Abstract
Treatment of o-alkynylphenylsydnones with triflic acid or trifluoroacetic acid provides a novel avenue to 3-arylcinnolines. With triflic acid the best results were obtained at room temperature with alkynylsydnones containing an electron-withdrawing group (or an H) whereas with TFA an electron-donating group and reflux conditions were required.
Highlights
Sydnones are dipolar, nonbenzenoid aromatic compounds, which are the most studied members of the class of heterocycles known as mesoionic.[1]
Sydno[3,4-a]quinoxalines were prepared by Coburn and O’Donnell using a lithiation protocol,[10] for us the most effective, general strategy to fused-ring sydnones has been to generate an electrophilic side-chain that can be trapped by the heteroaromatic sydnone ring
We postulated that this issue could be alleviated by the use of an organic acid and, we explored the use of trifluoromethanesulfonic acid (TfOH) and trifluoroacetic acid (TFA) as acid replacements
Summary
Nonbenzenoid aromatic compounds, which are the most studied members of the class of heterocycles known as mesoionic.[1]. We have synthesized a sydno[4,3-c]benzotriazine,[9] several 4-arylaminosydno[3,4-a]quinoxalines,[8] and a limited number of 5-substituted sydno[3,4-a]quinazolines.[11,12] In order to explore further this concept, and extend the limited range of fused-ring sydnones available, we elected to examine the reactions of o-alkynylphenylsydnones 1 (prepared by Sonogashira coupling with o-iodophenylsydnone)[13] with various acids in the expectation that sydnoquinoline derivatives 2 would result. While reaction of 1a with sulfuric acid did lead to the corresponding fused-ring sydnone 2a (Table 1, entry 1) the extremely low yield led us to consider alternative avenues.
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