Synthesis of 2,5-Dihydro-1,2,4-oxadiazoles through Formal [3 + 2] Cycloaddition of Oxazoles with Nitrosobenzene Derivatives
作者:Hiroyuki Suga、Xiaolan Shi、Toshikazu Ibata
DOI:10.1246/bcsj.71.1231
日期:1998.5
The reactions of substituted oxazoles with nitrosobenzene gave 2-phenyl-2,5-dihydro-1,2,4-oxadiazoles regioselectively through formal [3 + 2] cycloadditions proceeding via a ringopening of oxazoles by an attack of nitrosobenzene. The reactions with 1-chloro- and 1-methyl-4-nitrosobenzenes also produced the corresponding 2,5-dihydro-1,2,4-oxadiazoles regioselectivity.
Tin(IV) chloride-catalyzed formal [3 + 2] cycloaddition of 5-alkoxy-2-(p-methoxyphenyl)- or 2-phenyloxazoles with diethyl oxomalonate gave 4,5,5-tris(alkoxycarbonyl)-2-oxazolines in high regioselectivity. Under similar conditions, 4-substituted 5-alkoxy-2-methyloxazoles showed a trend to shift the regioselectivity in favor for 3-oxazolines rather than 2-oxazolines. Reaction of 5-alkoxy-4-ethoxycarbonyloxazoles with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone proceeded only under high pressure to give 2-oxazolines exclusively.