GENERATION OF SELENABENZENES BEARING AN ELECTRON-WITHDRAWING GROUP ATTHE 2-POSITION

3,6-Dihydro-2H-selenopyrans 2 with an electron-withdrawing group at th e 2-position were prepared by the Diels-Alder reaction of butadienes w ith selenoaldehydes generated in situ from selenocyanates 1 and trieth ylamine. Oxidation of the dihydroselenopyrans 2 with 1.5eq of m-chloro perbenzoic acid provided 2H-selenopyrans 9 and 3,6-dihydro-2H-selenopy ran-2-yl m-chlorobenzoates 10. The benzoates 10 were smoothly converte d into the selenopyrans 9 using polyphosphoric acid trimethylsilyl est er. The selenopyrans 9 were methylated with methyl trifluoromethanesul fonate to give Se-methyl selenopyranium trifluoromethanesulfonates 12. Deprotonation of the selenonium salts 12 with sodium hydride or triet hylamine generated the selenabenzene derivatives 13, but they were too unstable to be isolated. Therefore, we confirmed the generation of 13 by H-1- and C-13-NMR spectroscopy at -30 degrees C.