the steric repulsion and ?-? interaction of the phenyl ring 3 and the substituent on the N-atom of maleimides.The cycloadditions of 1-methoxy-3-methyl-2-benzopyrylium-4-olate with N-substituted maleimides were also explained in a similar manner.