NaH Promoted One-Pot Tandem Reactions of 3‑(1-Alkynyl) Chromones to Form 2‑Nitrogen-Substituted Xanthones

A silver-catalyzed
dimerization of ethyl isocyanoacetates could
trigger the tandem reaction of 3-(1-alkynyl) chromones under the basic
condition in a one-pot reaction to afford xanthone skeletons with
2-imidazolyl substitution in an efficient manner. With the control
experiment in hand, a mechanism including dimerization of isocyanoacetate/deprotonation/Michael
addition/ring-opening/cyclization 1,2-elimination was deduced. Further
investigation for the base was carried out, resulting in NaH as an
optimal base to avoid the dimerization of 3-(1-alkynyl) chromones.
The scope of this methodology was extended on the different substituents
of 3-(1-alkynyl)-chromones and the potential of other N-heterocycle
glycine ester anions to give the novel functional 2-nitrogen-derived
xanthones.