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The Marschalk reaction is the sodium dithionite promoted reaction of a phenolic anthraquinone with an aldehyde to yield a substituted phenolic anthraquinone after the addition of acid.
The mechanism can be found in the book Named Reactions in Organic Chemistry, and its more intuitive version is provided below:
One of the first applications of this reaction was reported in 1985.
References
Marschalk reaction Wikipedia(Text) CC BY-SA