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Dialkylamino-substituted indolin-2(3H)-ones were prepared by cyclisation of the required mandelanilides with hot polyphosphoric acid. A new reagent (PPEt) obtained from polyphosphoric acid and ethanol produced imidazoquinolines from aminobenzimidazoles and β-keto-esters and a 1,8-naphthyridine from 2-aminopyridine and ethyl α-methylacetoacetate, reactions which could not be brought about by PPA. The scope of the new reagent is discussed.It was also shown that PPA offers no advantage over sulphuric acid in various Skraup reactions.
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Mullock et al. (1970) studied this question.
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