4-Chloro-7-nitrobenzofurazan as a Diels–Alder reagent. A facile access to highly functionalized naphthofurazans

Abstract

4-Chloro-7-nitrobenzo-2-oxa-1,3-diazole (4-chloro-7-nitrobenzofurazan or NBD chloride) is found to react instantaneously and quantitatively with trans-1-methoxy-3-(trimethylsilyloxy)-1,3-butadiene (Danishefsky's diene), to form regioselectively the silyl enol ether 4, according to a normal electron-demand Diels–Alder (NEDDA) reaction. This provides the first evidence that this strongly electrophilic heterocycle can also exhibit pericyclic reactivity. Unmasking the enol 4 followed by treatment in the presence of 2 equiv. of base (DBN) leads to 4-chloro-7-hydroxynaphtho[1,2-c]furazan 7. We are thus reporting an easy access to a functionalized hydroxynaphthofurazan, which is obtained under mild conditions in three steps with greater than 80% overall yield.