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In biochemistry laboratory, it's common for undergraduate students to perform identification of carbohydrates and its functional groups. Bial's test, a method to check if any pentose is present, is one of some ways for the identification. The overall reaction can be found in Wikipedia page (https://en.m.wikipedia.org/wiki/Bial's_test).

The conversion of pentose to furfural has known to me (because I've found the paper) but not the second reaction. I guess some organometallic reactions are involved but not sure to write the mechanism.

I've tried to search the research articles related to the mechanism in Google, Google Scholar, ... or even, Scopus but can't find any paper related to it. This is indeed surprising to me because I thought this classical test should be investigated exhaustively either by spectrometry or computational perspectives.

Any thought or idea is appreciated to present the mechanism (even the mechanism is produced by direct computation).

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The acid-catalyzed condensation of furfural with orcinol to form structure 1 is explained here for Molisch's test: Clarification in the mechanism for Molisch's test for glucose. Ferric chloride is the oxidant of 1 (see 2 ----> 3). Acid catalyzes the ring closure of quinoid structure 3 to intermediate 4, which rearomatizes to 5 with loss of water. Direct cyclization of 2 with acid is unlikely because it would interrupt the aromaticity of one of the orcinol rings.

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