The reaction of 2-methylisoselenochromanium salt with Grignardreagents afforded the reductive ring-opened product by the single electron transfer (SET) mechanism, not by the self-decomposition of σ-selenurane. The same reduction was observed in the reaction of the selenonium salt with magnesium metal. Some other selenonium salts were easily reduced by magnesium metal to give ring-opened products.
acetals 3 and 21 in the presence of BF(3).Et(2)O to give alpha-alkoxyalkyl enones 4, 5 and 22, 23 in good yields. When the reaction mixtures were worked up with a saturated NaHCO(3) solution instead of Et(3)N, onium salts 6 and 7 were obtained together with 4 and 5. Reactions with cyclic acetal 14 gave alpha-(beta-hydroxyethoxy) enones 15 and 16 accompanied by dimeric products 17 and 18.
The tandemMichael-aldolreaction of 1-[2-(methylsulfanyl)-phenyl]prop-2-en-1-one (1) or the seleno congener 4 with p-nitrobenzaldehyde in the presence of BF3.Et2O gave the Baylis-Hillman adduct 2 or 5 and onium salt 3 or 6, respectively, and selenochromanone 7 from 4.