逆Ｄｉｅｌｓ−Ａｌｄｅｒ反応によるセレノアルデヒドの発生と各種エノフィルとの反応

Abstract

Selenoaldehydes have been generally considered to be highly reactive species as a dienophile for cycloaddition reactions because of long bond length and poor overlap in the carbon-selenium π bond, so they are one of the important intermediates in the synthesis of selenium-containing heterocycles. Selenoaldehydes, directly generated by the reaction of aromatic and aliphatic aldehydes with bis (dimethylaluminum) selenide as a selenating reagent, were in situ effectively trapped by anthracene to give [4 + 2] cycloadducts (1) in good yields as a stable solid in the most cases. 1 decomposed quantitatively via thermal retro Diels-Alder reaction to regenerate the corresponding selenoaldehydes under neutral conditions. Thus 1 can serve as convenient and clean precursors of selenoaldehydes. Cycloadducts 1 were heated with 2-trimethylsilyloxy-1, 3-butadiene in toluene at reflux temperature, followed by addition of trifluoroacetic acid to afford a mixture of 3-and 4-selenacyclohexanones in good yields. The regiochemistry in this cycloaddition reaction was influenced by electronic character of a substituent on selenoaldehydes. Thermally generated selenoaldehydes from 1 reacted with 2-methoxyfuran to give 5-substituted penta-2, 4-dienoates as a mixture of four geometrical isomers in excellent yields along with the deposition of elemental selenium. On the other hand, the reaction of selenoaldehydes with 5-alkoxyoxazoles afforded 1 : 1 adducts having 3-selenazoline structure as a mixture of diastereomers in good yields, regioselectively. Furthermore, aromatic selenoaldehydes reacted with nitrile oxides or nitrile imines under neutral conditions to afford the [3 + 2] cycloadducts with a complete regioselectivity in excellent yields.